From 4206df8fb660730e769508cbf35015e74057ba69 Mon Sep 17 00:00:00 2001
From: Max Onishi <max@insight.com>
Date: Mon, 3 May 2021 15:03:25 +1000
Subject: [PATCH 1/2] null safety

---
 .gitignore                                    |   1 +
 CHANGELOG.md                                  |   9 +-
 README.md                                     |   4 +-
 example/android/app/build.gradle              |  12 +-
 .../android/app/src/main/AndroidManifest.xml  |  19 +-
 .../reorderablesexample/MainActivity.java     |  12 +-
 example/android/build.gradle                  |   2 +-
 example/android/gradle.properties             |  17 +-
 .../gradle/wrapper/gradle-wrapper.properties  |   4 +-
 example/ios/Runner.xcodeproj/project.pbxproj  |  21 +-
 .../contents.xcworkspacedata                  |   2 +-
 example/lib/column_example1.dart              |  18 +-
 example/lib/column_example2.dart              |  15 +-
 example/lib/main.dart                         |  57 +--
 example/lib/nested_wrap_example.dart          |   8 +-
 example/lib/row_example.dart                  |   2 +-
 example/lib/sliver_example.dart               |  26 +-
 example/lib/table_example.dart                |   2 +-
 example/lib/wrap_example.dart                 |   2 +-
 example/pubspec.yaml                          |   6 +-
 example/web/favicon.png                       | Bin 0 -> 917 bytes
 example/web/icons/Icon-192.png                | Bin 0 -> 5292 bytes
 example/web/icons/Icon-512.png                | Bin 0 -> 8252 bytes
 example/web/index.html                        |  45 ++
 example/web/manifest.json                     |  23 +
 lib/generated/i18n.dart                       | 126 ------
 lib/src/rendering/tabluar_flex.dart           | 225 ++++------
 lib/src/rendering/transitions.dart            |  11 +-
 lib/src/rendering/wrap.dart                   | 118 ++----
 lib/src/widgets/basic.dart                    |   7 +-
 lib/src/widgets/passthrough_overlay.dart      | 113 +++--
 lib/src/widgets/reorderable_flex.dart         | 238 ++++++-----
 lib/src/widgets/reorderable_mixin.dart        |   4 +-
 lib/src/widgets/reorderable_sliver.dart       | 192 +++++----
 lib/src/widgets/reorderable_table.dart        |  57 +--
 lib/src/widgets/reorderable_widget.dart       |  12 +-
 lib/src/widgets/reorderable_wrap.dart         | 393 +++++++-----------
 lib/src/widgets/tabluar_flex.dart             |  26 +-
 lib/src/widgets/transitions.dart              |   6 +-
 lib/src/widgets/wrap.dart                     |   8 +-
 pubspec.yaml                                  |   4 +-
 41 files changed, 815 insertions(+), 1032 deletions(-)
 create mode 100644 example/web/favicon.png
 create mode 100644 example/web/icons/Icon-192.png
 create mode 100644 example/web/icons/Icon-512.png
 create mode 100644 example/web/index.html
 create mode 100644 example/web/manifest.json
 delete mode 100644 lib/generated/i18n.dart

diff --git a/.gitignore b/.gitignore
index 47e0b4d..62a5b66 100644
--- a/.gitignore
+++ b/.gitignore
@@ -59,6 +59,7 @@ build/
 **/ios/Flutter/Generated.xcconfig
 **/ios/Flutter/app.flx
 **/ios/Flutter/app.zip
+**/ios/Flutter/flutter_export_environment.sh
 **/ios/Flutter/flutter_assets/
 **/ios/ServiceDefinitions.json
 **/ios/Runner/GeneratedPluginRegistrant.*
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 40e4714..b5849cb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,10 @@
+## [0.4.1] - 11 April 2021.
+* Addresses Issue [#111](https://github.com/hanshengchiu/reorderables/issues/111). Resolves ReorderableSliverList ScrollController conflict (thanks [qAison](https://github.com/qAison)).
+
+## [0.4.0] - 24 March 2021.
+
+* Initial Null-Safety release.
+
 ## [0.3.2] - 10 March 2020.
 * Fix health suggestions.
 
@@ -5,7 +12,7 @@
 * Supports making individual child non-reorderable. See ReorderableColumn example 1.
 
 ## [0.3.0] - 10 Jan 2020.
-* Fix: Bad type in onLeave 
+* Fix: Bad type in onLeave
 
 ## [0.2.12] - 22 June 2019.
 * Removed dependency of FlutterErrorDetails and other ErrorXXX classes.
diff --git a/README.md b/README.md
index 0bc4e77..26c6bce 100644
--- a/README.md
+++ b/README.md
@@ -1,11 +1,11 @@
-** Kindly submit PR if you encounter issues and please make sure you're using stable channel releases. Updates might be slow...
+** Kindly submit PR if you encounter issues and please make sure you're using stable channel releases. <br>
+** Maintaining open source software ain't easy. If you've commercially used this software, please consider [support](https://www.buymeacoffee.com/q5gkeA4t2).
 
 # reorderables
 
 [![pub package](https://img.shields.io/pub/v/reorderables.svg)](https://pub.dartlang.org/packages/reorderables)
 [![Awesome Flutter](https://img.shields.io/badge/Awesome-Flutter-blue.svg?longCache=true&style=flat-square)](https://github.com/Solido/awesome-flutter)
 [![Buy Me A Coffee](https://img.shields.io/badge/Donate-Buy%20Me%20A%20Coffee-yellow.svg)](https://www.buymeacoffee.com/q5gkeA4t2)
-[![Donate](https://img.shields.io/badge/Donate-PayPal-green.svg)](https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=2L56VGH228QJE)
 
 
 Various reorderable, a.k.a. drag and drop, Flutter widgets, including reorderable table, row, column, wrap, and sliver list, that make their children draggable and 
diff --git a/example/android/app/build.gradle b/example/android/app/build.gradle
index ca7fe59..3703fd4 100644
--- a/example/android/app/build.gradle
+++ b/example/android/app/build.gradle
@@ -25,7 +25,7 @@ apply plugin: 'com.android.application'
 apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
 android {
-    compileSdkVersion 28
+    compileSdkVersion 30
 
     lintOptions {
         disable 'InvalidPackage'
@@ -34,11 +34,11 @@ android {
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "com.example.reorderablesexample"
-        minSdkVersion 16
-        targetSdkVersion 28
+        minSdkVersion 23
+        targetSdkVersion 30
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName
-        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
+        testInstrumentationRunner 'androidx.test.runner.AndroidJUnitRunner'
     }
 
     buildTypes {
@@ -56,6 +56,6 @@ flutter {
 
 dependencies {
     testImplementation 'junit:junit:4.12'
-    androidTestImplementation 'com.android.support.test:runner:1.0.2'
-    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
+    androidTestImplementation 'androidx.test.ext:junit:1.1.2'
+    androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'
 }
diff --git a/example/android/app/src/main/AndroidManifest.xml b/example/android/app/src/main/AndroidManifest.xml
index 7d46aa3..e19ed50 100644
--- a/example/android/app/src/main/AndroidManifest.xml
+++ b/example/android/app/src/main/AndroidManifest.xml
@@ -7,15 +7,13 @@
     -->
     <uses-permission android:name="android.permission.INTERNET"/>
 
-    <!-- io.flutter.app.FlutterApplication is an android.app.Application that
-         calls FlutterMain.startInitialization(this); in its onCreate method.
-         In most cases you can leave this as-is, but you if you want to provide
-         additional functionality it is fine to subclass or reimplement
-         FlutterApplication and put your custom class here. -->
     <application
-        android:name="io.flutter.app.FlutterApplication"
         android:label="reorderables_example"
-        android:icon="@mipmap/ic_launcher">
+        android:icon="@mipmap/ic_launcher"
+        android:allowBackup="false">
+        <meta-data
+            android:name="flutterEmbedding"
+            android:value="2" />
         <activity
             android:name=".MainActivity"
             android:launchMode="singleTop"
@@ -23,13 +21,6 @@
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density"
             android:hardwareAccelerated="true"
             android:windowSoftInputMode="adjustResize">
-            <!-- This keeps the window background of the activity showing
-                 until Flutter renders its first frame. It can be removed if
-                 there is no splash screen (such as the default splash screen
-                 defined in @style/LaunchTheme). -->
-            <meta-data
-                android:name="io.flutter.app.android.SplashScreenUntilFirstFrame"
-                android:value="true" />
             <intent-filter>
                 <action android:name="android.intent.action.MAIN"/>
                 <category android:name="android.intent.category.LAUNCHER"/>
diff --git a/example/android/app/src/main/java/com/example/reorderablesexample/MainActivity.java b/example/android/app/src/main/java/com/example/reorderablesexample/MainActivity.java
index dd9ced6..ada12b8 100644
--- a/example/android/app/src/main/java/com/example/reorderablesexample/MainActivity.java
+++ b/example/android/app/src/main/java/com/example/reorderablesexample/MainActivity.java
@@ -1,13 +1,5 @@
 package com.example.reorderablesexample;
 
-import android.os.Bundle;
-import io.flutter.app.FlutterActivity;
-import io.flutter.plugins.GeneratedPluginRegistrant;
+import io.flutter.embedding.android.FlutterActivity;
 
-public class MainActivity extends FlutterActivity {
-  @Override
-  protected void onCreate(Bundle savedInstanceState) {
-    super.onCreate(savedInstanceState);
-    GeneratedPluginRegistrant.registerWith(this);
-  }
-}
+public class MainActivity extends FlutterActivity { }
diff --git a/example/android/build.gradle b/example/android/build.gradle
index bb8a303..fab3c2e 100644
--- a/example/android/build.gradle
+++ b/example/android/build.gradle
@@ -5,7 +5,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.2.1'
+        classpath 'com.android.tools.build:gradle:4.1.3'
     }
 }
 
diff --git a/example/android/gradle.properties b/example/android/gradle.properties
index 7be3d8b..8d7e07a 100644
--- a/example/android/gradle.properties
+++ b/example/android/gradle.properties
@@ -1,2 +1,17 @@
-org.gradle.jvmargs=-Xmx1536M
+## For more details on how to configure your build environment visit
+# http://www.gradle.org/docs/current/userguide/build_environment.html
+#
+# Specifies the JVM arguments used for the daemon process.
+# The setting is particularly useful for tweaking memory settings.
+# Default value: -Xmx1024m -XX:MaxPermSize=256m
+# org.gradle.jvmargs=-Xmx2048m -XX:MaxPermSize=512m -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8
+#
+# When configured, Gradle will run in incubating parallel mode.
+# This option should only be used with decoupled projects. More details, visit
+# http://www.gradle.org/docs/current/userguide/multi_project_builds.html#sec:decoupled_projects
+# org.gradle.parallel=true
+#Sat Mar 20 19:11:05 EDT 2021
+android.enableJetifier=true
 android.enableR8=true
+android.useAndroidX=true
+org.gradle.jvmargs=-Xmx1536M
diff --git a/example/android/gradle/wrapper/gradle-wrapper.properties b/example/android/gradle/wrapper/gradle-wrapper.properties
index 2819f02..8413cf2 100644
--- a/example/android/gradle/wrapper/gradle-wrapper.properties
+++ b/example/android/gradle/wrapper/gradle-wrapper.properties
@@ -1,6 +1,6 @@
-#Fri Jun 23 08:50:38 CEST 2017
+#Sat Mar 20 19:06:23 EDT 2021
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.10.2-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.5-all.zip
diff --git a/example/ios/Runner.xcodeproj/project.pbxproj b/example/ios/Runner.xcodeproj/project.pbxproj
index 5ca2738..3c3e6a0 100644
--- a/example/ios/Runner.xcodeproj/project.pbxproj
+++ b/example/ios/Runner.xcodeproj/project.pbxproj
@@ -8,12 +8,7 @@
 
 /* Begin PBXBuildFile section */
 		1498D2341E8E89220040F4C2 /* GeneratedPluginRegistrant.m in Sources */ = {isa = PBXBuildFile; fileRef = 1498D2331E8E89220040F4C2 /* GeneratedPluginRegistrant.m */; };
-		2D5378261FAA1A9400D5DBA9 /* flutter_assets in Resources */ = {isa = PBXBuildFile; fileRef = 2D5378251FAA1A9400D5DBA9 /* flutter_assets */; };
 		3B3967161E833CAA004F5970 /* AppFrameworkInfo.plist in Resources */ = {isa = PBXBuildFile; fileRef = 3B3967151E833CAA004F5970 /* AppFrameworkInfo.plist */; };
-		3B80C3941E831B6300D905FE /* App.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 3B80C3931E831B6300D905FE /* App.framework */; };
-		3B80C3951E831B6300D905FE /* App.framework in Embed Frameworks */ = {isa = PBXBuildFile; fileRef = 3B80C3931E831B6300D905FE /* App.framework */; settings = {ATTRIBUTES = (CodeSignOnCopy, RemoveHeadersOnCopy, ); }; };
-		9705A1C61CF904A100538489 /* Flutter.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 9740EEBA1CF902C7004384FC /* Flutter.framework */; };
-		9705A1C71CF904A300538489 /* Flutter.framework in Embed Frameworks */ = {isa = PBXBuildFile; fileRef = 9740EEBA1CF902C7004384FC /* Flutter.framework */; settings = {ATTRIBUTES = (CodeSignOnCopy, RemoveHeadersOnCopy, ); }; };
 		9740EEB41CF90195004384FC /* Debug.xcconfig in Resources */ = {isa = PBXBuildFile; fileRef = 9740EEB21CF90195004384FC /* Debug.xcconfig */; };
 		978B8F6F1D3862AE00F588F7 /* AppDelegate.m in Sources */ = {isa = PBXBuildFile; fileRef = 7AFFD8EE1D35381100E5BB4D /* AppDelegate.m */; };
 		97C146F31CF9000F007C117D /* main.m in Sources */ = {isa = PBXBuildFile; fileRef = 97C146F21CF9000F007C117D /* main.m */; };
@@ -29,8 +24,6 @@
 			dstPath = "";
 			dstSubfolderSpec = 10;
 			files = (
-				3B80C3951E831B6300D905FE /* App.framework in Embed Frameworks */,
-				9705A1C71CF904A300538489 /* Flutter.framework in Embed Frameworks */,
 			);
 			name = "Embed Frameworks";
 			runOnlyForDeploymentPostprocessing = 0;
@@ -40,15 +33,12 @@
 /* Begin PBXFileReference section */
 		1498D2321E8E86230040F4C2 /* GeneratedPluginRegistrant.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = GeneratedPluginRegistrant.h; sourceTree = "<group>"; };
 		1498D2331E8E89220040F4C2 /* GeneratedPluginRegistrant.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GeneratedPluginRegistrant.m; sourceTree = "<group>"; };
-		2D5378251FAA1A9400D5DBA9 /* flutter_assets */ = {isa = PBXFileReference; lastKnownFileType = folder; name = flutter_assets; path = Flutter/flutter_assets; sourceTree = SOURCE_ROOT; };
 		3B3967151E833CAA004F5970 /* AppFrameworkInfo.plist */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.plist.xml; name = AppFrameworkInfo.plist; path = Flutter/AppFrameworkInfo.plist; sourceTree = "<group>"; };
-		3B80C3931E831B6300D905FE /* App.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = App.framework; path = Flutter/App.framework; sourceTree = "<group>"; };
 		7AFA3C8E1D35360C0083082E /* Release.xcconfig */ = {isa = PBXFileReference; lastKnownFileType = text.xcconfig; name = Release.xcconfig; path = Flutter/Release.xcconfig; sourceTree = "<group>"; };
 		7AFFD8ED1D35381100E5BB4D /* AppDelegate.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = AppDelegate.h; sourceTree = "<group>"; };
 		7AFFD8EE1D35381100E5BB4D /* AppDelegate.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = AppDelegate.m; sourceTree = "<group>"; };
 		9740EEB21CF90195004384FC /* Debug.xcconfig */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.xcconfig; name = Debug.xcconfig; path = Flutter/Debug.xcconfig; sourceTree = "<group>"; };
 		9740EEB31CF90195004384FC /* Generated.xcconfig */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.xcconfig; name = Generated.xcconfig; path = Flutter/Generated.xcconfig; sourceTree = "<group>"; };
-		9740EEBA1CF902C7004384FC /* Flutter.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = Flutter.framework; path = Flutter/Flutter.framework; sourceTree = "<group>"; };
 		97C146EE1CF9000F007C117D /* Runner.app */ = {isa = PBXFileReference; explicitFileType = wrapper.application; includeInIndex = 0; path = Runner.app; sourceTree = BUILT_PRODUCTS_DIR; };
 		97C146F21CF9000F007C117D /* main.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = main.m; sourceTree = "<group>"; };
 		97C146FB1CF9000F007C117D /* Base */ = {isa = PBXFileReference; lastKnownFileType = file.storyboard; name = Base; path = Base.lproj/Main.storyboard; sourceTree = "<group>"; };
@@ -62,8 +52,6 @@
 			isa = PBXFrameworksBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
-				9705A1C61CF904A100538489 /* Flutter.framework in Frameworks */,
-				3B80C3941E831B6300D905FE /* App.framework in Frameworks */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};
@@ -73,10 +61,7 @@
 		9740EEB11CF90186004384FC /* Flutter */ = {
 			isa = PBXGroup;
 			children = (
-				2D5378251FAA1A9400D5DBA9 /* flutter_assets */,
-				3B80C3931E831B6300D905FE /* App.framework */,
 				3B3967151E833CAA004F5970 /* AppFrameworkInfo.plist */,
-				9740EEBA1CF902C7004384FC /* Flutter.framework */,
 				9740EEB21CF90195004384FC /* Debug.xcconfig */,
 				7AFA3C8E1D35360C0083082E /* Release.xcconfig */,
 				9740EEB31CF90195004384FC /* Generated.xcconfig */,
@@ -190,7 +175,6 @@
 				3B3967161E833CAA004F5970 /* AppFrameworkInfo.plist in Resources */,
 				9740EEB41CF90195004384FC /* Debug.xcconfig in Resources */,
 				97C146FE1CF9000F007C117D /* Assets.xcassets in Resources */,
-				2D5378261FAA1A9400D5DBA9 /* flutter_assets in Resources */,
 				97C146FC1CF9000F007C117D /* Main.storyboard in Resources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
@@ -210,7 +194,7 @@
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 			shellPath = /bin/sh;
-			shellScript = "/bin/sh \"$FLUTTER_ROOT/packages/flutter_tools/bin/xcode_backend.sh\" thin";
+			shellScript = "/bin/sh \"$FLUTTER_ROOT/packages/flutter_tools/bin/xcode_backend.sh\" embed_and_thin";
 		};
 		9740EEB61CF901F6004384FC /* Run Script */ = {
 			isa = PBXShellScriptBuildPhase;
@@ -263,7 +247,6 @@
 /* Begin XCBuildConfiguration section */
 		249021D3217E4FDB00AE95B9 /* Profile */ = {
 			isa = XCBuildConfiguration;
-			baseConfigurationReference = 7AFA3C8E1D35360C0083082E /* Release.xcconfig */;
 			buildSettings = {
 				ALWAYS_SEARCH_USER_PATHS = NO;
 				CLANG_ANALYZER_NONNULL = YES;
@@ -335,7 +318,6 @@
 		};
 		97C147031CF9000F007C117D /* Debug */ = {
 			isa = XCBuildConfiguration;
-			baseConfigurationReference = 9740EEB21CF90195004384FC /* Debug.xcconfig */;
 			buildSettings = {
 				ALWAYS_SEARCH_USER_PATHS = NO;
 				CLANG_ANALYZER_NONNULL = YES;
@@ -389,7 +371,6 @@
 		};
 		97C147041CF9000F007C117D /* Release */ = {
 			isa = XCBuildConfiguration;
-			baseConfigurationReference = 7AFA3C8E1D35360C0083082E /* Release.xcconfig */;
 			buildSettings = {
 				ALWAYS_SEARCH_USER_PATHS = NO;
 				CLANG_ANALYZER_NONNULL = YES;
diff --git a/example/ios/Runner.xcodeproj/project.xcworkspace/contents.xcworkspacedata b/example/ios/Runner.xcodeproj/project.xcworkspace/contents.xcworkspacedata
index 1d526a1..919434a 100644
--- a/example/ios/Runner.xcodeproj/project.xcworkspace/contents.xcworkspacedata
+++ b/example/ios/Runner.xcodeproj/project.xcworkspace/contents.xcworkspacedata
@@ -2,6 +2,6 @@
 <Workspace
    version = "1.0">
    <FileRef
-      location = "group:Runner.xcodeproj">
+      location = "self:">
    </FileRef>
 </Workspace>
diff --git a/example/lib/column_example1.dart b/example/lib/column_example1.dart
index ed9e476..5c508fb 100644
--- a/example/lib/column_example1.dart
+++ b/example/lib/column_example1.dart
@@ -7,7 +7,7 @@ class ColumnExample1 extends StatefulWidget {
 }
 
 class _ColumnExample1State extends State<ColumnExample1> {
-  List<Widget> _rows;
+  late List<Widget> _rows;
 
   @override
   void initState() {
@@ -18,7 +18,11 @@ class _ColumnExample1State extends State<ColumnExample1> {
         (int index) => ReorderableWidget(
               reorderable: true,
               key: ValueKey(index),
-              child: Text('This is row $index', textScaleFactor: 1.5),
+              child: Container(
+                  width: double.infinity,
+                  child: Align(
+                      alignment: Alignment.centerLeft,
+                      child: Text('This is row $index', textScaleFactor: 1.5))),
             ));
 
     _rows += <ReorderableWidget>[
@@ -34,7 +38,12 @@ class _ColumnExample1State extends State<ColumnExample1> {
         (int index) => ReorderableWidget(
               reorderable: true,
               key: ValueKey(11 + index),
-              child: Text('This is row $index', textScaleFactor: 1.5),
+              child: Container(
+                width: double.infinity,
+                child: Align(
+                    alignment: Alignment.centerLeft,
+                    child: Text('This is row $index', textScaleFactor: 1.5)),
+              ),
             ));
   }
 
@@ -55,7 +64,8 @@ class _ColumnExample1State extends State<ColumnExample1> {
       onReorder: _onReorder,
       onNoReorder: (int index) {
         //this callback is optional
-        debugPrint('${DateTime.now().toString().substring(5, 22)} reorder cancelled. index:$index');
+        debugPrint(
+            '${DateTime.now().toString().substring(5, 22)} reorder cancelled. index:$index');
       },
     );
   }
diff --git a/example/lib/column_example2.dart b/example/lib/column_example2.dart
index 93e8922..6ffd4a5 100644
--- a/example/lib/column_example2.dart
+++ b/example/lib/column_example2.dart
@@ -1,5 +1,4 @@
 import 'package:flutter/material.dart';
-
 import 'package:reorderables/reorderables.dart';
 
 class ColumnExample2 extends StatefulWidget {
@@ -8,15 +7,20 @@ class ColumnExample2 extends StatefulWidget {
 }
 
 class _ColumnExample2State extends State<ColumnExample2> {
-  List<Widget> _rows;
+  late List<Widget> _rows;
 
   @override
   void initState() {
     super.initState();
     _rows = List<Widget>.generate(
         10,
-        (int index) => Text('This is row $index',
-            key: ValueKey(index), textScaleFactor: 1.5));
+        (int index) => Container(
+              key: ValueKey(index),
+              width: double.infinity,
+              child: Center(
+                child: Text('This is row $index', textScaleFactor: 1.5),
+              ),
+            ));
   }
 
   @override
@@ -36,7 +40,8 @@ class _ColumnExample2State extends State<ColumnExample2> {
       onReorder: _onReorder,
       onNoReorder: (int index) {
         //this callback is optional
-        debugPrint('${DateTime.now().toString().substring(5, 22)} reorder cancelled. index:$index');
+        debugPrint(
+            '${DateTime.now().toString().substring(5, 22)} reorder cancelled. index:$index');
       },
     ));
 
diff --git a/example/lib/main.dart b/example/lib/main.dart
index 442d55f..5de16e4 100644
--- a/example/lib/main.dart
+++ b/example/lib/main.dart
@@ -1,12 +1,12 @@
 import 'package:flutter/material.dart';
 
-import './table_example.dart';
-import './wrap_example.dart';
-import './nested_wrap_example.dart';
 import './column_example1.dart';
 import './column_example2.dart';
+import './nested_wrap_example.dart';
 import './row_example.dart';
 import './sliver_example.dart';
+import './table_example.dart';
+import './wrap_example.dart';
 
 void main() => runApp(MyApp());
 
@@ -22,7 +22,7 @@ class MyApp extends StatelessWidget {
 }
 
 class MyHomePage extends StatefulWidget {
-  MyHomePage({Key key, this.title}) : super(key: key);
+  MyHomePage({Key? key, required this.title}) : super(key: key);
 
   // This widget is the home page of your application. It is stateful, meaning
   // that it has a State object (defined below) that contains fields that affect
@@ -62,37 +62,42 @@ class _MyHomePageState extends State<MyHomePage> {
       ),
       body: _examples[_currentIndex],
       bottomNavigationBar: BottomNavigationBar(
-        currentIndex: _currentIndex, // this will be set when a new tab is tapped
+        fixedColor: _bottomNavigationColor,
+        showSelectedLabels: true,
+        showUnselectedLabels: false,
+        currentIndex: _currentIndex,
+        // this will be set when a new tab is tapped
 //        type: BottomNavigationBarType.fixed,
         items: [
           BottomNavigationBarItem(
-            icon: Icon(Icons.grid_on, color: _bottomNavigationColor),
-            title: Text('ReroderableTable', maxLines: 2, style: TextStyle(color: _bottomNavigationColor)),
-          ),
+              icon: Icon(Icons.grid_on, color: _bottomNavigationColor),
+              tooltip: "ReorderableTable",
+              label: "Table"),
           BottomNavigationBarItem(
-            icon: Icon(Icons.apps, color: _bottomNavigationColor),
-            title: Text('ReroderableWrap', maxLines: 2, style: TextStyle(color: _bottomNavigationColor)),
-          ),
+              icon: Icon(Icons.apps, color: _bottomNavigationColor),
+              tooltip: "ReorderableWrap",
+              label: "Wrap"),
           BottomNavigationBarItem(
-            icon: Icon(Icons.view_quilt, color: _bottomNavigationColor),
-            title: Text('Nested ReroderableWrap', maxLines: 3, style: TextStyle(color: _bottomNavigationColor)),
-          ),
+              icon: Icon(Icons.view_quilt, color: _bottomNavigationColor),
+              tooltip: 'Nested ReroderableWrap',
+              label: "Nested"),
           BottomNavigationBarItem(
-            icon: Icon(Icons.more_vert, color: _bottomNavigationColor),
-            title: Text('ReroderableColumn 1', maxLines: 2, style: TextStyle(color: _bottomNavigationColor))
-          ),
+              icon: Icon(Icons.more_vert, color: _bottomNavigationColor),
+              tooltip: "ReorderableColumn 1",
+              label: "Column 1"),
           BottomNavigationBarItem(
-            icon: Icon(Icons.more_vert, color: _bottomNavigationColor),
-            title: Text('ReroderableColumn 2', maxLines: 2, style: TextStyle(color: _bottomNavigationColor))
-          ),
+              icon: Icon(Icons.more_vert, color: _bottomNavigationColor),
+              tooltip: "ReroderableColumn 2",
+              label: "Column 2"),
           BottomNavigationBarItem(
-            icon: Icon(Icons.more_horiz, color: _bottomNavigationColor),
-            title: Text('ReroderableRow', maxLines: 2, style: TextStyle(color: _bottomNavigationColor))
-          ),
+              icon: Icon(Icons.more_horiz, color: _bottomNavigationColor),
+              tooltip: "ReorderableRow",
+              label: "Row"),
           BottomNavigationBarItem(
-            icon: Icon(Icons.calendar_view_day, color: _bottomNavigationColor),
-            title: Text('ReroderableSliverList', maxLines: 2, style: TextStyle(color: _bottomNavigationColor))
-          ),
+              icon:
+                  Icon(Icons.calendar_view_day, color: _bottomNavigationColor),
+              tooltip: "ReroderableSliverList",
+              label: "SliverList"),
         ],
         onTap: (int index) {
           setState(() {
diff --git a/example/lib/nested_wrap_example.dart b/example/lib/nested_wrap_example.dart
index bff76c8..e1b4f21 100644
--- a/example/lib/nested_wrap_example.dart
+++ b/example/lib/nested_wrap_example.dart
@@ -4,14 +4,14 @@ import 'package:reorderables/reorderables.dart';
 
 class NestedWrapExample extends StatefulWidget {
   NestedWrapExample({
-    Key key,
+    Key? key,
     this.depth = 0,
     this.valuePrefix = '',
     this.color,
   }) : super(key: key);
   final int depth;
   final String valuePrefix;
-  final Color color;
+  final Color? color;
   final List<Widget> _tiles = [];
 
   @override
@@ -20,8 +20,8 @@ class NestedWrapExample extends StatefulWidget {
 
 class _NestedWrapExampleState extends State<NestedWrapExample> {
 //  List<Widget> _tiles;
-  Color _color;
-  Color _colorBrighter;
+  Color? _color;
+  Color? _colorBrighter;
 
   @override
   void initState() {
diff --git a/example/lib/row_example.dart b/example/lib/row_example.dart
index 2054928..7872509 100644
--- a/example/lib/row_example.dart
+++ b/example/lib/row_example.dart
@@ -8,7 +8,7 @@ class RowExample extends StatefulWidget {
 }
 
 class _RowExampleState extends State<RowExample> {
-  List<Widget> _columns;
+  late List<Widget> _columns;
 
   @override
   void initState() {
diff --git a/example/lib/sliver_example.dart b/example/lib/sliver_example.dart
index 14087c7..d311d94 100644
--- a/example/lib/sliver_example.dart
+++ b/example/lib/sliver_example.dart
@@ -1,5 +1,4 @@
 import 'package:flutter/material.dart';
-
 import 'package:reorderables/reorderables.dart';
 
 class SliverExample extends StatefulWidget {
@@ -8,14 +7,19 @@ class SliverExample extends StatefulWidget {
 }
 
 class _SliverExampleState extends State<SliverExample> {
-  List<Widget> _rows;
+  late List<Widget> _rows;
 
   @override
   void initState() {
     super.initState();
-    _rows = List<Widget>.generate(50,
-        (int index) => Text('This is sliver child $index', textScaleFactor: 2)
-    );
+    _rows = List<Widget>.generate(
+        50,
+        (int index) => Container(
+            width: double.infinity,
+            child: Align(
+                alignment: Alignment.centerLeft,
+                child:
+                    Text('This is sliver child $index', textScaleFactor: 2))));
   }
 
   @override
@@ -26,9 +30,11 @@ class _SliverExampleState extends State<SliverExample> {
         _rows.insert(newIndex, row);
       });
     }
+
     // Make sure there is a scroll controller attached to the scroll view that contains ReorderableSliverList.
     // Otherwise an error will be thrown.
-    ScrollController _scrollController = PrimaryScrollController.of(context) ?? ScrollController();
+    ScrollController _scrollController =
+        PrimaryScrollController.of(context) ?? ScrollController();
 
     return CustomScrollView(
       // A ScrollController must be included in CustomScrollView, otherwise
@@ -40,7 +46,7 @@ class _SliverExampleState extends State<SliverExample> {
           flexibleSpace: FlexibleSpaceBar(
             title: Text('ReorderableSliverList'),
             background: Image.network(
-              'https://upload.wikimedia.org/wikipedia/commons/thumb/6/68/Yushan'
+                'https://upload.wikimedia.org/wikipedia/commons/thumb/6/68/Yushan'
                 '_main_east_peak%2BHuang_Chung_Yu%E9%BB%83%E4%B8%AD%E4%BD%91%2B'
                 '9030.png/640px-Yushan_main_east_peak%2BHuang_Chung_Yu%E9%BB%83'
                 '%E4%B8%AD%E4%BD%91%2B9030.png'),
@@ -56,10 +62,12 @@ class _SliverExampleState extends State<SliverExample> {
           onReorder: _onReorder,
           onNoReorder: (int index) {
             //this callback is optional
-            debugPrint('${DateTime.now().toString().substring(5, 22)} reorder cancelled. index:$index');
+            debugPrint(
+                '${DateTime.now().toString().substring(5, 22)} reorder cancelled. index:$index');
           },
           onReorderStarted: (int index) {
-            debugPrint('${DateTime.now().toString().substring(5, 22)} reorder started. index:$index');
+            debugPrint(
+                '${DateTime.now().toString().substring(5, 22)} reorder started. index:$index');
           },
         )
       ],
diff --git a/example/lib/table_example.dart b/example/lib/table_example.dart
index 037335f..6615d04 100644
--- a/example/lib/table_example.dart
+++ b/example/lib/table_example.dart
@@ -9,7 +9,7 @@ class TableExample extends StatefulWidget {
 
 class _TableExampleState extends State<TableExample> {
   //The children of ReorderableTableRow must be of type ReorderableTableRow
-  List<ReorderableTableRow> _itemRows;
+  late List<ReorderableTableRow> _itemRows;
 
   @override
   void initState() {
diff --git a/example/lib/wrap_example.dart b/example/lib/wrap_example.dart
index 9b9148e..b168269 100644
--- a/example/lib/wrap_example.dart
+++ b/example/lib/wrap_example.dart
@@ -9,7 +9,7 @@ class WrapExample extends StatefulWidget {
 
 class _WrapExampleState extends State<WrapExample> {
   final double _iconSize = 90;
-  List<Widget> _tiles;
+  late List<Widget> _tiles;
 
   @override
   void initState() {
diff --git a/example/pubspec.yaml b/example/pubspec.yaml
index ee7d80c..62009df 100644
--- a/example/pubspec.yaml
+++ b/example/pubspec.yaml
@@ -1,6 +1,6 @@
 name: reorderables_example
 description: Demonstrates how to use the reorderables package..
-
+publish_to: none
 # The following defines the version and build number for your application.
 # A version number is three numbers separated by dots, like 1.2.43
 # followed by an optional build number separated by a +.
@@ -10,7 +10,7 @@ description: Demonstrates how to use the reorderables package..
 version: 1.2.0
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   reorderables:
@@ -21,7 +21,7 @@ dependencies:
 
   # The following adds the Cupertino Icons font to your application.
   # Use with the CupertinoIcons class for iOS style icons.
-  cupertino_icons: ^0.1.2
+  cupertino_icons: ^1.0.2
 
 dev_dependencies:
   flutter_test:
diff --git a/example/web/favicon.png b/example/web/favicon.png
new file mode 100644
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GIT binary patch
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literal 0
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diff --git a/example/web/icons/Icon-192.png b/example/web/icons/Icon-192.png
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zW~&yp@scb-+Qj-EmErY+Tu=dUmf@*BoXY2&oKT8U?8?s1d}4a`Aq>7SV800m$FE~?
zjmz(LY+Xx9sDX$;vU`xgw*jLw7dWOnWWCO8o|;}f>cu0Q&`0I{YudMn;P;L3R-uz#
zfns_mZED_IakFBPP2r_S8XM$X)@O-xVKi4`7373Jkd5{2$M#%cRhWer3M(vr{S6>h
zj{givZJ3(`yFL@``(afn&~iNx@B1|-qfYiZu?-_&Z8+R~v`d6R-}EX9IVXWO-!hL5
z*k6T#^2zAXdardU3Ao~I)4DGdAv2bx{4nOK`20rJo>rmk3S2ZDu}))8Z1m}CKigf0
z3L`3Y`{huj`xj9@`$xTZzZc3je?n^yG<8sw$`Y%}9mUsjUR%T!?k^(q)6FH6Af^b6
zlPg~IEwg0y;`t9y;#D+uz!oE4VP&Je!<#q*F?m5L5?J3i@!0J6q#eu<H4#Z=wE5*>
z!RRU`-)HeqGi_UJZ(n~|PSNsv+Wgl{P-TvaUQ9j?ZC<g8ZiqhE3zDij95zlU{WNzY
zGy)v6`56SoiaGT2p?}>tvb^37U$sFpBrkT{7Jpd<ypu^Vm&l^4a`{KZ84xzZzo;KE
z-=A4+9_nb)Gbn{Qhi?_aB2;ywx^+Pf5T`lalHm9uiA_!U){D6XA1(b=Z<?``I_qA6
zZWTBHX|9X!%D5<=PH}$OLaXYW@n2Z;FvYVJKC4Vca$Pt%jnYqueP>?HpIvj2!}RIq
zH{9~+gErN2+}J`>Jvng2hwM`=PLNkc7pkjblKW|+Fk9rc)G1R>Ww>RC=r-|!m-u7(
zc(a$9NG}w#PjWNMS~)o=i~WA&4L(YIW25@AL9+H9!?3Y}sv#MOdY{bb9j>p`{?O(P
zIvb`n?_(gP2w3P#&91JX*md+bBEr%xUHMVqfB;(f?OPtMnAZ#rm5q5mh;a2f_si2_
z3oXWB?{NF(JtkAn6F(O{z@b76OIqMC$&oJ_&S|YbFJ*)3qVX_uNf5b8(!vGX19hsG
z(OP>RmZp29KH9Ge2kKjKigUmOe^K_!UXP`von)PR8Qz$%=EmOB9xS(ZxE_tnyzo}7
z=6~$~9k0M~v}`w={AeqF?_)9q{m8K#6M{a&(;u;O41j)I$^T?lx5(zlebpY@NT&#N
zR+1bB)-1-xj}R8uwqwf=iP1GbxBjneCC%UrSdSxK1vM^i9;bUkS#iRZw2H>rS<2<$
zNT3|sDH>{tXb=zq7XZi*K?#Zsa1h1{h5!Tq_YbKFm_*=A5-<~j63he;4`77!|LBlo
zR^~tR3yxcU=gDFbshyF6>o0bdp$qmHS7D}m3;^QZq9kBBU|9$N-~oU?G5;jyFR7>z
hN`IR97YZXIo@y!QgFWddJ3|0`sjFx!m))><{BI=FK%f8s

literal 0
HcmV?d00001

diff --git a/example/web/index.html b/example/web/index.html
new file mode 100644
index 0000000..1460b5e
--- /dev/null
+++ b/example/web/index.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <!--
+    If you are serving your web app in a path other than the root, change the
+    href value below to reflect the base path you are serving from.
+
+    The path provided below has to start and end with a slash "/" in order for
+    it to work correctly.
+
+    Fore more details:
+    * https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base
+  -->
+  <base href="/">
+
+  <meta charset="UTF-8">
+  <meta content="IE=Edge" http-equiv="X-UA-Compatible">
+  <meta name="description" content="A new Flutter project.">
+
+  <!-- iOS meta tags & icons -->
+  <meta name="apple-mobile-web-app-capable" content="yes">
+  <meta name="apple-mobile-web-app-status-bar-style" content="black">
+  <meta name="apple-mobile-web-app-title" content="example">
+  <link rel="apple-touch-icon" href="icons/Icon-192.png">
+
+  <!-- Favicon -->
+  <link rel="icon" type="image/png" href="favicon.png"/>
+
+  <title>example</title>
+  <link rel="manifest" href="manifest.json">
+</head>
+<body>
+  <!-- This script installs service_worker.js to provide PWA functionality to
+       application. For more information, see:
+       https://developers.google.com/web/fundamentals/primers/service-workers -->
+  <script>
+    if ('serviceWorker' in navigator) {
+      window.addEventListener('flutter-first-frame', function () {
+        navigator.serviceWorker.register('flutter_service_worker.js');
+      });
+    }
+  </script>
+  <script src="main.dart.js" type="application/javascript"></script>
+</body>
+</html>
diff --git a/example/web/manifest.json b/example/web/manifest.json
new file mode 100644
index 0000000..8c01291
--- /dev/null
+++ b/example/web/manifest.json
@@ -0,0 +1,23 @@
+{
+    "name": "example",
+    "short_name": "example",
+    "start_url": ".",
+    "display": "standalone",
+    "background_color": "#0175C2",
+    "theme_color": "#0175C2",
+    "description": "A new Flutter project.",
+    "orientation": "portrait-primary",
+    "prefer_related_applications": false,
+    "icons": [
+        {
+            "src": "icons/Icon-192.png",
+            "sizes": "192x192",
+            "type": "image/png"
+        },
+        {
+            "src": "icons/Icon-512.png",
+            "sizes": "512x512",
+            "type": "image/png"
+        }
+    ]
+}
diff --git a/lib/generated/i18n.dart b/lib/generated/i18n.dart
deleted file mode 100644
index 30fdb7a..0000000
--- a/lib/generated/i18n.dart
+++ /dev/null
@@ -1,126 +0,0 @@
-import 'dart:async';
-
-import 'package:flutter/foundation.dart';
-import 'package:flutter/material.dart';
-
-// ignore_for_file: non_constant_identifier_names
-// ignore_for_file: camel_case_types
-// ignore_for_file: prefer_single_quotes
-
-// This file is automatically generated. DO NOT EDIT, all your changes would be lost.
-class S implements WidgetsLocalizations {
-  const S();
-
-  static S current;
-
-  static const GeneratedLocalizationsDelegate delegate =
-    GeneratedLocalizationsDelegate();
-
-  static S of(BuildContext context) => Localizations.of<S>(context, S);
-
-  @override
-  TextDirection get textDirection => TextDirection.ltr;
-
-}
-
-class $en extends S {
-  const $en();
-}
-
-class GeneratedLocalizationsDelegate extends LocalizationsDelegate<S> {
-  const GeneratedLocalizationsDelegate();
-
-  List<Locale> get supportedLocales {
-    return const <Locale>[
-      Locale("en", ""),
-    ];
-  }
-
-  LocaleListResolutionCallback listResolution({Locale fallback, bool withCountry = true}) {
-    return (List<Locale> locales, Iterable<Locale> supported) {
-      if (locales == null || locales.isEmpty) {
-        return fallback ?? supported.first;
-      } else {
-        return _resolve(locales.first, fallback, supported, withCountry);
-      }
-    };
-  }
-
-  LocaleResolutionCallback resolution({Locale fallback, bool withCountry = true}) {
-    return (Locale locale, Iterable<Locale> supported) {
-      return _resolve(locale, fallback, supported, withCountry);
-    };
-  }
-
-  @override
-  Future<S> load(Locale locale) {
-    final String lang = getLang(locale);
-    if (lang != null) {
-      switch (lang) {
-        case "en":
-          S.current = const $en();
-          return SynchronousFuture<S>(S.current);
-        default:
-          // NO-OP.
-      }
-    }
-    S.current = const S();
-    return SynchronousFuture<S>(S.current);
-  }
-
-  @override
-  bool isSupported(Locale locale) => _isSupported(locale, true);
-
-  @override
-  bool shouldReload(GeneratedLocalizationsDelegate old) => false;
-
-  ///
-  /// Internal method to resolve a locale from a list of locales.
-  ///
-  Locale _resolve(Locale locale, Locale fallback, Iterable<Locale> supported, bool withCountry) {
-    if (locale == null || !_isSupported(locale, withCountry)) {
-      return fallback ?? supported.first;
-    }
-
-    final Locale languageLocale = Locale(locale.languageCode, "");
-    if (supported.contains(locale)) {
-      return locale;
-    } else if (supported.contains(languageLocale)) {
-      return languageLocale;
-    } else {
-      final Locale fallbackLocale = fallback ?? supported.first;
-      return fallbackLocale;
-    }
-  }
-
-  ///
-  /// Returns true if the specified locale is supported, false otherwise.
-  ///
-  bool _isSupported(Locale locale, bool withCountry) {
-    if (locale != null) {
-      for (Locale supportedLocale in supportedLocales) {
-        // Language must always match both locales.
-        if (supportedLocale.languageCode != locale.languageCode) {
-          continue;
-        }
-
-        // If country code matches, return this locale.
-        if (supportedLocale.countryCode == locale.countryCode) {
-          return true;
-        }
-
-        // If no country requirement is requested, check if this locale has no country.
-        if (true != withCountry && (supportedLocale.countryCode == null || supportedLocale.countryCode.isEmpty)) {
-          return true;
-        }
-      }
-    }
-    return false;
-  }
-}
-
-String getLang(Locale l) => l == null
-  ? null
-  : l.countryCode != null && l.countryCode.isEmpty
-    ? l.languageCode
-    : l.toString();
diff --git a/lib/src/rendering/tabluar_flex.dart b/lib/src/rendering/tabluar_flex.dart
index 9c5b2ab..5202348 100644
--- a/lib/src/rendering/tabluar_flex.dart
+++ b/lib/src/rendering/tabluar_flex.dart
@@ -1,32 +1,15 @@
-import 'dart:math' as math;
 import 'dart:collection';
+import 'dart:math' as math;
 
-import 'package:flutter/rendering.dart';
 import 'package:flutter/foundation.dart';
+import 'package:flutter/rendering.dart';
 
-bool _startIsTopLeft(Axis direction, TextDirection textDirection,
+bool? _startIsTopLeft(Axis direction, TextDirection? textDirection,
     VerticalDirection verticalDirection) {
-  assert(direction != null);
   // If the relevant value of textDirection or verticalDirection is null, this returns null too.
-  switch (direction) {
-    case Axis.horizontal:
-      switch (textDirection) {
-        case TextDirection.ltr:
-          return true;
-        case TextDirection.rtl:
-          return false;
-      }
-      break;
-    case Axis.vertical:
-      switch (verticalDirection) {
-        case VerticalDirection.down:
-          return true;
-        case VerticalDirection.up:
-          return false;
-      }
-      break;
-  }
-  return null;
+  return direction == Axis.horizontal
+      ? (textDirection == null ? null : textDirection == TextDirection.ltr)
+      : verticalDirection == VerticalDirection.down;
 }
 
 typedef _ChildSizingFunction = double Function(RenderBox child, double extent);
@@ -37,15 +20,15 @@ class RenderTabluarFlex extends RenderFlex {
   /// By default, the flex layout is horizontal and children are aligned to the
   /// start of the main axis and the center of the cross axis.
   RenderTabluarFlex({
-    List<RenderBox> children,
+    List<RenderBox>? children,
     Axis direction = Axis.horizontal,
     MainAxisSize mainAxisSize = MainAxisSize.max,
     MainAxisAlignment mainAxisAlignment = MainAxisAlignment.start,
     CrossAxisAlignment crossAxisAlignment = CrossAxisAlignment.center,
-    TextDirection textDirection,
+    TextDirection? textDirection,
     VerticalDirection verticalDirection = VerticalDirection.down,
-    TextBaseline textBaseline,
-    Decoration decoration,
+    TextBaseline? textBaseline,
+    Decoration? decoration,
     ImageConfiguration configuration = ImageConfiguration.empty,
   })  : _decoration = decoration,
         _configuration = configuration,
@@ -60,15 +43,15 @@ class RenderTabluarFlex extends RenderFlex {
           textBaseline: textBaseline,
         );
 
-  BoxPainter _painter;
+  BoxPainter? _painter;
 
   /// What decoration to paint.
   ///
   /// Commonly a [BoxDecoration].
-  Decoration get decoration => _decoration;
-  Decoration _decoration;
+  Decoration? get decoration => _decoration;
+  Decoration? _decoration;
 
-  set decoration(Decoration value) {
+  set decoration(Decoration? value) {
 //    assert(value != null);
     if (value == _decoration) return;
     _painter?.dispose();
@@ -87,7 +70,6 @@ class RenderTabluarFlex extends RenderFlex {
   ImageConfiguration _configuration;
 
   set configuration(ImageConfiguration value) {
-    assert(value != null);
     if (value == _configuration) return;
     _configuration = value;
     markNeedsPaint();
@@ -107,52 +89,37 @@ class RenderTabluarFlex extends RenderFlex {
   }
 
   bool get _debugHasNecessaryDirections {
-    assert(direction != null);
-    assert(crossAxisAlignment != null);
     if (firstChild != null && lastChild != firstChild) {
       // i.e. there's more than one child
-      switch (direction) {
-        case Axis.horizontal:
-          assert(textDirection != null,
-              'Horizontal $runtimeType with multiple children has a null textDirection, so the layout order is undefined.');
-          break;
-        case Axis.vertical:
-          assert(verticalDirection != null,
-              'Vertical $runtimeType with multiple children has a null verticalDirection, so the layout order is undefined.');
-          break;
-      }
+      assert(
+          direction == Axis.vertical || textDirection != null,
+          'Horizontal $runtimeType with multiple children has a '
+          'null textDirection, so the layout order is undefined.');
     }
     if (mainAxisAlignment == MainAxisAlignment.start ||
         mainAxisAlignment == MainAxisAlignment.end) {
-      switch (direction) {
-        case Axis.horizontal:
-          assert(textDirection != null,
-              'Horizontal $runtimeType with $mainAxisAlignment has a null textDirection, so the alignment cannot be resolved.');
-          break;
-        case Axis.vertical:
-          assert(verticalDirection != null,
-              'Vertical $runtimeType with $mainAxisAlignment has a null verticalDirection, so the alignment cannot be resolved.');
-          break;
-      }
+      assert(
+          direction == Axis.vertical || textDirection != null,
+          'Horizontal $runtimeType with $mainAxisAlignment has a null '
+          'textDirection, so the alignment cannot be resolved.');
     }
     if (crossAxisAlignment == CrossAxisAlignment.start ||
         crossAxisAlignment == CrossAxisAlignment.end) {
-      switch (direction) {
-        case Axis.horizontal:
-          assert(verticalDirection != null,
-              'Horizontal $runtimeType with $crossAxisAlignment has a null verticalDirection, so the alignment cannot be resolved.');
-          break;
-        case Axis.vertical:
-          assert(textDirection != null,
-              'Vertical $runtimeType with $crossAxisAlignment has a null textDirection, so the alignment cannot be resolved.');
-          break;
-      }
+      assert(
+          direction == Axis.horizontal || textDirection != null,
+          'Vertical $runtimeType with $crossAxisAlignment has a null '
+          'textDirection, so the alignment cannot be resolved.');
     }
     return true;
   }
 
   // Set during layout if overflow occurred on the main axis.
-  double _overflow;
+  double _overflow = 0;
+
+  ///
+  /// Determines whether the current overflow value is greater than zero.
+  ///
+  bool get _hasOverflow => _overflow > 0.0;
 
   final ListQueue<LayoutCallback<BoxConstraints>> layoutCallbackQueue =
       ListQueue<LayoutCallback<BoxConstraints>>();
@@ -168,10 +135,10 @@ class RenderTabluarFlex extends RenderFlex {
 //  }
 
   double _getIntrinsicSize(
-      {Axis sizingDirection,
-      double
+      {required Axis sizingDirection,
+      required double
           extent, // the extent in the direction that isn't the sizing direction
-      _ChildSizingFunction
+      required _ChildSizingFunction
           childSize // a method to find the size in the sizing direction
       }) {
     if (direction == sizingDirection) {
@@ -181,7 +148,7 @@ class RenderTabluarFlex extends RenderFlex {
       double totalFlex = 0.0;
       double inflexibleSpace = 0.0;
       double maxFlexFractionSoFar = 0.0;
-      RenderBox child = firstChild;
+      RenderBox? child = firstChild;
       while (child != null) {
         final int flex = _getFlex(child);
         totalFlex += flex;
@@ -192,7 +159,8 @@ class RenderTabluarFlex extends RenderFlex {
         } else {
           inflexibleSpace += childSize(child, extent);
         }
-        final FlexParentData childParentData = child.parentData;
+        final FlexParentData childParentData =
+            child.parentData! as FlexParentData;
         child = childParentData.nextSibling;
       }
       return maxFlexFractionSoFar * totalFlex + inflexibleSpace;
@@ -208,7 +176,7 @@ class RenderTabluarFlex extends RenderFlex {
       int totalFlex = 0;
       double inflexibleSpace = 0.0;
       double maxCrossSize = 0.0;
-      RenderBox child = firstChild;
+      RenderBox? child = firstChild;
       Map<int, double> maxGrandchildCrossSize = {};
       while (child != null) {
         final int flex = _getFlex(child);
@@ -227,7 +195,7 @@ class RenderTabluarFlex extends RenderFlex {
               break;
           }
 
-          RenderTabluarFlex tabluarFlexChild =
+          RenderTabluarFlex? tabluarFlexChild =
               _findTabluarFlexDescendant(child);
           if (tabluarFlexChild is RenderTabluarFlex) {
 //            RenderTabluarFlex _child = child as RenderTabluarFlex;
@@ -245,7 +213,8 @@ class RenderTabluarFlex extends RenderFlex {
           inflexibleSpace += mainSize;
           maxCrossSize = math.max(maxCrossSize, crossSize);
         }
-        final FlexParentData childParentData = child.parentData;
+        final FlexParentData childParentData =
+            child.parentData! as FlexParentData;
         child = childParentData.nextSibling;
       }
 
@@ -260,7 +229,7 @@ class RenderTabluarFlex extends RenderFlex {
         final int flex = _getFlex(child);
         if (flex > 0) {
           double mainSize = spacePerFlex * flex;
-          RenderTabluarFlex tabluarFlexChild =
+          RenderTabluarFlex? tabluarFlexChild =
               _findTabluarFlexDescendant(child);
           if (tabluarFlexChild is RenderTabluarFlex) {
             List<RenderBox> grandchildren =
@@ -275,7 +244,8 @@ class RenderTabluarFlex extends RenderFlex {
 //          maxCrossSize = math.max(maxCrossSize, childSize(child, spacePerFlex * flex));
           maxCrossSize = math.max(maxCrossSize, childSize(child, mainSize));
         }
-        final FlexParentData childParentData = child.parentData;
+        final FlexParentData childParentData =
+            child.parentData! as FlexParentData;
         child = childParentData.nextSibling;
       }
 
@@ -327,41 +297,27 @@ class RenderTabluarFlex extends RenderFlex {
   }
 
   int _getFlex(RenderBox child) {
-    final FlexParentData childParentData = child.parentData;
-    return childParentData.flex ?? 0;
+    final FlexParentData? childParentData = child.parentData as FlexParentData;
+    return childParentData?.flex ?? 0;
   }
 
-  FlexFit _getFit(RenderBox child) {
-    final FlexParentData childParentData = child.parentData;
-    return childParentData.fit ?? FlexFit.tight;
+  FlexFit _getFit(RenderBox? child) {
+    final FlexParentData? childParentData = child?.parentData as FlexParentData;
+    return childParentData?.fit ?? FlexFit.tight;
   }
 
-  double _getCrossSize(RenderBox child) {
-    switch (direction) {
-      case Axis.horizontal:
-        return child.size.height;
-      case Axis.vertical:
-        return child.size.width;
-    }
-    return null;
-  }
+  double _getCrossSize(RenderBox child) =>
+      direction == Axis.horizontal ? child.size.height : child.size.width;
 
-  double _getMainSize(RenderBox child) {
-    switch (direction) {
-      case Axis.horizontal:
-        return child.size.width;
-      case Axis.vertical:
-        return child.size.height;
-    }
-    return null;
-  }
+  double _getMainSize(RenderBox child) =>
+      direction == Axis.horizontal ? child.size.width : child.size.height;
 
-  RenderTabluarFlex _findTabluarFlexDescendant(RenderBox child) {
-    RenderObject curDescendant = child;
+  RenderTabluarFlex? _findTabluarFlexDescendant(RenderBox child) {
+    RenderObject? curDescendant = child;
     ListQueue<RenderObject> childrenQueue = ListQueue<RenderObject>();
     while (curDescendant != null && curDescendant is! RenderTabluarFlex) {
 //      RenderObject firstChildRenderer;
-      curDescendant.visitChildren((RenderObject renderObject) {
+      curDescendant!.visitChildren((RenderObject renderObject) {
 //        firstChildRenderer ??= renderObject;
         if (curDescendant is! RenderTabluarFlex) {
           if (renderObject is RenderTabluarFlex) {
@@ -378,8 +334,7 @@ class RenderTabluarFlex extends RenderFlex {
       }
     }
 
-    RenderTabluarFlex tabluarFlexDescendant = curDescendant;
-    return tabluarFlexDescendant;
+    return curDescendant == null ? null : curDescendant as RenderTabluarFlex;
   }
 
   @override
@@ -388,7 +343,6 @@ class RenderTabluarFlex extends RenderFlex {
     // Determine used flex factor, size inflexible items, calculate free space.
     int totalFlex = 0;
     int totalChildren = 0;
-    assert(constraints != null);
     final double maxMainSize = direction == Axis.horizontal
         ? constraints.maxWidth
         : constraints.maxHeight;
@@ -399,7 +353,7 @@ class RenderTabluarFlex extends RenderFlex {
     Map<RenderBox, RenderTabluarFlex> tabluarFlexDescendants = {};
 
     void _layoutChild(RenderBox child, BoxConstraints constraints) {
-      RenderTabluarFlex tabluarFlexChild = _findTabluarFlexDescendant(child);
+      RenderTabluarFlex? tabluarFlexChild = _findTabluarFlexDescendant(child);
 //      debugPrint('this:$this _layoutChild: child:$child tabluarFlexChild:$tabluarFlexChild');
       if (tabluarFlexChild != null) {
         //when this is laying out its child (and descendants), this function will be called. So that we can get the grandchild's size
@@ -466,12 +420,12 @@ class RenderTabluarFlex extends RenderFlex {
           case Axis.horizontal:
             innerConstraints = innerConstraints.copyWith(
                 minWidth: math.max(innerConstraints.minWidth,
-                    minChildrenMainSize[childIndex]));
+                    minChildrenMainSize[childIndex]!));
             break;
           case Axis.vertical:
             innerConstraints = innerConstraints.copyWith(
                 minHeight: math.max(innerConstraints.minHeight,
-                    minChildrenMainSize[childIndex]));
+                    minChildrenMainSize[childIndex]!));
             break;
         }
       }
@@ -484,11 +438,12 @@ class RenderTabluarFlex extends RenderFlex {
     double crossSize = 0.0;
     double allocatedSize =
         0.0; // Sum of the sizes of the non-flexible children.
-    RenderBox child = firstChild;
-    RenderBox lastFlexChild;
+    RenderBox? child = firstChild;
+    RenderBox? lastFlexChild;
     int childIndex = 0;
     while (child != null) {
-      final FlexParentData childParentData = child.parentData;
+      final FlexParentData childParentData =
+          child.parentData! as FlexParentData;
       totalChildren++;
       final int flex = _getFlex(child);
       if (flex > 0) {
@@ -512,16 +467,16 @@ class RenderTabluarFlex extends RenderFlex {
                 'axis. Setting a flex on a child (e.g. using Expanded) indicates that the child is to '
                 'expand to fill the remaining space in the $axis direction.';
             final StringBuffer information = StringBuffer();
-            RenderBox node = this;
+            RenderBox? node = this;
             switch (direction) {
               case Axis.horizontal:
-                while (!node.constraints.hasBoundedWidth &&
-                    node.parent is RenderBox) node = node.parent;
+                while (!node!.constraints.hasBoundedWidth &&
+                    node.parent is RenderBox) node = node.parent as RenderBox;
                 if (!node.constraints.hasBoundedWidth) node = null;
                 break;
               case Axis.vertical:
-                while (!node.constraints.hasBoundedHeight &&
-                    node.parent is RenderBox) node = node.parent;
+                while (!node!.constraints.hasBoundedHeight &&
+                    node.parent is RenderBox) node = node.parent as RenderBox;
                 if (!node.constraints.hasBoundedHeight) node = null;
                 break;
             }
@@ -556,7 +511,7 @@ class RenderTabluarFlex extends RenderFlex {
               'If none of the above helps enough to fix this problem, please don\'t hesitate to file a bug:\n'
               '  https://github.com/flutter/flutter/issues/new?template=BUG.md');
         }());
-        totalFlex += childParentData.flex;
+        totalFlex += childParentData.flex!;
         lastFlexChild = child;
       } else {
         BoxConstraints innerConstraints = _innerConstraints(childIndex);
@@ -616,7 +571,6 @@ class RenderTabluarFlex extends RenderFlex {
               minChildExtent = 0.0;
               break;
           }
-          assert(minChildExtent != null);
           BoxConstraints innerConstraints;
           if (crossAxisAlignment == CrossAxisAlignment.stretch) {
             switch (direction) {
@@ -657,12 +611,12 @@ class RenderTabluarFlex extends RenderFlex {
               case Axis.horizontal:
                 innerConstraints = innerConstraints.copyWith(
                     minWidth: math.max(innerConstraints.minWidth,
-                        minChildrenMainSize[childIndex]));
+                        minChildrenMainSize[childIndex]!));
                 break;
               case Axis.vertical:
                 innerConstraints = innerConstraints.copyWith(
                     minHeight: math.max(innerConstraints.minHeight,
-                        minChildrenMainSize[childIndex]));
+                        minChildrenMainSize[childIndex]!));
                 break;
             }
           }
@@ -684,12 +638,13 @@ class RenderTabluarFlex extends RenderFlex {
                   'To use FlexAlignItems.baseline, you must also specify which baseline to use using the "baseline" argument.');
             return true;
           }());
-          final double distance =
-              child.getDistanceToBaseline(textBaseline, onlyReal: true);
+          final double? distance =
+              child.getDistanceToBaseline(textBaseline!, onlyReal: true);
           if (distance != null)
             maxBaselineDistance = math.max(maxBaselineDistance, distance);
         }
-        final FlexParentData childParentData = child.parentData;
+        final FlexParentData childParentData =
+            child.parentData! as FlexParentData;
         child = childParentData.nextSibling;
         childIndex++;
       }
@@ -706,7 +661,7 @@ class RenderTabluarFlex extends RenderFlex {
       child = firstChild;
       while (child != null) {
 //          debugPrint('this:$this relayout child:$child');
-        BoxConstraints innerConstraints = childrenConstraints[child];
+        BoxConstraints innerConstraints = childrenConstraints[child]!;
         switch (direction) {
           case Axis.horizontal:
             innerConstraints = innerConstraints.copyWith(
@@ -725,7 +680,7 @@ class RenderTabluarFlex extends RenderFlex {
             callbackCalled = true;
           }
 
-          RenderTabluarFlex tabluarFlexChild = tabluarFlexDescendants[child];
+          RenderTabluarFlex tabluarFlexChild = tabluarFlexDescendants[child]!;
           tabluarFlexChild.layoutCallbackQueue.addLast(_childLayoutCallback);
           tabluarFlexChild.minMainSizesQueue.addLast(maxGrandchildrenCrossSize);
           tabluarFlexChild.layout(innerConstraints, parentUsesSize: true);
@@ -740,7 +695,8 @@ class RenderTabluarFlex extends RenderFlex {
         }
         crossSize = math.max(crossSize, _getCrossSize(child));
 
-        final FlexParentData childParentData = child.parentData;
+        final FlexParentData childParentData =
+            child.parentData! as FlexParentData;
         child = childParentData.nextSibling;
       }
 //    debugPrint('this:$this updated crossSize:$crossSize');
@@ -810,7 +766,8 @@ class RenderTabluarFlex extends RenderFlex {
         flipMainAxis ? actualSize - leadingSpace : leadingSpace;
     child = firstChild;
     while (child != null) {
-      final FlexParentData childParentData = child.parentData;
+      final FlexParentData childParentData =
+          child.parentData! as FlexParentData;
       double childCrossPosition;
       switch (crossAxisAlignment) {
         case CrossAxisAlignment.start:
@@ -831,8 +788,8 @@ class RenderTabluarFlex extends RenderFlex {
           childCrossPosition = 0.0;
           if (direction == Axis.horizontal) {
             assert(textBaseline != null);
-            final double distance =
-                child.getDistanceToBaseline(textBaseline, onlyReal: true);
+            final double? distance =
+                child.getDistanceToBaseline(textBaseline!, onlyReal: true);
             if (distance != null)
               childCrossPosition = maxBaselineDistance - distance;
           }
@@ -872,7 +829,7 @@ class RenderTabluarFlex extends RenderFlex {
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (_overflow <= 0.0) {
+    if (!_hasOverflow) {
       defaultPaint(context, offset);
       return;
     }
@@ -881,10 +838,10 @@ class RenderTabluarFlex extends RenderFlex {
     if (size.isEmpty) return;
 
     if (_decoration != null) {
-      _painter ??= _decoration.createBoxPainter(markNeedsPaint);
+      _painter ??= _decoration!.createBoxPainter(markNeedsPaint);
       final ImageConfiguration filledConfiguration =
           configuration.copyWith(size: size);
-      _painter.paint(context.canvas, offset, filledConfiguration);
+      _painter!.paint(context.canvas, offset, filledConfiguration);
     }
 
     // We have overflow. Clip it.
@@ -934,13 +891,13 @@ class RenderTabluarFlex extends RenderFlex {
   }
 
   @override
-  Rect describeApproximatePaintClip(RenderObject child) =>
-      _overflow > 0.0 ? Offset.zero & size : null;
+  Rect? describeApproximatePaintClip(RenderObject child) =>
+      _hasOverflow ? Offset.zero & size : null;
 
   @override
   String toStringShort() {
     String header = super.toStringShort();
-    if (_overflow is double && _overflow > 0.0) header += ' OVERFLOWING';
+    if (_hasOverflow) header += ' OVERFLOWING';
     return header;
   }
 }
diff --git a/lib/src/rendering/transitions.dart b/lib/src/rendering/transitions.dart
index 99d99d7..e2b5c6f 100644
--- a/lib/src/rendering/transitions.dart
+++ b/lib/src/rendering/transitions.dart
@@ -1,13 +1,12 @@
 import 'package:flutter/rendering.dart';
 import 'package:flutter/animation.dart';
-import 'package:flutter/foundation.dart';
 
 class RenderSizeTransitionWithIntrinsicSize extends RenderProxyBox {
   RenderSizeTransitionWithIntrinsicSize({
     this.axis = Axis.vertical,
-    @required this.sizeFactor,
-    RenderBox child,
-  })  : assert(sizeFactor != null),
+    required this.sizeFactor,
+    RenderBox? child,
+  })  :
 //       _axis = axis,
 //       _sizeFactor = sizeFactor,
         super(child);
@@ -26,6 +25,7 @@ class RenderSizeTransitionWithIntrinsicSize extends RenderProxyBox {
 
   @override
   double computeMinIntrinsicWidth(double height) {
+    final child = this.child;
     if (child != null) {
       double childWidth = child.getMinIntrinsicWidth(height);
       return axis == Axis.horizontal
@@ -37,6 +37,7 @@ class RenderSizeTransitionWithIntrinsicSize extends RenderProxyBox {
 
   @override
   double computeMaxIntrinsicWidth(double height) {
+    final child = this.child;
     if (child != null) {
       double childWidth = child.getMaxIntrinsicWidth(height);
       return axis == Axis.horizontal
@@ -48,6 +49,7 @@ class RenderSizeTransitionWithIntrinsicSize extends RenderProxyBox {
 
   @override
   double computeMinIntrinsicHeight(double width) {
+    final child = this.child;
     if (child != null) {
       double childHeight = child.getMinIntrinsicHeight(width);
       return axis == Axis.vertical
@@ -59,6 +61,7 @@ class RenderSizeTransitionWithIntrinsicSize extends RenderProxyBox {
 
   @override
   double computeMaxIntrinsicHeight(double width) {
+    final child = this.child;
     if (child != null) {
       double childHeight = child.getMaxIntrinsicHeight(width);
       return axis == Axis.vertical
diff --git a/lib/src/rendering/wrap.dart b/lib/src/rendering/wrap.dart
index 4b739d7..c41ba9c 100644
--- a/lib/src/rendering/wrap.dart
+++ b/lib/src/rendering/wrap.dart
@@ -19,14 +19,14 @@ class WrapWithMainAxisCountParentData extends WrapParentData {
 
 class RenderWrapWithMainAxisCount extends RenderWrap {
   RenderWrapWithMainAxisCount({
-    List<RenderBox> children,
+    List<RenderBox>? children,
     Axis direction = Axis.horizontal,
     WrapAlignment alignment = WrapAlignment.start,
     double spacing = 0.0,
     WrapAlignment runAlignment = WrapAlignment.start,
     double runSpacing = 0.0,
     WrapCrossAlignment crossAxisAlignment = WrapCrossAlignment.start,
-    TextDirection textDirection,
+    TextDirection? textDirection,
     VerticalDirection verticalDirection = VerticalDirection.down,
     this.minMainAxisCount,
     this.maxMainAxisCount,
@@ -36,24 +36,25 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
 //       _minMainAxisCount = minMainAxisCount,
 //       _maxMainAxisCount = maxMainAxisCount,
         super(
-            children: children,
-            direction: direction,
-            alignment: alignment,
-            spacing: spacing,
-            runAlignment: runAlignment,
-            runSpacing: runSpacing,
-            crossAxisAlignment: crossAxisAlignment,
-            textDirection: textDirection,
-            verticalDirection: verticalDirection);
-
-  int minMainAxisCount;
+          children: children,
+          direction: direction,
+          alignment: alignment,
+          spacing: spacing,
+          runAlignment: runAlignment,
+          runSpacing: runSpacing,
+          crossAxisAlignment: crossAxisAlignment,
+          textDirection: textDirection,
+          verticalDirection: verticalDirection,
+        );
+
+  int? minMainAxisCount;
 
 //  int get minMainAxisCount => _minMainAxisCount;
 //  set minMainAxisCount(int value) {
 //    _minMainAxisCount = value;
 //  }
 
-  int maxMainAxisCount;
+  int? maxMainAxisCount;
 
 //  int get maxMainAxisCount => _maxMainAxisCount;
 //  set maxMainAxisCount(int value) {
@@ -61,60 +62,32 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
 //  }
 
   bool get _debugHasNecessaryDirections {
-    assert(direction != null);
-    assert(alignment != null);
-    assert(runAlignment != null);
-    assert(crossAxisAlignment != null);
     if (firstChild != null && lastChild != firstChild) {
       // i.e. there's more than one child
-      switch (direction) {
-        case Axis.horizontal:
-          assert(textDirection != null,
-              'Horizontal $runtimeType with multiple children has a null textDirection, so the layout order is undefined.');
-          break;
-        case Axis.vertical:
-          assert(verticalDirection != null,
-              'Vertical $runtimeType with multiple children has a null verticalDirection, so the layout order is undefined.');
-          break;
-      }
+      assert(
+          direction == Axis.vertical || textDirection != null,
+          'Horizontal $runtimeType with multiple children has a null '
+          'textDirection, so the layout order is undefined.');
     }
     if (alignment == WrapAlignment.start || alignment == WrapAlignment.end) {
-      switch (direction) {
-        case Axis.horizontal:
-          assert(textDirection != null,
-              'Horizontal $runtimeType with alignment $alignment has a null textDirection, so the alignment cannot be resolved.');
-          break;
-        case Axis.vertical:
-          assert(verticalDirection != null,
-              'Vertical $runtimeType with alignment $alignment has a null verticalDirection, so the alignment cannot be resolved.');
-          break;
-      }
+      assert(
+          direction == Axis.vertical || textDirection != null,
+          'Horizontal $runtimeType with alignment $alignment has a null '
+          'textDirection, so the alignment cannot be resolved.');
     }
     if (runAlignment == WrapAlignment.start ||
         runAlignment == WrapAlignment.end) {
-      switch (direction) {
-        case Axis.horizontal:
-          assert(verticalDirection != null,
-              'Horizontal $runtimeType with runAlignment $runAlignment has a null verticalDirection, so the alignment cannot be resolved.');
-          break;
-        case Axis.vertical:
-          assert(textDirection != null,
-              'Vertical $runtimeType with runAlignment $runAlignment has a null textDirection, so the alignment cannot be resolved.');
-          break;
-      }
+      assert(
+          direction == Axis.horizontal || textDirection != null,
+          'Horizontal $runtimeType with runAlignment $runAlignment has a null '
+          'verticalDirection, so the alignment cannot be resolved.');
     }
     if (crossAxisAlignment == WrapCrossAlignment.start ||
         crossAxisAlignment == WrapCrossAlignment.end) {
-      switch (direction) {
-        case Axis.horizontal:
-          assert(verticalDirection != null,
-              'Horizontal $runtimeType with crossAxisAlignment $crossAxisAlignment has a null verticalDirection, so the alignment cannot be resolved.');
-          break;
-        case Axis.vertical:
-          assert(textDirection != null,
-              'Vertical $runtimeType with crossAxisAlignment $crossAxisAlignment has a null textDirection, so the alignment cannot be resolved.');
-          break;
-      }
+      assert(
+          direction == Axis.horizontal || textDirection != null,
+          'Vertical $runtimeType with crossAxisAlignment $crossAxisAlignment '
+          'has a null textDirection, so the alignment cannot be resolved.');
     }
     return true;
   }
@@ -132,7 +105,7 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
     double runWidth = 0.0;
     double runHeight = 0.0;
     int childCount = 0;
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     int minChildCount = minMainAxisCount ?? 1;
     int maxChildCount = maxMainAxisCount ?? -1;
     while (child != null) {
@@ -167,7 +140,7 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
     double runHeight = 0.0;
     double runWidth = 0.0;
     int childCount = 0;
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     int minChildCount = minMainAxisCount ?? 1;
     int maxChildCount = maxMainAxisCount ?? -1;
     while (child != null) {
@@ -199,14 +172,14 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
       {
 //    Axis sizingDirection,
 //    double extent, // the extent in the direction that isn't the sizing direction
-      int childCountAlongMainAxis,
-      _ChildSizingFunction
+      required int childCountAlongMainAxis,
+      required _ChildSizingFunction
           childSize // a method to find the size in the sizing direction
       }) {
     double runMainAxisExtent = 0.0;
     double maxRunMainAxisExtent = 0.0;
     int childCount = 0;
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
 //    final List<double> runMainAxisExtents = [];
     while (child != null) {
       final double childMainAxisExtent = childSize(child, double.infinity);
@@ -238,7 +211,6 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
       case Axis.vertical:
         return _computeIntrinsicWidthForHeight(height);
     }
-    return null;
   }
 
   @override
@@ -252,7 +224,6 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
       case Axis.vertical:
         return _computeIntrinsicWidthForHeight(height);
     }
-    return null;
   }
 
   @override
@@ -266,7 +237,6 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
             childSize: (RenderBox child, double extent) =>
                 child.getMinIntrinsicHeight(extent));
     }
-    return null;
   }
 
   @override
@@ -280,7 +250,6 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
             childSize: (RenderBox child, double extent) =>
                 child.getMaxIntrinsicHeight(extent));
     }
-    return null;
   }
 
   double _getMainAxisExtent(RenderBox child) {
@@ -290,7 +259,6 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
       case Axis.vertical:
         return child.size.height;
     }
-    return 0.0;
   }
 
   double _getCrossAxisExtent(RenderBox child) {
@@ -300,7 +268,6 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
       case Axis.vertical:
         return child.size.width;
     }
-    return 0.0;
   }
 
   Offset _getOffset(double mainAxisOffset, double crossAxisOffset) {
@@ -310,7 +277,6 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
       case Axis.vertical:
         return Offset(crossAxisOffset, mainAxisOffset);
     }
-    return Offset.zero;
   }
 
   double _getChildCrossAxisOffset(bool flipCrossAxis, double runCrossAxisExtent,
@@ -324,17 +290,16 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
       case WrapCrossAlignment.center:
         return freeSpace / 2.0;
     }
-    return 0.0;
   }
 
   bool _hasVisualOverflow = false;
-  List<int> childRunIndexes;
+  late List<int> childRunIndexes;
 
   @override
   void performLayout() {
     assert(_debugHasNecessaryDirections);
     _hasVisualOverflow = false;
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     if (child == null) {
       size = constraints.smallest;
       return;
@@ -357,8 +322,6 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
         if (textDirection == TextDirection.rtl) flipCrossAxis = true;
         break;
     }
-    assert(childConstraints != null);
-    assert(mainAxisLimit != null);
     final double spacing = this.spacing;
     final double runSpacing = this.runSpacing;
     final List<_RunMetrics> runMetrics = <_RunMetrics>[];
@@ -394,7 +357,8 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
       if (childCount > 0) runMainAxisExtent += spacing;
       runCrossAxisExtent = math.max(runCrossAxisExtent, childCrossAxisExtent);
       childCount += 1;
-      final WrapWithMainAxisCountParentData childParentData = child.parentData;
+      final WrapWithMainAxisCountParentData childParentData =
+          child.parentData! as WrapWithMainAxisCountParentData;
       childParentData._runIndex = runMetrics.length;
       child = childParentData.nextSibling;
       childRunIndexes.add(runIndex);
@@ -505,7 +469,7 @@ class RenderWrapWithMainAxisCount extends RenderWrap {
 
       while (child != null) {
         final WrapWithMainAxisCountParentData childParentData =
-            child.parentData;
+            child.parentData! as WrapWithMainAxisCountParentData;
         if (childParentData._runIndex != i) break;
         final double childMainAxisExtent = _getMainAxisExtent(child);
         final double childCrossAxisExtent = _getCrossAxisExtent(child);
diff --git a/lib/src/widgets/basic.dart b/lib/src/widgets/basic.dart
index 5762ff2..62fab31 100644
--- a/lib/src/widgets/basic.dart
+++ b/lib/src/widgets/basic.dart
@@ -12,10 +12,9 @@ class SafeStatefulBuilder extends StatefulWidget {
   ///
   /// The [builder] argument must not be null.
   const SafeStatefulBuilder({
-    Key key,
-    @required this.builder,
-  })  : assert(builder != null),
-        super(key: key);
+    required this.builder,
+    Key? key,
+  }) : super(key: key);
 
   /// Called to obtain the child widget.
   ///
diff --git a/lib/src/widgets/passthrough_overlay.dart b/lib/src/widgets/passthrough_overlay.dart
index 3df491f..7226627 100644
--- a/lib/src/widgets/passthrough_overlay.dart
+++ b/lib/src/widgets/passthrough_overlay.dart
@@ -5,9 +5,8 @@
 import 'dart:async';
 import 'dart:collection';
 
-import 'package:flutter/material.dart';
-
 import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 
@@ -61,13 +60,10 @@ class PassthroughOverlayEntry {
   /// [Overlay.of] and then call [OverlayState.insert]. To remove the entry,
   /// call [remove] on the overlay entry itself.
   PassthroughOverlayEntry({
-    @required this.builder,
+    required this.builder,
     bool opaque = false,
     bool maintainState = false,
-  })  : assert(builder != null),
-        assert(opaque != null),
-        assert(maintainState != null),
-        _opaque = opaque,
+  })  : _opaque = opaque,
         _maintainState = maintainState;
 
   /// This entry will include the widget built by this builder in the overlay at
@@ -84,11 +80,12 @@ class PassthroughOverlayEntry {
   /// set.
   bool get opaque => _opaque;
   bool _opaque;
+
   set opaque(bool value) {
     if (_opaque == value) return;
     _opaque = value;
     assert(_overlay != null);
-    _overlay._didChangeEntryOpacity();
+    _overlay!._didChangeEntryOpacity();
   }
 
   /// Whether this entry must be included in the tree even if there is a fully
@@ -107,15 +104,15 @@ class PassthroughOverlayEntry {
   /// from subsequent routes will be handled properly when they complete.
   bool get maintainState => _maintainState;
   bool _maintainState;
+
   set maintainState(bool value) {
-    assert(_maintainState != null);
     if (_maintainState == value) return;
     _maintainState = value;
     assert(_overlay != null);
-    _overlay._didChangeEntryOpacity();
+    _overlay!._didChangeEntryOpacity();
   }
 
-  PassthroughOverlayState _overlay;
+  PassthroughOverlayState? _overlay;
   final GlobalKey<_OverlayEntryState> _key = GlobalKey<_OverlayEntryState>();
 
   /// Remove this entry from the overlay.
@@ -131,11 +128,11 @@ class PassthroughOverlayEntry {
   /// the next frame (i.e. many milliseconds later).
   void remove() {
     assert(_overlay != null);
-    final PassthroughOverlayState overlay = _overlay;
+    final PassthroughOverlayState overlay = _overlay!;
     _overlay = null;
-    if (SchedulerBinding.instance.schedulerPhase ==
+    if (SchedulerBinding.instance!.schedulerPhase ==
         SchedulerPhase.persistentCallbacks) {
-      SchedulerBinding.instance.addPostFrameCallback((Duration duration) {
+      SchedulerBinding.instance!.addPostFrameCallback((Duration duration) {
         overlay._remove(this);
       });
     } else {
@@ -156,9 +153,7 @@ class PassthroughOverlayEntry {
 }
 
 class _OverlayEntry extends StatefulWidget {
-  _OverlayEntry(this.entry)
-      : assert(entry != null),
-        super(key: entry._key);
+  _OverlayEntry(this.entry) : super(key: entry._key);
 
   final PassthroughOverlayEntry entry;
 
@@ -173,7 +168,9 @@ class _OverlayEntryState extends State<_OverlayEntry> {
   }
 
   void _markNeedsBuild() {
-    setState(() {/* the state that changed is in the builder */});
+    setState(() {
+      /* the state that changed is in the builder */
+    });
   }
 }
 
@@ -202,10 +199,10 @@ class PassthroughOverlay extends StatefulWidget {
   ///
   /// Rather than creating an overlay, consider using the overlay that is
   /// created by the [WidgetsApp] or the [MaterialApp] for the application.
-  const PassthroughOverlay(
-      {Key key, this.initialEntries = const <PassthroughOverlayEntry>[]})
-      : assert(initialEntries != null),
-        super(key: key);
+  const PassthroughOverlay({
+    this.initialEntries = const <PassthroughOverlayEntry>[],
+    Key? key,
+  }) : super(key: key);
 
   /// The entries to include in the overlay initially.
   ///
@@ -236,8 +233,8 @@ class PassthroughOverlay extends StatefulWidget {
   /// OverlayState overlay = Overlay.of(context);
   /// ```
   static PassthroughOverlayState of(BuildContext context,
-      {Widget debugRequiredFor}) {
-    final PassthroughOverlayState result =
+      {Widget? debugRequiredFor}) {
+    final PassthroughOverlayState? result =
         context.findAncestorStateOfType<PassthroughOverlayState>();
     assert(() {
       if (debugRequiredFor != null && result == null) {
@@ -253,7 +250,7 @@ class PassthroughOverlay extends StatefulWidget {
       }
       return true;
     }());
-    return result;
+    return result!;
   }
 
   @override
@@ -278,7 +275,7 @@ class PassthroughOverlayState extends State<PassthroughOverlay>
   ///
   /// If [above] is non-null, the entry is inserted just above [above].
   /// Otherwise, the entry is inserted on top.
-  void insert(PassthroughOverlayEntry entry, {PassthroughOverlayEntry above}) {
+  void insert(PassthroughOverlayEntry entry, {PassthroughOverlayEntry? above}) {
     assert(entry._overlay == null);
     assert(
         above == null || (above._overlay == this && _entries.contains(above)));
@@ -295,7 +292,7 @@ class PassthroughOverlayState extends State<PassthroughOverlay>
   /// If [above] is non-null, the entries are inserted just above [above].
   /// Otherwise, the entries are inserted on top.
   void insertAll(Iterable<PassthroughOverlayEntry> entries,
-      {PassthroughOverlayEntry above}) {
+      {PassthroughOverlayEntry? above}) {
     assert(
         above == null || (above._overlay == this && _entries.contains(above)));
     if (entries.isEmpty) return;
@@ -313,7 +310,9 @@ class PassthroughOverlayState extends State<PassthroughOverlay>
   void _remove(PassthroughOverlayEntry entry) {
     if (mounted) {
       _entries.remove(entry);
-      setState(() {/* entry was removed */});
+      setState(() {
+        /* entry was removed */
+      });
     }
   }
 
@@ -367,8 +366,8 @@ class PassthroughOverlayState extends State<PassthroughOverlay>
     }
     return _Theatre(
       onstage: Stack(
-        fit: StackFit
-            .passthrough, //HanSheng changed it to passthrough so that this widget doesn't change layout constraints
+        fit: StackFit.passthrough,
+        //HanSheng changed it to passthrough so that this widget doesn't change layout constraints
         children: onstageChildren.reversed.toList(growable: false),
       ),
       offstage: offstageChildren,
@@ -395,12 +394,11 @@ class PassthroughOverlayState extends State<PassthroughOverlay>
 /// widgets.
 class _Theatre extends RenderObjectWidget {
   _Theatre({
+    required this.offstage,
     this.onstage,
-    @required this.offstage,
-  })  : assert(offstage != null),
-        assert(!offstage.any((Widget child) => child == null));
+  });
 
-  final Stack onstage;
+  final Stack? onstage;
 
   final List<Widget> offstage;
 
@@ -417,23 +415,23 @@ class _TheatreElement extends RenderObjectElement {
         super(widget);
 
   @override
-  _Theatre get widget => super.widget;
+  _Theatre get widget => super.widget as _Theatre;
 
   @override
-  _RenderTheatre get renderObject => super.renderObject;
+  _RenderTheatre get renderObject => super.renderObject as _RenderTheatre;
 
-  Element _onstage;
+  Element? _onstage;
   static final Object _onstageSlot = Object();
 
-  List<Element> _offstage;
+  late List<Element> _offstage;
   final Set<Element> _forgottenOffstageChildren = HashSet<Element>();
 
   @override
-  void insertChildRenderObject(RenderBox child, dynamic slot) {
+  void insertRenderObjectChild(RenderBox child, dynamic slot) {
     assert(renderObject.debugValidateChild(child));
     if (slot == _onstageSlot) {
       assert(child is RenderStack);
-      renderObject.child = child;
+      renderObject.child = child as RenderStack?;
     } else {
       assert(slot == null || slot is Element);
       renderObject.insert(child, after: slot?.renderObject);
@@ -441,11 +439,11 @@ class _TheatreElement extends RenderObjectElement {
   }
 
   @override
-  void moveChildRenderObject(RenderBox child, dynamic slot) {
+  void moveRenderObjectChild(RenderBox child, dynamic oldSlot, dynamic slot) {
     if (slot == _onstageSlot) {
       renderObject.remove(child);
       assert(child is RenderStack);
-      renderObject.child = child;
+      renderObject.child = child as RenderStack?;
     } else {
       assert(slot == null || slot is Element);
       if (renderObject.child == child) {
@@ -458,7 +456,7 @@ class _TheatreElement extends RenderObjectElement {
   }
 
   @override
-  void removeChildRenderObject(RenderBox child) {
+  void removeRenderObjectChild(RenderBox child, dynamic slot) {
     if (renderObject.child == child) {
       renderObject.child = null;
     } else {
@@ -468,7 +466,7 @@ class _TheatreElement extends RenderObjectElement {
 
   @override
   void visitChildren(ElementVisitor visitor) {
-    if (_onstage != null) visitor(_onstage);
+    if (_onstage != null) visitor(_onstage!);
     for (Element child in _offstage) {
       if (!_forgottenOffstageChildren.contains(child)) visitor(child);
     }
@@ -476,11 +474,11 @@ class _TheatreElement extends RenderObjectElement {
 
   @override
   void debugVisitOnstageChildren(ElementVisitor visitor) {
-    if (_onstage != null) visitor(_onstage);
+    if (_onstage != null) visitor(_onstage!);
   }
 
   @override
-  bool forgetChild(Element child) {
+  void forgetChild(Element child) {
     if (child == _onstage) {
       _onstage = null;
     } else {
@@ -488,15 +486,15 @@ class _TheatreElement extends RenderObjectElement {
       assert(!_forgottenOffstageChildren.contains(child));
       _forgottenOffstageChildren.add(child);
     }
-    return true;
+    super.forgetChild(child);
   }
 
   @override
-  void mount(Element parent, dynamic newSlot) {
+  void mount(Element? parent, dynamic newSlot) {
     super.mount(parent, newSlot);
     _onstage = updateChild(_onstage, widget.onstage, _onstageSlot);
-    _offstage = List<Element>(widget.offstage.length);
-    Element previousChild;
+    _offstage = [];
+    Element? previousChild;
     for (int i = 0; i < _offstage.length; i += 1) {
       final Element newChild = inflateWidget(widget.offstage[i], previousChild);
       _offstage[i] = newChild;
@@ -552,13 +550,13 @@ class _RenderTheatre extends RenderBox
 
   @override
   void redepthChildren() {
-    if (child != null) redepthChild(child);
+    if (child != null) redepthChild(child!);
     super.redepthChildren();
   }
 
   @override
   void visitChildren(RenderObjectVisitor visitor) {
-    if (child != null) visitor(child);
+    if (child != null) visitor(child!);
     super.visitChildren(visitor);
   }
 
@@ -566,10 +564,10 @@ class _RenderTheatre extends RenderBox
   List<DiagnosticsNode> debugDescribeChildren() {
     final List<DiagnosticsNode> children = <DiagnosticsNode>[];
 
-    if (child != null) children.add(child.toDiagnosticsNode(name: 'onstage'));
+    if (child != null) children.add(child!.toDiagnosticsNode(name: 'onstage'));
 
     if (firstChild != null) {
-      RenderBox child = firstChild;
+      RenderBox child = firstChild!;
 
       int count = 1;
       while (true) {
@@ -580,8 +578,9 @@ class _RenderTheatre extends RenderBox
           ),
         );
         if (child == lastChild) break;
-        final StackParentData childParentData = child.parentData;
-        child = childParentData.nextSibling;
+        final StackParentData childParentData =
+            child.parentData! as StackParentData;
+        child = childParentData.nextSibling!;
         count += 1;
       }
     } else {
@@ -597,6 +596,6 @@ class _RenderTheatre extends RenderBox
 
   @override
   void visitChildrenForSemantics(RenderObjectVisitor visitor) {
-    if (child != null) visitor(child);
+    if (child != null) visitor(child!);
   }
 }
diff --git a/lib/src/widgets/reorderable_flex.dart b/lib/src/widgets/reorderable_flex.dart
index 7abc888..da3d245 100644
--- a/lib/src/widgets/reorderable_flex.dart
+++ b/lib/src/widgets/reorderable_flex.dart
@@ -38,12 +38,12 @@ import './typedefs.dart';
 class ReorderableFlex extends StatefulWidget {
   /// Creates a reorderable list.
   ReorderableFlex({
-    Key key,
+    Key? key,
     this.header,
     this.footer,
-    @required this.children,
-    @required this.onReorder,
-    @required this.direction,
+    required this.children,
+    required this.onReorder,
+    required this.direction,
     this.scrollDirection = Axis.vertical,
     this.padding,
     this.buildItemsContainer,
@@ -53,13 +53,10 @@ class ReorderableFlex extends StatefulWidget {
     this.scrollController,
     this.needsLongPressDraggable = true,
     this.draggingWidgetOpacity = 0.2,
-    this.reorderAnimationDuration = const Duration(milliseconds: 200),
-    this.scrollAnimationDuration = const Duration(milliseconds: 200),
+    this.reorderAnimationDuration,
+    this.scrollAnimationDuration,
     this.ignorePrimaryScrollController = false,
-  })  : assert(direction != null),
-        assert(onReorder != null),
-        assert(children != null),
-        assert(
+  })  : assert(
           children.every((Widget w) => w.key != null),
           'All children of this widget must have a key.',
         ),
@@ -68,12 +65,12 @@ class ReorderableFlex extends StatefulWidget {
   /// A non-reorderable header widget to show before the list.
   ///
   /// If null, no header will appear at the top/left of the widget.
-  final Widget header;
+  final Widget? header;
 
   /// A non-reorderable footer widget to show after the list.
   ///
   /// If null, no footer will appear at the bottom/right of the widget.
-  final Widget footer;
+  final Widget? footer;
 
   /// The widgets to display.
   final List<Widget> children;
@@ -83,26 +80,26 @@ class ReorderableFlex extends StatefulWidget {
   /// List [children] can only drag along this [Axis].
   final Axis direction;
   final Axis scrollDirection;
-  final ScrollController scrollController;
+  final ScrollController? scrollController;
 
   /// The amount of space by which to inset the [children].
-  final EdgeInsets padding;
+  final EdgeInsets? padding;
 
   /// Called when a child is dropped into a new position to shuffle the
   /// children.
   final ReorderCallback onReorder;
-  final NoReorderCallback onNoReorder;
+  final NoReorderCallback? onNoReorder;
 
-  final BuildItemsContainer buildItemsContainer;
-  final BuildDraggableFeedback buildDraggableFeedback;
+  final BuildItemsContainer? buildItemsContainer;
+  final BuildDraggableFeedback? buildDraggableFeedback;
 
   final MainAxisAlignment mainAxisAlignment;
 
   final bool needsLongPressDraggable;
   final double draggingWidgetOpacity;
 
-  final Duration reorderAnimationDuration;
-  final Duration scrollAnimationDuration;
+  final Duration? reorderAnimationDuration;
+  final Duration? scrollAnimationDuration;
   final bool ignorePrimaryScrollController;
 
   @override
@@ -124,7 +121,7 @@ class _ReorderableFlexState extends State<ReorderableFlex> {
       GlobalKey(debugLabel: '$ReorderableFlex overlay key');
 
   // This entry contains the scrolling list itself.
-  PassthroughOverlayEntry _listOverlayEntry;
+  late PassthroughOverlayEntry _listOverlayEntry;
 
   @override
   void initState() {
@@ -147,8 +144,10 @@ class _ReorderableFlexState extends State<ReorderableFlex> {
           scrollController: widget.scrollController,
           needsLongPressDraggable: widget.needsLongPressDraggable,
           draggingWidgetOpacity: widget.draggingWidgetOpacity,
-          reorderAnimationDuration: widget.reorderAnimationDuration,
-          scrollAnimationDuration: widget.scrollAnimationDuration,
+          reorderAnimationDuration: widget.reorderAnimationDuration ??
+              const Duration(milliseconds: 200),
+          scrollAnimationDuration: widget.scrollAnimationDuration ??
+              const Duration(milliseconds: 200),
         );
       },
     );
@@ -173,33 +172,33 @@ class _ReorderableFlexContent extends StatefulWidget {
   const _ReorderableFlexContent({
     this.header,
     this.footer,
-    @required this.children,
-    @required this.direction,
-    @required this.scrollDirection,
-    @required this.padding,
-    @required this.onReorder,
-    @required this.onNoReorder,
-    @required this.buildItemsContainer,
-    @required this.buildDraggableFeedback,
-    @required this.mainAxisAlignment,
-    @required this.scrollController,
-    @required this.needsLongPressDraggable,
-    @required this.draggingWidgetOpacity,
-    @required this.reorderAnimationDuration,
-    @required this.scrollAnimationDuration,
+    required this.children,
+    required this.direction,
+    required this.scrollDirection,
+    required this.onReorder,
+    required this.mainAxisAlignment,
+    required this.scrollController,
+    required this.needsLongPressDraggable,
+    required this.draggingWidgetOpacity,
+    required this.onNoReorder,
+    required this.buildItemsContainer,
+    required this.buildDraggableFeedback,
+    required this.padding,
+    this.reorderAnimationDuration = const Duration(milliseconds: 200),
+    this.scrollAnimationDuration = const Duration(milliseconds: 200),
   });
 
-  final Widget header;
-  final Widget footer;
+  final Widget? header;
+  final Widget? footer;
   final List<Widget> children;
   final Axis direction;
   final Axis scrollDirection;
-  final ScrollController scrollController;
-  final EdgeInsets padding;
   final ReorderCallback onReorder;
-  final NoReorderCallback onNoReorder;
-  final BuildItemsContainer buildItemsContainer;
-  final BuildDraggableFeedback buildDraggableFeedback;
+  final NoReorderCallback? onNoReorder;
+  final BuildItemsContainer? buildItemsContainer;
+  final BuildDraggableFeedback? buildDraggableFeedback;
+  final ScrollController? scrollController;
+  final EdgeInsets? padding;
 
   final MainAxisAlignment mainAxisAlignment;
   final bool needsLongPressDraggable;
@@ -224,31 +223,31 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
   static const double _dropAreaMargin = 0.0;
 
   // How long an animation to reorder an element in the list takes.
-  Duration _reorderAnimationDuration;
+  late Duration _reorderAnimationDuration;
 
   // How long an animation to scroll to an off-screen element in the
   // list takes.
-  Duration _scrollAnimationDuration;
+  late Duration _scrollAnimationDuration;
 
   // Controls scrolls and measures scroll progress.
-  ScrollController _scrollController;
-  ScrollPosition _attachedScrollPosition;
+  late ScrollController _scrollController;
+  ScrollPosition? _attachedScrollPosition;
 
   // This controls the entrance of the dragging widget into a new place.
-  AnimationController _entranceController;
+  late AnimationController _entranceController;
 
   // This controls the 'ghost' of the dragging widget, which is left behind
   // where the widget used to be.
-  AnimationController _ghostController;
+  late AnimationController _ghostController;
 
   // The member of widget.children currently being dragged.
   //
   // Null if no drag is underway.
-  Key _dragging;
-  Widget _draggingWidget;
+  Key? _dragging;
+  Widget? _draggingWidget;
 
   // The last computed size of the feedback widget being dragged.
-  Size _draggingFeedbackSize = Size(0, 0);
+  Size? _draggingFeedbackSize = Size(0, 0);
 
   // The location that the dragging widget occupied before it started to drag.
   int _dragStartIndex = -1;
@@ -272,7 +271,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
     if (_draggingFeedbackSize == null) {
       return Size(0, 0);
     }
-    return _draggingFeedbackSize + Offset(_dropAreaMargin, _dropAreaMargin);
+    return _draggingFeedbackSize! + Offset(_dropAreaMargin, _dropAreaMargin);
 //    double dropAreaWithoutMargin;
 //    switch (widget.direction) {
 //      case Axis.horizontal:
@@ -300,8 +299,8 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
 
   @override
   void didChangeDependencies() {
-    if (_scrollController != null && _attachedScrollPosition != null) {
-      _scrollController.detach(_attachedScrollPosition);
+    if (_attachedScrollPosition != null) {
+      _scrollController.detach(_attachedScrollPosition!);
       _attachedScrollPosition = null;
     }
 
@@ -310,14 +309,13 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
         ScrollController();
 
     if (_scrollController.hasClients) {
-      ScrollableState scrollableState = Scrollable.of(context);
-      _attachedScrollPosition = scrollableState?.position;
+      _attachedScrollPosition = Scrollable.of(context)?.position;
     } else {
       _attachedScrollPosition = null;
     }
 
     if (_attachedScrollPosition != null) {
-      _scrollController.attach(_attachedScrollPosition);
+      _scrollController.attach(_attachedScrollPosition!);
     }
 
     super.didChangeDependencies();
@@ -325,8 +323,8 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
 
   @override
   void dispose() {
-    if (_scrollController != null && _attachedScrollPosition != null) {
-      _scrollController.detach(_attachedScrollPosition);
+    if (_attachedScrollPosition != null) {
+      _scrollController.detach(_attachedScrollPosition!);
       _attachedScrollPosition = null;
     }
     _entranceController.dispose();
@@ -364,10 +362,9 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
   // Scrolls to a target context if that context is not on the screen.
   void _scrollTo(BuildContext context) {
     if (_scrolling) return;
-    final RenderObject contextObject = context.findRenderObject();
+    final RenderObject contextObject = context.findRenderObject()!;
     final RenderAbstractViewport viewport =
-        RenderAbstractViewport.of(contextObject);
-    assert(viewport != null);
+        RenderAbstractViewport.of(contextObject)!;
     // If and only if the current scroll offset falls in-between the offsets
     // necessary to reveal the selected context at the top or bottom of the
     // screen, then it is already on-screen.
@@ -407,7 +404,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
 
   // Wraps children in Row or Column, so that the children flow in
   // the widget's scrollDirection.
-  Widget _buildContainerForMainAxis({List<Widget> children}) {
+  Widget _buildContainerForMainAxis({required List<Widget> children}) {
     switch (widget.direction) {
       case Axis.horizontal:
         return Row(
@@ -427,7 +424,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
   // Handles up the logic for dragging and reordering items in the list.
   Widget _wrap(Widget toWrap, int index) {
     assert(toWrap.key != null);
-    final GlobalObjectKey keyIndexGlobalKey = GlobalObjectKey(toWrap.key);
+    final GlobalObjectKey keyIndexGlobalKey = GlobalObjectKey(toWrap.key!);
     // We pass the toWrapWithGlobalKey into the Draggable so that when a list
     // item gets dragged, the accessibility framework can preserve the selected
     // state of the dragging item.
@@ -441,7 +438,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
         _ghostIndex = index;
         _currentIndex = index;
         _entranceController.value = 1.0;
-        _draggingFeedbackSize = keyIndexGlobalKey.currentContext.size;
+        _draggingFeedbackSize = keyIndexGlobalKey.currentContext?.size;
       });
     }
 
@@ -450,7 +447,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
 //      debugPrint('startIndex:$startIndex endIndex:$endIndex');
       if (startIndex != endIndex)
         widget.onReorder(startIndex, endIndex);
-      else if (widget.onNoReorder != null) widget.onNoReorder(startIndex);
+      else if (widget.onNoReorder != null) widget.onNoReorder!(startIndex);
       // Animates leftover space in the drop area closed.
       // TODO(djshuckerow): bring the animation in line with the Material
       // specifications.
@@ -495,34 +492,31 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
       final MaterialLocalizations localizations =
           MaterialLocalizations.of(context);
 
-      if (localizations != null) {
-        // If the item can move to before its current position in the list.
-        if (index > 0) {
-          semanticsActions[CustomSemanticsAction(
-              label: localizations.reorderItemToStart)] = moveToStart;
-          String reorderItemBefore = localizations.reorderItemUp;
-          if (widget.direction == Axis.horizontal) {
-            reorderItemBefore = Directionality.of(context) == TextDirection.ltr
-                ? localizations.reorderItemLeft
-                : localizations.reorderItemRight;
-          }
-          semanticsActions[CustomSemanticsAction(label: reorderItemBefore)] =
-              moveBefore;
+      if (index > 0) {
+        semanticsActions[CustomSemanticsAction(
+            label: localizations.reorderItemToStart)] = moveToStart;
+        String reorderItemBefore = localizations.reorderItemUp;
+        if (widget.direction == Axis.horizontal) {
+          reorderItemBefore = Directionality.of(context) == TextDirection.ltr
+              ? localizations.reorderItemLeft
+              : localizations.reorderItemRight;
         }
+        semanticsActions[CustomSemanticsAction(label: reorderItemBefore)] =
+            moveBefore;
+      }
 
-        // If the item can move to after its current position in the list.
-        if (index < widget.children.length - 1) {
-          String reorderItemAfter = localizations.reorderItemDown;
-          if (widget.direction == Axis.horizontal) {
-            reorderItemAfter = Directionality.of(context) == TextDirection.ltr
-                ? localizations.reorderItemRight
-                : localizations.reorderItemLeft;
-          }
-          semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] =
-              moveAfter;
-          semanticsActions[CustomSemanticsAction(
-              label: localizations.reorderItemToEnd)] = moveToEnd;
+      // If the item can move to after its current position in the list.
+      if (index < widget.children.length - 1) {
+        String reorderItemAfter = localizations.reorderItemDown;
+        if (widget.direction == Axis.horizontal) {
+          reorderItemAfter = Directionality.of(context) == TextDirection.ltr
+              ? localizations.reorderItemRight
+              : localizations.reorderItemLeft;
         }
+        semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] =
+            moveAfter;
+        semanticsActions[CustomSemanticsAction(
+            label: localizations.reorderItemToEnd)] = moveToEnd;
       }
 
       // We pass toWrap with a GlobalKey into the Draggable so that when a list
@@ -566,7 +560,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
       );
     }
 
-    Widget buildDragTarget(BuildContext context, List<Key> acceptedCandidates,
+    Widget buildDragTarget(BuildContext context, List<Key?> acceptedCandidates,
         List<dynamic> rejectedCandidates) {
       final Widget toWrapWithSemantics = wrapWithSemantics();
 
@@ -574,7 +568,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
 //          RenderRepaintBoundary renderObject = _contentKey.currentContext.findRenderObject();
 //          BoxConstraints contentSizeConstraints = BoxConstraints.loose(renderObject.size);
         BoxConstraints contentSizeConstraints = BoxConstraints.loose(
-            _draggingFeedbackSize); //renderObject.constraints
+            _draggingFeedbackSize!); //renderObject.constraints
 //          debugPrint('${DateTime.now().toString().substring(5, 22)} reorderable_flex.dart(515) $this.buildDragTarget: contentConstraints:$contentSizeConstraints _draggingFeedbackSize:$_draggingFeedbackSize');
         return (widget.buildDraggableFeedback ?? defaultBuildDraggableFeedback)(
             context, contentSizeConstraints, toWrap);
@@ -716,7 +710,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
     return Builder(builder: (BuildContext context) {
       Widget dragTarget = DragTarget<Key>(
         builder: buildDragTarget,
-        onWillAccept: (Key toAccept) {
+        onWillAccept: (Key? toAccept) {
           bool willAccept = _dragging == toAccept && toAccept != toWrap.key;
 
 //          debugPrint('${DateTime.now().toString().substring(5, 22)} reorderable_flex.dart(609) $this._wrap: '
@@ -744,7 +738,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
           return willAccept; //_dragging == toAccept && toAccept != toWrap.key;
         },
         onAccept: (Key accepted) {},
-        onLeave: (Object leaving) {},
+        onLeave: (Object? leaving) {},
       );
 
       dragTarget = KeyedSubtree(key: keyIndexGlobalKey, child: dragTarget);
@@ -831,13 +825,13 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
 //    return LayoutBuilder(builder: (BuildContext context, BoxConstraints constraints) {
     final List<Widget> wrappedChildren = <Widget>[];
     if (widget.header != null) {
-      wrappedChildren.add(widget.header);
+      wrappedChildren.add(widget.header!);
     }
     for (int i = 0; i < widget.children.length; i += 1) {
       wrappedChildren.add(_wrap(widget.children[i], i));
     }
     if (widget.footer != null) {
-      wrappedChildren.add(widget.footer);
+      wrappedChildren.add(widget.footer!);
     }
 //      const Key endWidgetKey = Key('DraggableList - End Widget');
 //      Widget finalDropArea;
@@ -960,25 +954,25 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
 ///  * [ReorderableColumn], for a version of this widget that is always vertical.
 class ReorderableRow extends ReorderableFlex {
   ReorderableRow({
-    Key key,
-    Widget header,
-    Widget footer,
-    ReorderCallback onReorder,
-    EdgeInsets padding,
+    required ReorderCallback onReorder,
+    Key? key,
+    Widget? header,
+    Widget? footer,
+    EdgeInsets? padding,
     MainAxisAlignment mainAxisAlignment = MainAxisAlignment.start,
     MainAxisSize mainAxisSize = MainAxisSize.max,
     CrossAxisAlignment crossAxisAlignment = CrossAxisAlignment.center,
-    TextDirection textDirection,
+    TextDirection? textDirection,
     VerticalDirection verticalDirection = VerticalDirection.down,
-    TextBaseline textBaseline,
+    TextBaseline? textBaseline,
     List<Widget> children = const <Widget>[],
-    BuildDraggableFeedback buildDraggableFeedback,
-    NoReorderCallback onNoReorder,
-    ScrollController scrollController,
+    BuildDraggableFeedback? buildDraggableFeedback,
+    NoReorderCallback? onNoReorder,
+    ScrollController? scrollController,
     bool needsLongPressDraggable = true,
     double draggingWidgetOpacity = 0.2,
-    Duration reorderAnimationDuration,
-    Duration scrollAnimationDuration,
+    Duration? reorderAnimationDuration,
+    Duration? scrollAnimationDuration,
     bool ignorePrimaryScrollController = false,
   }) : super(
             key: key,
@@ -1039,25 +1033,25 @@ class ReorderableRow extends ReorderableFlex {
 ///  * [ReorderableRow], for a version of this widget that is always horizontal.
 class ReorderableColumn extends ReorderableFlex {
   ReorderableColumn({
-    Key key,
-    Widget header,
-    Widget footer,
-    ReorderCallback onReorder,
-    EdgeInsets padding,
+    required ReorderCallback onReorder,
+    Key? key,
+    Widget? header,
+    Widget? footer,
+    EdgeInsets? padding,
     MainAxisAlignment mainAxisAlignment = MainAxisAlignment.start,
     MainAxisSize mainAxisSize = MainAxisSize.max,
     CrossAxisAlignment crossAxisAlignment = CrossAxisAlignment.center,
-    TextDirection textDirection,
+    TextDirection? textDirection,
     VerticalDirection verticalDirection = VerticalDirection.down,
-    TextBaseline textBaseline,
+    TextBaseline? textBaseline,
     List<Widget> children = const <Widget>[],
-    BuildDraggableFeedback buildDraggableFeedback,
-    NoReorderCallback onNoReorder,
-    ScrollController scrollController,
+    BuildDraggableFeedback? buildDraggableFeedback,
+    NoReorderCallback? onNoReorder,
+    ScrollController? scrollController,
     bool needsLongPressDraggable = true,
     double draggingWidgetOpacity = 0.2,
-    Duration reorderAnimationDuration = const Duration(milliseconds: 200),
-    Duration scrollAnimationDuration = const Duration(milliseconds: 200),
+    Duration? reorderAnimationDuration,
+    Duration? scrollAnimationDuration,
     bool ignorePrimaryScrollController = false,
   }) : super(
             key: key,
diff --git a/lib/src/widgets/reorderable_mixin.dart b/lib/src/widgets/reorderable_mixin.dart
index 96bc273..cbe44d0 100644
--- a/lib/src/widgets/reorderable_mixin.dart
+++ b/lib/src/widgets/reorderable_mixin.dart
@@ -7,7 +7,7 @@ mixin ReorderableMixin {
   Widget makeAppearingWidget(
     Widget child,
     AnimationController entranceController,
-    Size draggingFeedbackSize,
+    Size? draggingFeedbackSize,
     Axis direction,
   ) {
     if (null == draggingFeedbackSize) {
@@ -35,7 +35,7 @@ mixin ReorderableMixin {
   Widget makeDisappearingWidget(
       Widget child,
       AnimationController ghostController,
-      Size draggingFeedbackSize,
+      Size? draggingFeedbackSize,
       Axis direction,
       ) {
     if (null == draggingFeedbackSize) {
diff --git a/lib/src/widgets/reorderable_sliver.dart b/lib/src/widgets/reorderable_sliver.dart
index 3366b1f..8044f61 100644
--- a/lib/src/widgets/reorderable_sliver.dart
+++ b/lib/src/widgets/reorderable_sliver.dart
@@ -19,10 +19,10 @@ import './reorderable_mixin.dart';
 int _kDefaultSemanticIndexCallback(Widget _, int localIndex) => localIndex;
 
 mixin _ReorderableSliverChildDelegateMixin<T extends SliverChildDelegate> {
-  Widget Function(Widget toWrap, int index) _wrap;
+  Widget Function(Widget toWrap, int index)? _wrap;
 
   set wrap(Function value) {
-    _wrap = value;
+    _wrap = value as Widget Function(Widget toWrap, int index)?;
   }
 }
 
@@ -48,7 +48,7 @@ class ReorderableSliverChildBuilderDelegate extends SliverChildBuilderDelegate
   /// null.
   ReorderableSliverChildBuilderDelegate(
     IndexedWidgetBuilder builder, {
-    int childCount,
+    int? childCount,
     bool addAutomaticKeepAlives = true,
     bool addRepaintBoundaries = true,
     bool addSemanticIndexes = true,
@@ -84,11 +84,10 @@ class ReorderableSliverChildBuilderDelegate extends SliverChildBuilderDelegate
 //  }
 
   @override
-  Widget build(BuildContext context, int index) {
+  Widget? build(BuildContext context, int index) {
 //    Widget child = super.build(context, index);
-    assert(builder != null);
-    if (index < 0 || (childCount != null && index >= childCount)) return null;
-    Widget child = builder(context, index);
+    if (index < 0 || (childCount != null && index >= childCount!)) return null;
+    Widget? child = builder(context, index);
 //    try {
 //      child = builder(context, index);
 //    } catch (exception, stackTrace) {
@@ -97,7 +96,7 @@ class ReorderableSliverChildBuilderDelegate extends SliverChildBuilderDelegate
     if (child == null) return null;
     if (addRepaintBoundaries) child = RepaintBoundary.wrap(child, index);
     if (addSemanticIndexes) {
-      final int semanticIndex = semanticIndexCallback(child, index);
+      final int? semanticIndex = semanticIndexCallback(child, index);
       if (semanticIndex != null)
         child = IndexedSemantics(
             index: semanticIndex + semanticIndexOffset, child: child);
@@ -110,7 +109,7 @@ class ReorderableSliverChildBuilderDelegate extends SliverChildBuilderDelegate
 //      child: child
 //    );
 
-    child = _wrap(child, index);
+    child = _wrap!(child, index);
     if (addAutomaticKeepAlives) child = AutomaticKeepAlive(child: child);
 
     return child;
@@ -159,16 +158,14 @@ class ReorderableSliverChildListDelegate extends SliverChildListDelegate
       true;
 
   @override
-  Widget build(BuildContext context, int index) {
+  Widget? build(BuildContext context, int index) {
 //    Widget child = super.build(context, index);
-    assert(children != null);
     if (index < 0 || index >= children.length) return null;
     Widget child = children[index];
 //    debugPrint('${DateTime.now().toString().substring(5, 22)} reorderable_sliver.dart(97) $this.build: index:$index child:$child');
-    assert(child != null);
     if (addRepaintBoundaries) child = RepaintBoundary.wrap(child, index);
     if (addSemanticIndexes) {
-      final int semanticIndex = semanticIndexCallback(child, index);
+      final int? semanticIndex = semanticIndexCallback(child, index);
       if (semanticIndex != null)
         child = IndexedSemantics(
             index: semanticIndex + semanticIndexOffset, child: child);
@@ -182,7 +179,7 @@ class ReorderableSliverChildListDelegate extends SliverChildListDelegate
 //      child: child
 //    );
 
-    child = _wrap(child, index);
+    child = _wrap!(child, index);
     if (addAutomaticKeepAlives) child = AutomaticKeepAlive(child: child);
 
     return child;
@@ -241,9 +238,8 @@ class ReorderableSliverChildListDelegate extends SliverChildListDelegate
 class ReorderableSliverList extends StatefulWidget {
   /// Creates a reorderable list.
   ReorderableSliverList({
-    Key key,
-    @required this.delegate,
-    @required this.onReorder,
+    required this.delegate,
+    required this.onReorder,
     this.buildItemsContainer,
     this.buildDraggableFeedback,
     this.onNoReorder,
@@ -251,8 +247,10 @@ class ReorderableSliverList extends StatefulWidget {
     this.onDragStart,
     this.onDragEnd,
     this.enabled = true,
-  }): assert(onReorder != null && delegate != null),
-      super(key: key);
+    this.controller,
+    Key? key,
+  })  : super(key: key);
+
   /// The delegate that provides the children for this widget.
   ///
   /// The children are constructed lazily using this widget to avoid creating
@@ -268,30 +266,31 @@ class ReorderableSliverList extends StatefulWidget {
   /// Called when a child is dropped into a new position to shuffle the
   /// children.
   final ReorderCallback onReorder;
-  final NoReorderCallback onNoReorder;
+  final NoReorderCallback? onNoReorder;
 
   /// Called when a drag process is started
-  final VoidCallback onDragStart;
+  final VoidCallback? onDragStart;
+
   /// Called when the drag process has ended, either via [Draggable.onDraggableCanceled] or [Draggable.onDragCompleted]
-  final VoidCallback onDragEnd;
+  final VoidCallback? onDragEnd;
 
   /// Called when the draggable starts being dragged.
-  final ReorderStartedCallback onReorderStarted;
+  final ReorderStartedCallback? onReorderStarted;
 
-  final BuildItemsContainer buildItemsContainer;
-  final BuildDraggableFeedback buildDraggableFeedback;
+  final BuildItemsContainer? buildItemsContainer;
+  final BuildDraggableFeedback? buildDraggableFeedback;
 
   /// Sets whether the children are reorderable or not
   final bool enabled;
 
+  final ScrollController? controller;
+
   @override
   _ReorderableSliverListState createState() => _ReorderableSliverListState();
 }
 
 class _ReorderableSliverListState extends State<ReorderableSliverList>
-  with TickerProviderStateMixin<ReorderableSliverList>, ReorderableMixin
-{
-
+    with TickerProviderStateMixin<ReorderableSliverList>, ReorderableMixin {
   // The extent along the [widget.scrollDirection] axis to allow a child to
   // drop into when the user reorders list children.
   //
@@ -310,24 +309,24 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
   static const Duration _scrollAnimationDuration = Duration(milliseconds: 200);
 
   // Controls scrolls and measures scroll progress.
-  ScrollController _scrollController;
-  ScrollPosition _attachedScrollPosition;
+  late ScrollController _scrollController;
+  ScrollPosition? _attachedScrollPosition;
 
   // This controls the entrance of the dragging widget into a new place.
-  AnimationController _entranceController;
+  late AnimationController _entranceController;
 
   // This controls the 'ghost' of the dragging widget, which is left behind
   // where the widget used to be.
-  AnimationController _ghostController;
+  late AnimationController _ghostController;
 
   // The member of widget.children currently being dragged.
   //
   // Null if no drag is underway.
 //  Key _dragging;
-  Widget _draggingWidget;
+  Widget? _draggingWidget;
 
   // The last computed size of the feedback widget being dragged.
-  Size _draggingFeedbackSize;
+  Size? _draggingFeedbackSize;
 
 //  BuildContext _draggingContext;
 
@@ -349,7 +348,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
 
 //  final GlobalKey _contentKey = GlobalKey(debugLabel: '$ReorderableSliverList content key');
 
-  int _childCount;
+  late int _childCount;
 
   final Map<int, StateSetter> _setStateMap = <int, StateSetter>{};
   final List<int> _spacedIndexes = <int>[];
@@ -358,7 +357,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
     if (_draggingFeedbackSize == null) {
       return Size(0, 0);
     }
-    return _draggingFeedbackSize + Offset(_dropAreaMargin, _dropAreaMargin);
+    return _draggingFeedbackSize! + Offset(_dropAreaMargin, _dropAreaMargin);
   }
 
   @override
@@ -379,23 +378,19 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
 
   @override
   void didChangeDependencies() {
-    if (_scrollController != null && _attachedScrollPosition != null) {
-      _scrollController.detach(_attachedScrollPosition);
+    if (_attachedScrollPosition != null) {
+      _scrollController.detach(_attachedScrollPosition!);
       _attachedScrollPosition = null;
     }
 
     _scrollController =
-        PrimaryScrollController.of(context) ?? ScrollController();
+        widget.controller ?? PrimaryScrollController.of(context) ?? ScrollController();
 
-    if (!_scrollController.hasClients) {
-      ScrollableState scrollableState = Scrollable.of(context);
-      _attachedScrollPosition = scrollableState?.position;
-    } else {
-      _attachedScrollPosition = null;
-    }
+    _attachedScrollPosition =
+        _scrollController.hasClients ? null : Scrollable.of(context)?.position;
 
     if (_attachedScrollPosition != null) {
-      _scrollController.attach(_attachedScrollPosition);
+      _scrollController.attach(_attachedScrollPosition!);
     }
 
     super.didChangeDependencies();
@@ -403,8 +398,8 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
 
   @override
   void dispose() {
-    if (_scrollController != null && _attachedScrollPosition != null) {
-      _scrollController.detach(_attachedScrollPosition);
+    if (_attachedScrollPosition != null) {
+      _scrollController.detach(_attachedScrollPosition!);
       _attachedScrollPosition = null;
     }
     _entranceController.dispose();
@@ -428,7 +423,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
 
   // Animates the droppable space from _currentIndex to _nextIndex.
   void _requestAnimationToNextIndex(
-      {bool isAcceptingNewTarget = false, int updatingIndex}) {
+      {bool isAcceptingNewTarget = false, int? updatingIndex}) {
 //    debugPrint('${DateTime.now().toString().substring(5, 22)} reorderable_sliver.dart(345) $this._requestAnimationToNextIndex: '
 //      '_dragStartIndex:$_dragStartIndex _ghostIndex:$_ghostIndex _currentIndex:$_currentIndex _nextIndex:$_nextIndex isAcceptingNewTarget:$isAcceptingNewTarget isCompleted:${_entranceController.isCompleted}');
 
@@ -478,7 +473,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
           if (_setStateMap[index] == null) {
 //            debugPrint('${DateTime.now().toString().substring(5, 22)} reorderable_sliver.dart(394) $this._setState: index:$index _setStateMap:$_setStateMap');
           }
-          _setStateMap[index](_setState);
+          _setStateMap[index]!(_setState);
         } else {
           _update();
         }
@@ -500,10 +495,9 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
   // Scrolls to a target context if that context is not on the screen.
   void _scrollTo(BuildContext context) {
     if (_scrolling) return;
-    final RenderObject contextObject = context.findRenderObject();
+    final RenderObject contextObject = context.findRenderObject()!;
     final RenderAbstractViewport viewport =
-        RenderAbstractViewport.of(contextObject);
-    assert(viewport != null);
+        RenderAbstractViewport.of(contextObject)!;
 
 //    if (_scrollController.positions.isEmpty) {
 //      debugPrint('${DateTime.now().toString().substring(5, 22)} reorderable_sliver.dart(537) $this._scrollTo: empty pos');
@@ -555,7 +549,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
 
   // Wraps children in Row or Column, so that the children flow in
   // the widget's scrollDirection.
-  Widget _buildContainerForMainAxis({List<Widget> children}) {
+  Widget _buildContainerForMainAxis({required List<Widget> children}) {
     var column = Column(
         mainAxisSize: MainAxisSize.min,
         crossAxisAlignment: CrossAxisAlignment.start,
@@ -582,14 +576,14 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
   Widget _statefulWrap(Widget toWrap, int index, StateSetter setState) {
 //    assert(toWrap.key != null);
 //    final GlobalObjectKey keyIndexGlobalKey = GlobalObjectKey(toWrap.key);
-    BuildContext draggableContext;
+    BuildContext? draggableContext;
     // We pass the toWrapWithGlobalKey into the Draggable so that when a list
     // item gets dragged, the accessibility framework can preserve the selected
     // state of the dragging item.
 
     // Starts dragging toWrap.
     void onDragStarted() {
-      if (widget.onDragStart != null) widget.onDragStart();
+      if (widget.onDragStart != null) widget.onDragStart!();
 //      debugPrint('${DateTime.now().toString().substring(5, 22)} reorderable_sliver.dart(419) $this.onDragStarted: index:$index');
       setState(() {
         _draggingWidget = toWrap;
@@ -599,11 +593,11 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
         _currentIndex = index;
         _entranceController.value = 1.0;
         _draggingFeedbackSize =
-            draggableContext.size; //keyIndexGlobalKey.currentContext.size;
+            draggableContext!.size; //keyIndexGlobalKey.currentContext.size;
 //        _draggingContext = draggableContext;
       });
       if (widget.onReorderStarted != null) {
-        widget.onReorderStarted(index);
+        widget.onReorderStarted!(index);
       }
     }
 
@@ -612,7 +606,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
 //      debugPrint('startIndex:$startIndex endIndex:$endIndex');
       if (startIndex != endIndex)
         widget.onReorder(startIndex, endIndex);
-      else if (widget.onNoReorder != null) widget.onNoReorder(startIndex);
+      else if (widget.onNoReorder != null) widget.onNoReorder!(startIndex);
       // Animates leftover space in the drop area closed.
       // TODO(djshuckerow): bring the animation in line with the Material
       // specifications.
@@ -634,7 +628,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
     // Drops toWrap into the last position it was hovering over.
     void onDragEnded() {
 //      reorder(_dragStartIndex, _currentIndex);
-      if (widget.onDragEnd != null) widget.onDragEnd();
+      if (widget.onDragEnd != null) widget.onDragEnd!();
       this.setState(() {
         void _update() {
           _reorder(_dragStartIndex, _currentIndex);
@@ -653,7 +647,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
         void _setState() {
           if (updateIndexes.length > 0) {
             int index = updateIndexes.removeLast();
-            _setStateMap[index](_setState);
+            _setStateMap[index]!(_setState);
           } else {
             _update();
           }
@@ -679,34 +673,31 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
       final MaterialLocalizations localizations =
           MaterialLocalizations.of(context);
 
-      if (localizations != null) {
-        // If the item can move to before its current position in the list.
-        if (index > 0) {
-          semanticsActions[CustomSemanticsAction(
-              label: localizations.reorderItemToStart)] = moveToStart;
-          String reorderItemBefore = localizations.reorderItemUp;
+      if (index > 0) {
+        semanticsActions[CustomSemanticsAction(
+            label: localizations.reorderItemToStart)] = moveToStart;
+        String reorderItemBefore = localizations.reorderItemUp;
 //        if (widget.direction == Axis.horizontal) {
 //          reorderItemBefore = Directionality.of(context) == TextDirection.ltr
 //            ? localizations.reorderItemLeft
 //            : localizations.reorderItemRight;
 //        }
-          semanticsActions[CustomSemanticsAction(label: reorderItemBefore)] =
-              moveBefore;
-        }
+        semanticsActions[CustomSemanticsAction(label: reorderItemBefore)] =
+            moveBefore;
+      }
 
-        // If the item can move to after its current position in the list.
-        if (index < _childCount - 1) {
-          String reorderItemAfter = localizations.reorderItemDown;
+      // If the item can move to after its current position in the list.
+      if (index < _childCount - 1) {
+        String reorderItemAfter = localizations.reorderItemDown;
 //        if (widget.direction == Axis.horizontal) {
 //          reorderItemAfter = Directionality.of(context) == TextDirection.ltr
 //            ? localizations.reorderItemRight
 //            : localizations.reorderItemLeft;
 //        }
-          semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] =
-              moveAfter;
-          semanticsActions[CustomSemanticsAction(
-              label: localizations.reorderItemToEnd)] = moveToEnd;
-        }
+        semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] =
+            moveAfter;
+        semanticsActions[CustomSemanticsAction(
+            label: localizations.reorderItemToEnd)] = moveToEnd;
       }
 
       // We pass toWrap with a GlobalKey into the Draggable so that when a list
@@ -733,14 +724,24 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
     }
 
     Widget _makeAppearingWidget(Widget child) {
-      return makeAppearingWidget(child, _entranceController, _draggingFeedbackSize, Axis.vertical,);
+      return makeAppearingWidget(
+        child,
+        _entranceController,
+        _draggingFeedbackSize,
+        Axis.vertical,
+      );
     }
 
     Widget _makeDisappearingWidget(Widget child) {
-      return makeDisappearingWidget(child, _ghostController, _draggingFeedbackSize, Axis.vertical,);
+      return makeDisappearingWidget(
+        child,
+        _ghostController,
+        _draggingFeedbackSize,
+        Axis.vertical,
+      );
     }
 
-    Widget buildDragTarget(BuildContext context, List<int> acceptedCandidates,
+    Widget buildDragTarget(BuildContext context, List<int?> acceptedCandidates,
         List<dynamic> rejectedCandidates) {
       final Widget toWrapWithSemantics = wrapWithSemantics();
 
@@ -748,7 +749,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
 //          RenderRepaintBoundary renderObject = _contentKey.currentContext.findRenderObject();
 //          BoxConstraints contentSizeConstraints = BoxConstraints.loose(renderObject.size);
         BoxConstraints contentSizeConstraints = BoxConstraints.loose(
-            _draggingFeedbackSize); //renderObject.constraints
+            _draggingFeedbackSize!); //renderObject.constraints
 //          debugPrint('${DateTime.now().toString().substring(5, 22)} reorderable_flex.dart(515) $this.buildDragTarget: contentConstraints:$contentSizeConstraints _draggingFeedbackSize:$_draggingFeedbackSize');
         return (widget.buildDraggableFeedback ?? defaultBuildDraggableFeedback)(
             context, contentSizeConstraints, toWrap);
@@ -766,7 +767,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
         // We build the draggable inside of a layout builder so that we can
         // constrain the size of the feedback dragging widget.
         child = LongPressDraggable<int>(
-          maxSimultaneousDrags: widget.enabled?1:0,
+          maxSimultaneousDrags: widget.enabled ? 1 : 0,
           axis: Axis.vertical,
           //widget.direction,
           data: index,
@@ -799,17 +800,14 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
               child: toWrapWithSemantics, behavior: HitTestBehavior.opaque),
           //toWrapWithSemantics,//_dragging == toWrap.key ? const SizedBox() : toWrapWithSemantics,
           childWhenDragging: IgnorePointer(
-            ignoring: true,
-            child: SizedBox(
-              // Small values (<50) cause an error when used with ListTile.
-              width: double.infinity,
-              child: Opacity(
-                  opacity: 0,
+              ignoring: true,
+              child: SizedBox(
+                  // Small values (<50) cause an error when used with ListTile.
+                  width: double.infinity,
+                  child: Opacity(
+                      opacity: 0,
 //              child: _makeAppearingWidget(toWrap)
-                  child: Container(width: 0, height: 0, child: toWrap)
-              )
-            )
-          ),
+                      child: Container(width: 0, height: 0, child: toWrap)))),
           //ConstrainedBox(constraints: contentConstraints),//SizedBox(),
           dragAnchor: DragAnchor.child,
           onDragStarted: onDragStarted,
@@ -850,7 +848,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
     return Builder(builder: (BuildContext context) {
       Widget dragTarget = DragTarget<int>(
         builder: buildDragTarget,
-        onWillAccept: (int toAccept) {
+        onWillAccept: (int? toAccept) {
           bool willAccept = _dragStartIndex == toAccept && toAccept != index;
 //          debugPrint('${DateTime.now().toString().substring(5, 22)} reorderable_sliver.dart(679) $this._statefulWrap: '
 //            'onWillAccept: toAccept:$toAccept return:$willAccept _nextIndex:$_nextIndex index:$index _currentIndex:$_currentIndex _dragStartIndex:$_dragStartIndex');
@@ -880,7 +878,7 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
           return willAccept; //_dragging == toAccept && toAccept != toWrap.key;
         },
         onAccept: (int accepted) {},
-        onLeave: (Object leaving) {},
+        onLeave: (Object? leaving) {},
       );
 
 //      dragTarget = KeyedSubtree(
@@ -960,8 +958,8 @@ class _ReorderableSliverListState extends State<ReorderableSliverList>
     assert(widget.delegate is _ReorderableSliverChildDelegateMixin);
 
     if (widget.delegate is ReorderableSliverChildBuilderDelegate) {
-      _childCount =
-          (widget.delegate as ReorderableSliverChildBuilderDelegate).childCount;
+      _childCount = (widget.delegate as ReorderableSliverChildBuilderDelegate)
+          .childCount!;
     } else if (widget.delegate is ReorderableSliverChildListDelegate) {
       _childCount = (widget.delegate as ReorderableSliverChildListDelegate)
           .children
diff --git a/lib/src/widgets/reorderable_table.dart b/lib/src/widgets/reorderable_table.dart
index 7db1345..d94210b 100644
--- a/lib/src/widgets/reorderable_table.dart
+++ b/lib/src/widgets/reorderable_table.dart
@@ -8,18 +8,17 @@ import '../rendering/tabluar_flex.dart';
 
 class ReorderableTableRow extends TabluarRow {
   ReorderableTableRow({
-    Key key,
     MainAxisAlignment mainAxisAlignment = MainAxisAlignment.start,
     MainAxisSize mainAxisSize = MainAxisSize.max,
     CrossAxisAlignment crossAxisAlignment = CrossAxisAlignment.center,
-    TextDirection textDirection,
+    TextDirection? textDirection,
     VerticalDirection verticalDirection = VerticalDirection.down,
-    TextBaseline textBaseline,
+    TextBaseline? textBaseline,
     List<Widget> children = const <Widget>[],
-    Decoration decoration,
+    Decoration? decoration,
+    Key? key,
   }) : super(
           children: children,
-          key: key,
           mainAxisAlignment: mainAxisAlignment,
           mainAxisSize: mainAxisSize,
           crossAxisAlignment: crossAxisAlignment,
@@ -27,6 +26,7 @@ class ReorderableTableRow extends TabluarRow {
           verticalDirection: verticalDirection,
           textBaseline: textBaseline,
           decoration: decoration,
+          key: key,
         );
 }
 
@@ -56,7 +56,7 @@ class ReorderableTable extends StatelessWidget {
   /// The [children], [defaultColumnWidth], and [defaultVerticalAlignment]
   /// arguments must not be null.
   ReorderableTable({
-    Key key,
+    required this.onReorder,
     this.children = const <ReorderableTableRow>[],
     this.columnWidths,
     this.defaultColumnWidth = const FlexColumnWidth(1.0),
@@ -66,26 +66,13 @@ class ReorderableTable extends StatelessWidget {
     this.textBaseline,
     this.header,
     this.footer,
-    @required this.onReorder,
     this.decorateDraggableFeedback,
     this.onNoReorder,
-    thi,
     this.reorderAnimationDuration,
     this.scrollAnimationDuration,
     this.ignorePrimaryScrollController = false,
-  })  : assert(children != null),
-        assert(defaultColumnWidth != null),
-        assert(defaultVerticalAlignment != null),
-        assert(() {
-          if (children.any((ReorderableTableRow row) =>
-              row.children.any((Widget cell) => cell == null))) {
-            throw FlutterError(
-                'One of the children of one of the rows of the table was null.\n'
-                'The children of a ReorderableTableRow must not be null.');
-          }
-          return true;
-        }()),
-        assert(() {
+    Key? key,
+  })  : assert(() {
           if (children.any((ReorderableTableRow row1) =>
               row1.key != null &&
               children.any((ReorderableTableRow row2) =>
@@ -145,7 +132,7 @@ class ReorderableTable extends StatelessWidget {
   /// sizing algorithms are used here. In particular, [IntrinsicColumnWidth] is
   /// quite expensive because it needs to measure each cell in the column to
   /// determine the intrinsic size of the column.
-  final Map<int, TableColumnWidth> columnWidths;
+  final Map<int, TableColumnWidth>? columnWidths;
 
   /// How to determine with widths of columns that don't have an explicit sizing algorithm.
   ///
@@ -156,32 +143,32 @@ class ReorderableTable extends StatelessWidget {
   /// The direction in which the columns are ordered.
   ///
   /// Defaults to the ambient [Directionality].
-  final TextDirection textDirection;
+  final TextDirection? textDirection;
 
   /// The style to use when painting the boundary and interior divisions of the table.
-  final TableBorder border;
+  final TableBorder? border;
 
   /// How cells that do not explicitly specify a vertical alignment are aligned vertically.
   final TableCellVerticalAlignment defaultVerticalAlignment;
 
   /// The text baseline to use when aligning rows using [TableCellVerticalAlignment.baseline].
-  final TextBaseline textBaseline;
+  final TextBaseline? textBaseline;
 
   /// Non-reorderable widget at top of the table. Cells in [header] also affects
   /// alignment of columns.
-  final Widget header;
+  final Widget? header;
 
   /// Non-reorderable widget at top of the table. Cells in [footer] also affects
   /// alignment of columns.
-  final Widget footer;
+  final Widget? footer;
 
   /// Called when a child is dropped into a new position to shuffle the
   /// children.
   final ReorderCallback onReorder;
-  final NoReorderCallback onNoReorder;
-  final DecorateDraggableFeedback decorateDraggableFeedback;
-  final Duration reorderAnimationDuration;
-  final Duration scrollAnimationDuration;
+  final NoReorderCallback? onNoReorder;
+  final DecorateDraggableFeedback? decorateDraggableFeedback;
+  final Duration? reorderAnimationDuration;
+  final Duration? scrollAnimationDuration;
   final bool ignorePrimaryScrollController;
 
   @override
@@ -220,9 +207,9 @@ class ReorderableTable extends StatelessWidget {
         buildDraggableFeedback: (BuildContext feedbackContext,
             BoxConstraints constraints, Widget child) {
           // The child is a ReorderableTableRow because children is a List<ReorderableTableRow>
-          ReorderableTableRow tableRow = child;
+          ReorderableTableRow tableRow = child as ReorderableTableRow;
           RenderTabluarFlex renderTabluarFlex =
-              tableKey.currentContext.findRenderObject();
+              tableKey.currentContext!.findRenderObject() as RenderTabluarFlex;
           int grandChildIndex = 0;
           for (;
               grandChildIndex < tableRow.children.length;
@@ -230,7 +217,7 @@ class ReorderableTable extends StatelessWidget {
             tableRow.children[grandChildIndex] = ConstrainedBox(
                 constraints: BoxConstraints(
                     minWidth: renderTabluarFlex
-                        .maxGrandchildrenCrossSize[grandChildIndex]),
+                        .maxGrandchildrenCrossSize[grandChildIndex]!),
                 child: tableRow.children[grandChildIndex]);
           }
           for (;
@@ -240,7 +227,7 @@ class ReorderableTable extends StatelessWidget {
             tableRow.children.add(ConstrainedBox(
               constraints: BoxConstraints(
                   minWidth: renderTabluarFlex
-                      .maxGrandchildrenCrossSize[grandChildIndex]),
+                      .maxGrandchildrenCrossSize[grandChildIndex]!),
             ));
           }
 
diff --git a/lib/src/widgets/reorderable_widget.dart b/lib/src/widgets/reorderable_widget.dart
index 74867c5..579e7b6 100644
--- a/lib/src/widgets/reorderable_widget.dart
+++ b/lib/src/widgets/reorderable_widget.dart
@@ -4,11 +4,11 @@ class ReorderableWidget extends StatelessWidget implements ReorderableItem {
   final Widget child;
   final bool reorderable;
 
-  ReorderableWidget(
-      {@required this.child, @required this.reorderable, @required Key key})
-      : assert(reorderable != null && reorderable is bool),
-        assert(key != null, 'A key is mandatory for each Widget'),
-        super(key: key);
+  ReorderableWidget({
+    required this.child,
+    required this.reorderable,
+    required Key key,
+  }) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
@@ -19,5 +19,5 @@ class ReorderableWidget extends StatelessWidget implements ReorderableItem {
 abstract class ReorderableItem extends Widget {
   final bool reorderable;
 
-  ReorderableItem({@required this.reorderable});
+  ReorderableItem({required this.reorderable});
 }
diff --git a/lib/src/widgets/reorderable_wrap.dart b/lib/src/widgets/reorderable_wrap.dart
index 6db7acc..c7dbb2d 100644
--- a/lib/src/widgets/reorderable_wrap.dart
+++ b/lib/src/widgets/reorderable_wrap.dart
@@ -9,11 +9,9 @@ import 'package:flutter/rendering.dart';
 import 'package:reorderables/reorderables.dart';
 
 import './passthrough_overlay.dart';
-
 //import './transitions.dart';
 import './typedefs.dart';
 import './wrap.dart';
-
 //import './transitions.dart';
 import '../rendering/wrap.dart';
 import 'reorderable_mixin.dart';
@@ -36,13 +34,11 @@ import 'reorderable_mixin.dart';
 class ReorderableWrap extends StatefulWidget {
   /// Creates a reorderable wrap.
   ReorderableWrap({
-    Key key,
+    required this.children,
+    required this.onReorder,
     this.header,
     this.footer,
     this.controller,
-    this.enabledDragging = true,
-    @required this.children,
-    @required this.onReorder,
     this.direction = Axis.horizontal,
     this.scrollDirection = Axis.vertical,
     this.padding = EdgeInsets.zero,
@@ -63,9 +59,9 @@ class ReorderableWrap extends StatefulWidget {
     this.reorderAnimationDuration = const Duration(milliseconds: 200),
     this.scrollAnimationDuration = const Duration(milliseconds: 200),
     this.ignorePrimaryScrollController = false,
-  })  : assert(direction != null),
-        assert(onReorder != null),
-        assert(children != null),
+    this.enabledDragging = true,
+    Key? key,
+  }) :
 //        assert(
 //          children.every((Widget w) => w.key != null),
 //          'All children of this widget must have a key.',
@@ -75,13 +71,13 @@ class ReorderableWrap extends StatefulWidget {
   /// A non-reorderable header widget to show before the list.
   ///
   /// If null, no header will appear before the list.
-  final List<Widget> header;
-  final Widget footer;
+  final List<Widget>? header;
+  final Widget? footer;
 
   /// A custom scroll [controller].
   /// To control the initial scroll offset of the scroll view, provide a
   /// [controller] with its [ScrollController.initialScrollOffset] property set.
-  final ScrollController controller;
+  final ScrollController? controller;
 
   /// The widgets to display.
   final List<Widget> children;
@@ -96,18 +92,18 @@ class ReorderableWrap extends StatefulWidget {
   final Axis scrollDirection;
 
   /// The amount of space by which to inset the [children].
-  final EdgeInsets padding;
+  final EdgeInsets? padding;
 
   /// Called when a child is dropped into a new position to shuffle the
   /// children.
   final ReorderCallback onReorder;
-  final NoReorderCallback onNoReorder;
+  final NoReorderCallback? onNoReorder;
 
   /// Called when the draggable starts being dragged.
-  final ReorderStartedCallback onReorderStarted;
+  final ReorderStartedCallback? onReorderStarted;
 
-  final BuildItemsContainer buildItemsContainer;
-  final BuildDraggableFeedback buildDraggableFeedback;
+  final BuildItemsContainer? buildItemsContainer;
+  final BuildDraggableFeedback? buildDraggableFeedback;
 
   /// The flag of whether needs long press to trigger dragging mode.
   /// true means it needs long press and false means no need.
@@ -211,7 +207,7 @@ class ReorderableWrap extends StatefulWidget {
   /// [crossAxisAlignment] is either [WrapCrossAlignment.start] or
   /// [WrapCrossAlignment.end], or there's more than one child, then the
   /// [textDirection] (or the ambient [Directionality]) must not be null.
-  final TextDirection textDirection;
+  final TextDirection? textDirection;
 
   /// Determines the order to lay children out vertically and how to interpret
   /// `start` and `end` in the vertical direction.
@@ -237,8 +233,8 @@ class ReorderableWrap extends StatefulWidget {
   /// [verticalDirection] must not be null.
   final VerticalDirection verticalDirection;
 
-  final int minMainAxisCount;
-  final int maxMainAxisCount;
+  final int? minMainAxisCount;
+  final int? maxMainAxisCount;
   final Duration reorderAnimationDuration;
   final Duration scrollAnimationDuration;
   final bool ignorePrimaryScrollController;
@@ -260,11 +256,10 @@ class ReorderableWrap extends StatefulWidget {
 // insert Draggables into the Overlay above itself.
 class _ReorderableWrapState extends State<ReorderableWrap> {
   // We use an inner overlay so that the dragging list item doesn't draw outside of the list itself.
-  final GlobalKey _overlayKey =
-      GlobalKey(debugLabel: '$ReorderableWrap overlay key');
+  final GlobalKey _overlayKey = GlobalKey(debugLabel: '$ReorderableWrap overlay key');
 
   // This entry contains the scrolling list itself.
-  PassthroughOverlayEntry _listOverlayEntry;
+  late PassthroughOverlayEntry _listOverlayEntry;
 
   @override
   void initState() {
@@ -281,7 +276,7 @@ class _ReorderableWrapState extends State<ReorderableWrap> {
           onReorder: widget.onReorder,
           onNoReorder: widget.onNoReorder,
           onReorderStarted: widget.onReorderStarted,
-          padding: widget.padding,
+          padding: widget.padding ?? EdgeInsets.zero,
           buildItemsContainer: widget.buildItemsContainer,
           buildDraggableFeedback: widget.buildDraggableFeedback,
           needsLongPressDraggable: widget.needsLongPressDraggable,
@@ -305,11 +300,10 @@ class _ReorderableWrapState extends State<ReorderableWrap> {
 
   @override
   Widget build(BuildContext context) {
-    final PassthroughOverlay passthroughOverlay = PassthroughOverlay(
-        key: _overlayKey,
-        initialEntries: <PassthroughOverlayEntry>[
-          _listOverlayEntry,
-        ]);
+    final PassthroughOverlay passthroughOverlay =
+        PassthroughOverlay(key: _overlayKey, initialEntries: <PassthroughOverlayEntry>[
+      _listOverlayEntry,
+    ]);
     return widget.ignorePrimaryScrollController
         ? PrimaryScrollController.none(child: passthroughOverlay)
         : passthroughOverlay;
@@ -320,45 +314,45 @@ class _ReorderableWrapState extends State<ReorderableWrap> {
 // ReorderableListView.
 class _ReorderableWrapContent extends StatefulWidget {
   const _ReorderableWrapContent({
+    required this.children,
+    required this.direction,
+    required this.scrollDirection,
+    required this.padding,
+    required this.onReorder,
+    required this.onNoReorder,
+    required this.onReorderStarted,
+    required this.buildItemsContainer,
+    required this.buildDraggableFeedback,
+    required this.needsLongPressDraggable,
+    required this.alignment,
+    required this.spacing,
+    required this.runAlignment,
+    required this.runSpacing,
+    required this.crossAxisAlignment,
+    required this.textDirection,
+    required this.verticalDirection,
+    required this.minMainAxisCount,
+    required this.maxMainAxisCount,
     this.header,
     this.footer,
     this.controller,
-    @required this.children,
-    @required this.direction,
-    @required this.scrollDirection,
-    @required this.padding,
-    @required this.onReorder,
-    @required this.onNoReorder,
-    @required this.onReorderStarted,
-    @required this.buildItemsContainer,
-    @required this.buildDraggableFeedback,
-    @required this.needsLongPressDraggable,
-    @required this.alignment,
-    @required this.spacing,
-    @required this.runAlignment,
-    @required this.runSpacing,
-    @required this.crossAxisAlignment,
-    @required this.textDirection,
-    @required this.verticalDirection,
-    @required this.minMainAxisCount,
-    @required this.maxMainAxisCount,
-    @required this.reorderAnimationDuration,
-    @required this.scrollAnimationDuration,
-    @required this.enableDragging,
+    this.reorderAnimationDuration = const Duration(milliseconds: 200),
+    this.scrollAnimationDuration = const Duration(milliseconds: 200),
+    this.enableDragging = true,
   });
 
-  final List<Widget> header;
-  final Widget footer;
-  final ScrollController controller;
+  final List<Widget>? header;
+  final Widget? footer;
+  final ScrollController? controller;
   final List<Widget> children;
   final Axis direction;
   final Axis scrollDirection;
   final EdgeInsets padding;
   final ReorderCallback onReorder;
-  final NoReorderCallback onNoReorder;
-  final ReorderStartedCallback onReorderStarted;
-  final BuildItemsContainer buildItemsContainer;
-  final BuildDraggableFeedback buildDraggableFeedback;
+  final NoReorderCallback? onNoReorder;
+  final ReorderStartedCallback? onReorderStarted;
+  final BuildItemsContainer? buildItemsContainer;
+  final BuildDraggableFeedback? buildDraggableFeedback;
   final bool needsLongPressDraggable;
 
   final WrapAlignment alignment;
@@ -366,13 +360,13 @@ class _ReorderableWrapContent extends StatefulWidget {
   final WrapAlignment runAlignment;
   final double runSpacing;
   final WrapCrossAlignment crossAxisAlignment;
-  final TextDirection textDirection;
+  final TextDirection? textDirection;
   final VerticalDirection verticalDirection;
-  final int minMainAxisCount;
-  final int maxMainAxisCount;
-  final bool enableDragging;
+  final int? minMainAxisCount;
+  final int? maxMainAxisCount;
   final Duration reorderAnimationDuration;
   final Duration scrollAnimationDuration;
+  final bool enableDragging;
 
   @override
   _ReorderableWrapContentState createState() => _ReorderableWrapContentState();
@@ -391,36 +385,36 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
   static const double _dropAreaMargin = 0.0;
 
   // How long an animation to reorder an element in the list takes.
-  Duration _reorderAnimationDuration;
+  late Duration _reorderAnimationDuration;
 
   // How long an animation to scroll to an off-screen element in the
   // list takes.
-  Duration _scrollAnimationDuration;
+  late Duration _scrollAnimationDuration;
 
   // Controls scrolls and measures scroll progress.
-  ScrollController _scrollController;
+  late ScrollController _scrollController;
 
   // This controls the entrance of the dragging widget into a new place.
-  AnimationController _entranceController;
+  late AnimationController _entranceController;
 
   // This controls the 'ghost' of the dragging widget, which is left behind
   // where the widget used to be.
-  AnimationController _ghostController;
+  late AnimationController _ghostController;
 
   // The member of widget.children currently being dragged.
   //
   // Null if no drag is underway.
 //  int _dragging;
-  Widget _draggingWidget;
+  Widget? _draggingWidget;
 
   // The last computed size of the feedback widget being dragged.
-  Size _draggingFeedbackSize;
+  Size? _draggingFeedbackSize;
 
 //  List<GlobalObjectKey> _childKeys;
-  List<BuildContext> _childContexts;
-  List<Size> _childSizes;
-  List<int> _childIndexToDisplayIndex;
-  List<int> _childDisplayIndexToIndex;
+  late List<BuildContext?> _childContexts;
+  late List<Size> _childSizes;
+  late List<int> _childIndexToDisplayIndex;
+  late List<int> _childDisplayIndexToIndex;
 
   // The location that the dragging widget occupied before it started to drag.
   int _dragStartIndex = -1;
@@ -439,16 +433,16 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
   bool _scrolling = false;
 
   final GlobalKey _wrapKey = GlobalKey(debugLabel: '$ReorderableWrap wrap key');
-  List<int> _wrapChildRunIndexes;
-  List<int> _childRunIndexes;
-  List<int> _nextChildRunIndexes;
-  List<Widget> _wrapChildren;
+  late List<int> _wrapChildRunIndexes;
+  late List<int> _childRunIndexes;
+  late List<int> _nextChildRunIndexes;
+  late List<Widget?> _wrapChildren;
 
   Size get _dropAreaSize {
     if (_draggingFeedbackSize == null) {
       return Size(0, 0);
     }
-    return _draggingFeedbackSize + Offset(_dropAreaMargin, _dropAreaMargin);
+    return _draggingFeedbackSize! + Offset(_dropAreaMargin, _dropAreaMargin);
 //    double dropAreaWithoutMargin;
 //    switch (widget.direction) {
 //      case Axis.horizontal:
@@ -467,10 +461,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
     super.initState();
     _reorderAnimationDuration = widget.reorderAnimationDuration;
     _scrollAnimationDuration = widget.scrollAnimationDuration;
-    _entranceController = AnimationController(
-        value: 1.0, vsync: this, duration: _reorderAnimationDuration);
-    _ghostController = AnimationController(
-        value: 0, vsync: this, duration: _reorderAnimationDuration);
+    _entranceController = AnimationController(value: 1.0, vsync: this, duration: _reorderAnimationDuration);
+    _ghostController = AnimationController(value: 0, vsync: this, duration: _reorderAnimationDuration);
     _entranceController.addStatusListener(_onEntranceStatusChanged);
 //    _childKeys = List.filled(widget.children.length, null);
     _childContexts = List.filled(widget.children.length, null);
@@ -487,9 +479,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 
   @override
   void didChangeDependencies() {
-    _scrollController = widget.controller ??
-        PrimaryScrollController.of(context) ??
-        ScrollController();
+    _scrollController = widget.controller ?? PrimaryScrollController.of(context) ?? ScrollController();
     super.didChangeDependencies();
   }
 
@@ -528,16 +518,12 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
   // Scrolls to a target context if that context is not on the screen.
   void _scrollTo(BuildContext context) {
     if (_scrolling) return;
-    final RenderObject contextObject = context.findRenderObject();
-    final RenderAbstractViewport viewport =
-        RenderAbstractViewport.of(contextObject);
-    assert(viewport != null);
+    final RenderObject contextObject = context.findRenderObject()!;
+    final RenderAbstractViewport viewport = RenderAbstractViewport.of(contextObject)!;
     // If and only if the current scroll offset falls in-between the offsets
     // necessary to reveal the selected context at the top or bottom of the
     // screen, then it is already on-screen.
-    final double margin = widget.direction == Axis.horizontal
-        ? _dropAreaSize.width
-        : _dropAreaSize.height;
+    final double margin = widget.direction == Axis.horizontal ? _dropAreaSize.width : _dropAreaSize.height;
     final double scrollOffset = _scrollController.offset;
     final double topOffset = max(
       _scrollController.position.minScrollExtent,
@@ -547,8 +533,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
       _scrollController.position.maxScrollExtent,
       viewport.getOffsetToReveal(contextObject, 1.0).offset + margin,
     );
-    final bool onScreen =
-        scrollOffset <= topOffset && scrollOffset >= bottomOffset;
+    final bool onScreen = scrollOffset <= topOffset && scrollOffset >= bottomOffset;
     // If the context is off screen, then we request a scroll to make it visible.
     if (!onScreen) {
       _scrolling = true;
@@ -568,7 +553,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 
   // Wraps children in Row or Column, so that the children flow in
   // the widget's scrollDirection.
-  Widget _buildContainerForMainAxis({List<Widget> children}) {
+  Widget _buildContainerForMainAxis({required List<Widget> children}) {
 //    MainAxisSize mainAxisSize = MainAxisSize.min;
 //    CrossAxisAlignment crossAxisAlignment;
     WrapAlignment runAlignment;
@@ -640,36 +625,32 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 //        for (int i = 0; i < widget.children.length; i++) {
 //          _childSizes[i] = _childKeys[i].currentContext.size;
 //        }
-        _draggingFeedbackSize = _childContexts[index].size;
+        _draggingFeedbackSize = _childContexts[index]!.size;
         for (int i = 0; i < widget.children.length; i++) {
-          _childSizes[i] = _childContexts[i].size;
+          _childSizes[i] = _childContexts[i]!.size!;
         }
 
         if (_wrapKey.currentContext != null) {
           RenderWrapWithMainAxisCount wrapRenderObject =
-              _wrapKey.currentContext.findRenderObject();
+              _wrapKey.currentContext!.findRenderObject() as RenderWrapWithMainAxisCount;
           _wrapChildRunIndexes = wrapRenderObject.childRunIndexes;
           for (int i = 0; i < _childRunIndexes.length; i++) {
-            _nextChildRunIndexes[i] =
-                _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
+            _nextChildRunIndexes[i] = _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
           }
         } else {
           if (widget.minMainAxisCount != null &&
               widget.maxMainAxisCount != null &&
               widget.minMainAxisCount == widget.maxMainAxisCount) {
-            _wrapChildRunIndexes = List.generate(widget.children.length,
-                (int index) => index ~/ widget.minMainAxisCount);
+            _wrapChildRunIndexes =
+                List.generate(widget.children.length, (int index) => index ~/ widget.minMainAxisCount!);
             for (int i = 0; i < _childRunIndexes.length; i++) {
-              _nextChildRunIndexes[i] =
-                  _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
+              _nextChildRunIndexes[i] = _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
             }
           }
         }
 
 //        debugPrint('onDragStarted: index:$index _ghostDisplayIndex:$_ghostDisplayIndex _currentDisplayIndex:$_currentDisplayIndex _dragStartIndex:$_dragStartIndex');
-        if (widget.onReorderStarted != null) {
-          widget.onReorderStarted(index);
-        }
+        widget.onReorderStarted?.call(index);
       });
     }
 
@@ -678,7 +659,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 //      debugPrint('_reorder: startIndex:$startIndex endIndex:$endIndex');
       if (startIndex != endIndex)
         widget.onReorder(startIndex, endIndex);
-      else if (widget.onNoReorder != null) widget.onNoReorder(startIndex);
+      else if (widget.onNoReorder != null) widget.onNoReorder!(startIndex);
       // Animates leftover space in the drop area closed.
       // TODO(djshuckerow): bring the animation in line with the Material
       // specifications.
@@ -711,8 +692,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 
     Widget wrapWithSemantics() {
       // First, determine which semantics actions apply.
-      final Map<CustomSemanticsAction, VoidCallback> semanticsActions =
-          <CustomSemanticsAction, VoidCallback>{};
+      final Map<CustomSemanticsAction, VoidCallback> semanticsActions = <CustomSemanticsAction, VoidCallback>{};
 
       // Create the appropriate semantics actions.
       void moveToStart() => reorder(index, 0);
@@ -722,37 +702,29 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
       // before index+2, which is after the space at index+1.
       void moveAfter() => reorder(index, index + 2);
 
-      final MaterialLocalizations localizations =
-          MaterialLocalizations.of(context);
-
-      if (localizations != null) {
-        // If the item can move to before its current position in the list.
-        if (index > 0) {
-          semanticsActions[CustomSemanticsAction(
-              label: localizations.reorderItemToStart)] = moveToStart;
-          String reorderItemBefore = localizations.reorderItemUp;
-          if (widget.direction == Axis.horizontal) {
-            reorderItemBefore = Directionality.of(context) == TextDirection.ltr
-                ? localizations.reorderItemLeft
-                : localizations.reorderItemRight;
-          }
-          semanticsActions[CustomSemanticsAction(label: reorderItemBefore)] =
-              moveBefore;
+      final MaterialLocalizations localizations = MaterialLocalizations.of(context);
+
+      if (index > 0) {
+        semanticsActions[CustomSemanticsAction(label: localizations.reorderItemToStart)] = moveToStart;
+        String reorderItemBefore = localizations.reorderItemUp;
+        if (widget.direction == Axis.horizontal) {
+          reorderItemBefore = Directionality.of(context) == TextDirection.ltr
+              ? localizations.reorderItemLeft
+              : localizations.reorderItemRight;
         }
+        semanticsActions[CustomSemanticsAction(label: reorderItemBefore)] = moveBefore;
+      }
 
-        // If the item can move to after its current position in the list.
-        if (index < widget.children.length - 1) {
-          String reorderItemAfter = localizations.reorderItemDown;
-          if (widget.direction == Axis.horizontal) {
-            reorderItemAfter = Directionality.of(context) == TextDirection.ltr
-                ? localizations.reorderItemRight
-                : localizations.reorderItemLeft;
-          }
-          semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] =
-              moveAfter;
-          semanticsActions[CustomSemanticsAction(
-              label: localizations.reorderItemToEnd)] = moveToEnd;
+      // If the item can move to after its current position in the list.
+      if (index < widget.children.length - 1) {
+        String reorderItemAfter = localizations.reorderItemDown;
+        if (widget.direction == Axis.horizontal) {
+          reorderItemAfter = Directionality.of(context) == TextDirection.ltr
+              ? localizations.reorderItemRight
+              : localizations.reorderItemLeft;
         }
+        semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] = moveAfter;
+        semanticsActions[CustomSemanticsAction(label: localizations.reorderItemToEnd)] = moveToEnd;
       }
 
       // We pass toWrap with a GlobalKey into the Draggable so that when a list
@@ -803,8 +775,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
       Widget feedbackBuilder = Builder(builder: (BuildContext context) {
 //          RenderRepaintBoundary renderObject = _contentKey.currentContext.findRenderObject();
 //          BoxConstraints contentSizeConstraints = BoxConstraints.loose(renderObject.size);
-        BoxConstraints contentSizeConstraints = BoxConstraints.loose(
-            _draggingFeedbackSize); //renderObject.constraints
+        BoxConstraints contentSizeConstraints = BoxConstraints.loose(_draggingFeedbackSize!); //renderObject.constraints
 //          debugPrint('feedbackBuilder: contentConstraints:$contentSizeConstraints');
         return (widget.buildDraggableFeedback ?? defaultBuildDraggableFeedback)(
             context, contentSizeConstraints, toWrap);
@@ -830,9 +801,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
                 feedback: feedbackBuilder,
                 // Wrap toWrapWithSemantics with a widget that supports HitTestBehavior
                 // to make sure the whole toWrapWithSemantics responds to pointer events, i.e. dragging
-                child: MetaData(
-                    child: toWrapWithSemantics,
-                    behavior: HitTestBehavior.opaque),
+                child: MetaData(child: toWrapWithSemantics, behavior: HitTestBehavior.opaque),
                 //toWrapWithSemantics,//_dragging == toWrap.key ? const SizedBox() : toWrapWithSemantics,
                 childWhenDragging: IgnorePointer(
                     ignoring: true,
@@ -849,8 +818,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
                 // When the drag does not end inside a DragTarget widget, the
                 // drag fails, but we still reorder the widget to the last position it
                 // had been dragged to.
-                onDraggableCanceled: (Velocity velocity, Offset offset) =>
-                    onDragEnded(),
+                onDraggableCanceled: (Velocity velocity, Offset offset) => onDragEnded(),
               )
             : Draggable<int>(
                 maxSimultaneousDrags: widget.enableDragging ? 1 : 0,
@@ -858,9 +826,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
                 //toWrap.key,
                 ignoringFeedbackSemantics: false,
                 feedback: feedbackBuilder,
-                child: MetaData(
-                    child: toWrapWithSemantics,
-                    behavior: HitTestBehavior.opaque),
+                child: MetaData(child: toWrapWithSemantics, behavior: HitTestBehavior.opaque),
                 childWhenDragging: IgnorePointer(
                   ignoring: true,
                   child: Opacity(
@@ -871,8 +837,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
                 dragAnchor: DragAnchor.child,
                 onDragStarted: onDragStarted,
                 onDragCompleted: onDragEnded,
-                onDraggableCanceled: (Velocity velocity, Offset offset) =>
-                    onDragEnded(),
+                onDraggableCanceled: (Velocity velocity, Offset offset) => onDragEnded(),
               );
       }
 
@@ -900,33 +865,27 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 //      debugPrint('index:$index displayIndex:$displayIndex _nextDisplayIndex:$_nextDisplayIndex _currentDisplayIndex:$_currentDisplayIndex _ghostDisplayIndex:$_ghostDisplayIndex _dragStartIndex:$_dragStartIndex');
 //      debugPrint(' _childRunIndexes:$_childRunIndexes _nextChildRunIndexes:$_nextChildRunIndexes _wrapChildRunIndexes:$_wrapChildRunIndexes');
 
-      List<Widget> _includeMovedAdjacentChildIfNeeded(
-          Widget child, int childDisplayIndex) {
+      List<Widget> _includeMovedAdjacentChildIfNeeded(Widget child, int childDisplayIndex) {
 //        debugPrint(' checking ${_childDisplayIndexToIndex[childDisplayIndex]}($childDisplayIndex)');
         int checkingTargetDisplayIndex = -1;
-        if (_ghostDisplayIndex < _currentDisplayIndex &&
-            childDisplayIndex > _ghostDisplayIndex) {
+        if (_ghostDisplayIndex < _currentDisplayIndex && childDisplayIndex > _ghostDisplayIndex) {
           checkingTargetDisplayIndex = childDisplayIndex - 1;
-        } else if (_ghostDisplayIndex > _currentDisplayIndex &&
-            childDisplayIndex < _ghostDisplayIndex) {
+        } else if (_ghostDisplayIndex > _currentDisplayIndex && childDisplayIndex < _ghostDisplayIndex) {
           checkingTargetDisplayIndex = childDisplayIndex + 1;
         }
         if (checkingTargetDisplayIndex == -1) {
           return [child];
         }
-        int checkingTargetIndex =
-            _childDisplayIndexToIndex[checkingTargetDisplayIndex];
+        int checkingTargetIndex = _childDisplayIndexToIndex[checkingTargetDisplayIndex];
         if (checkingTargetIndex == _dragStartIndex) {
           return [child];
         }
         if (_childRunIndexes[checkingTargetIndex] == -1 ||
-            _childRunIndexes[checkingTargetIndex] ==
-                _wrapChildRunIndexes[checkingTargetDisplayIndex]) {
+            _childRunIndexes[checkingTargetIndex] == _wrapChildRunIndexes[checkingTargetDisplayIndex]) {
           return [child];
         }
 //        debugPrint(' make $checkingTargetIndex($checkingTargetDisplayIndex) disappearing around $index');
-        Widget disappearingPreChild =
-            _makeDisappearingWidget(_wrapChildren[checkingTargetIndex]);
+        Widget disappearingPreChild = _makeDisappearingWidget(_wrapChildren[checkingTargetIndex]!);
 //        return _buildContainerForMainAxis(
 //          children: _ghostDisplayIndex < _currentDisplayIndex
 //            ? [disappearingPreChild, child]
@@ -943,11 +902,10 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
       if (_currentDisplayIndex == -1 || displayIndex == _currentDisplayIndex) {
         //we still wrap dragTarget with a container so that widget's depths are the same and it prevents layout alignment issue
         return _buildContainerForMainAxis(
-            children: _includeMovedAdjacentChildIfNeeded(
-                containedDraggable, displayIndex));
+            children: _includeMovedAdjacentChildIfNeeded(containedDraggable, displayIndex));
       }
 
-      bool _onWillAccept(int toAccept, bool isPre) {
+      bool _onWillAccept(int? toAccept, bool isPre) {
         int nextDisplayIndex;
         if (_currentDisplayIndex < displayIndex) {
           nextDisplayIndex = isPre ? displayIndex - 1 : displayIndex;
@@ -956,8 +914,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
         }
 
         bool movingToAdjacentChild =
-            nextDisplayIndex <= _currentDisplayIndex + 1 &&
-                nextDisplayIndex >= _currentDisplayIndex - 1;
+            nextDisplayIndex <= _currentDisplayIndex + 1 && nextDisplayIndex >= _currentDisplayIndex - 1;
         bool willAccept = _dragStartIndex == toAccept &&
 //          toAccept != toWrap.key &&
             toAccept != index &&
@@ -969,12 +926,13 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
         if (!willAccept) {
           return false;
         }
-        assert(_childDisplayIndexToIndex[_currentDisplayIndex] != index &&
-            _currentDisplayIndex != displayIndex);
+        if (!(_childDisplayIndexToIndex[_currentDisplayIndex] != index && _currentDisplayIndex != displayIndex)) {
+          return false;
+        }
 
         if (_wrapKey.currentContext != null) {
           RenderWrapWithMainAxisCount wrapRenderObject =
-              _wrapKey.currentContext.findRenderObject();
+              _wrapKey.currentContext!.findRenderObject() as RenderWrapWithMainAxisCount;
           _wrapChildRunIndexes = wrapRenderObject.childRunIndexes;
 //          for (int i=0; i<_childRunIndexes.length; i++) {
 //            _childRunIndexes[i] = _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
@@ -983,8 +941,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
           if (widget.minMainAxisCount != null &&
               widget.maxMainAxisCount != null &&
               widget.minMainAxisCount == widget.maxMainAxisCount) {
-            _wrapChildRunIndexes = List.generate(widget.children.length,
-                (int index) => index ~/ widget.minMainAxisCount);
+            _wrapChildRunIndexes =
+                List.generate(widget.children.length, (int index) => index ~/ widget.minMainAxisCount!);
 //            for (int i=0; i<_childRunIndexes.length; i++) {
 //              _childRunIndexes[i] = _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
 //            }
@@ -1002,47 +960,35 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
       }
 
       Widget preDragTarget = DragTarget<int>(
-        builder: (BuildContext context, List<int> acceptedCandidates,
-                List<dynamic> rejectedCandidates) =>
-            SizedBox(),
-        onWillAccept: (int toAccept) => _onWillAccept(toAccept, true),
+        builder: (BuildContext context, List<int?> acceptedCandidates, List<dynamic> rejectedCandidates) => SizedBox(),
+        onWillAccept: (int? toAccept) => _onWillAccept(toAccept, true),
         onAccept: (int accepted) {},
-        onLeave: (Object leaving) {},
+        onLeave: (Object? leaving) {},
       );
       Widget nextDragTarget = DragTarget<int>(
-        builder: (BuildContext context, List<int> acceptedCandidates,
-                List<dynamic> rejectedCandidates) =>
-            SizedBox(),
-        onWillAccept: (int toAccept) => _onWillAccept(toAccept, false),
+        builder: (BuildContext context, List<int?> acceptedCandidates, List<dynamic> rejectedCandidates) => SizedBox(),
+        onWillAccept: (int? toAccept) => _onWillAccept(toAccept, false),
         onAccept: (int accepted) {},
-        onLeave: (Object leaving) {},
+        onLeave: (Object? leaving) {},
       );
 
       Widget dragTarget = Stack(
 //        key: keyIndexGlobalKey,
 //        fit: StackFit.passthrough,
-        overflow: Overflow.clip,
+        clipBehavior: Clip.hardEdge,
         children: <Widget>[
           containedDraggable,
           Positioned(
               left: 0,
               top: 0,
-              width: widget.direction == Axis.horizontal
-                  ? _childSizes[index].width / 2
-                  : _childSizes[index].width,
-              height: widget.direction == Axis.vertical
-                  ? _childSizes[index].height / 2
-                  : _childSizes[index].height,
+              width: widget.direction == Axis.horizontal ? _childSizes[index].width / 2 : _childSizes[index].width,
+              height: widget.direction == Axis.vertical ? _childSizes[index].height / 2 : _childSizes[index].height,
               child: preDragTarget),
           Positioned(
               right: 0,
               bottom: 0,
-              width: widget.direction == Axis.horizontal
-                  ? _childSizes[index].width / 2
-                  : _childSizes[index].width,
-              height: widget.direction == Axis.vertical
-                  ? _childSizes[index].height / 2
-                  : _childSizes[index].height,
+              width: widget.direction == Axis.horizontal ? _childSizes[index].width / 2 : _childSizes[index].width,
+              height: widget.direction == Axis.vertical ? _childSizes[index].height / 2 : _childSizes[index].height,
               child: nextDragTarget),
         ],
       );
@@ -1093,8 +1039,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
           : Opacity(opacity: 0.2, child: _draggingWidget);
 //      Widget spacing = SizedBox.fromSize(size: _dropAreaSize, child: Container(color: Colors.red));
 
-      if (_childRunIndexes[index] != -1 &&
-          _childRunIndexes[index] != _wrapChildRunIndexes[displayIndex]) {
+      if (_childRunIndexes[index] != -1 && _childRunIndexes[index] != _wrapChildRunIndexes[displayIndex]) {
         dragTarget = _makeAppearingWidget(dragTarget);
       }
 
@@ -1103,20 +1048,15 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
         if (_ghostDisplayIndex < _currentDisplayIndex) {
           //ghost is on the left of current, so shift it to the right
           return _buildContainerForMainAxis(
-              children: [ghostSpacing] +
-                  _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex));
+              children: [ghostSpacing] + _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex));
         } else if (_ghostDisplayIndex > _currentDisplayIndex) {
           return _buildContainerForMainAxis(
-              children:
-                  _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex) +
-                      [ghostSpacing]);
+              children: _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex) + [ghostSpacing]);
         }
       }
 
       //we still wrap dragTarget with a container so that widget's depths are the same and it prevent's layout alignment issue
-      return _buildContainerForMainAxis(
-          children:
-              _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex));
+      return _buildContainerForMainAxis(children: _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex));
 
 //      if (shiftedIndex == _currentDisplayIndex) {
 //        Widget entranceSpacing = SizeTransitionWithIntrinsicSize(
@@ -1162,8 +1102,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 //    _childSizes = List.filled(widget.children.length, Size(0, 0));
     List<E> _resizeListMember<E>(List<E> listVar, E initValue) {
       if (listVar.length < widget.children.length) {
-        return listVar +
-            List.filled(widget.children.length - listVar.length, initValue);
+        return listVar + List.filled(widget.children.length - listVar.length, initValue);
       } else if (listVar.length > widget.children.length) {
         return listVar.sublist(0, widget.children.length);
       }
@@ -1174,15 +1113,10 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
     _childContexts = _resizeListMember(_childContexts, null);
     _childSizes = _resizeListMember(_childSizes, Size(0, 0));
 
-    _childDisplayIndexToIndex =
-        List.generate(widget.children.length, (int index) => index);
-    _childIndexToDisplayIndex =
-        List.generate(widget.children.length, (int index) => index);
-    if (_dragStartIndex >= 0 &&
-        _currentDisplayIndex >= 0 &&
-        _dragStartIndex != _currentDisplayIndex) {
-      _childDisplayIndexToIndex.insert(_currentDisplayIndex,
-          _childDisplayIndexToIndex.removeAt(_dragStartIndex));
+    _childDisplayIndexToIndex = List.generate(widget.children.length, (int index) => index);
+    _childIndexToDisplayIndex = List.generate(widget.children.length, (int index) => index);
+    if (_dragStartIndex >= 0 && _currentDisplayIndex >= 0 && _dragStartIndex != _currentDisplayIndex) {
+      _childDisplayIndexToIndex.insert(_currentDisplayIndex, _childDisplayIndexToIndex.removeAt(_dragStartIndex));
     }
     int index = 0;
     _childDisplayIndexToIndex.forEach((int element) {
@@ -1202,40 +1136,33 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
     for (int i = 0; i < widget.children.length; i++) {
       wrappedChildren.add(_wrap(widget.children[i], i));
     }
-    if (_dragStartIndex >= 0 &&
-        _currentDisplayIndex >= 0 &&
-        _dragStartIndex != _currentDisplayIndex) {
+    if (_dragStartIndex >= 0 && _currentDisplayIndex >= 0 && _dragStartIndex != _currentDisplayIndex) {
       //we are dragging an widget
-      wrappedChildren.insert(
-          _currentDisplayIndex, wrappedChildren.removeAt(_dragStartIndex));
+      wrappedChildren.insert(_currentDisplayIndex, wrappedChildren.removeAt(_dragStartIndex));
     }
     if (widget.header != null) {
-      wrappedChildren.insertAll(0, widget.header);
+      wrappedChildren.insertAll(0, widget.header!);
     }
     if (widget.footer != null) {
-      wrappedChildren.add(widget.footer);
+      wrappedChildren.add(widget.footer!);
     }
 
-
     if (widget.controller != null) {
       return Padding(
-        child: (widget.buildItemsContainer ?? defaultBuildItemsContainer)(
-            context, widget.direction, wrappedChildren),
+        child: (widget.buildItemsContainer ?? defaultBuildItemsContainer)(context, widget.direction, wrappedChildren),
         padding: widget.padding,
       );
     }
     return SingleChildScrollView(
 //      key: _contentKey,
       scrollDirection: widget.scrollDirection,
-      child: (widget.buildItemsContainer ?? defaultBuildItemsContainer)(
-          context, widget.direction, wrappedChildren),
+      child: (widget.buildItemsContainer ?? defaultBuildItemsContainer)(context, widget.direction, wrappedChildren),
       padding: widget.padding,
       controller: _scrollController,
     );
   }
 
-  Widget defaultBuildItemsContainer(
-      BuildContext context, Axis direction, List<Widget> children) {
+  Widget defaultBuildItemsContainer(BuildContext context, Axis direction, List<Widget> children) {
     return WrapWithMainAxisCount(
       key: _wrapKey,
       direction: direction,
@@ -1252,14 +1179,12 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
     );
   }
 
-  Widget defaultBuildDraggableFeedback(
-      BuildContext context, BoxConstraints constraints, Widget child) {
+  Widget defaultBuildDraggableFeedback(BuildContext context, BoxConstraints constraints, Widget child) {
     return Transform(
       transform: new Matrix4.rotationZ(0),
       alignment: FractionalOffset.topLeft,
       child: Material(
-        child:
-            Card(child: ConstrainedBox(constraints: constraints, child: child)),
+        child: Card(child: ConstrainedBox(constraints: constraints, child: child)),
         elevation: 6.0,
         color: Colors.transparent,
         borderRadius: BorderRadius.zero,
diff --git a/lib/src/widgets/tabluar_flex.dart b/lib/src/widgets/tabluar_flex.dart
index b95eb90..3638b6a 100644
--- a/lib/src/widgets/tabluar_flex.dart
+++ b/lib/src/widgets/tabluar_flex.dart
@@ -5,16 +5,16 @@ import '../rendering/tabluar_flex.dart';
 
 class TabluarFlex extends Flex {
   TabluarFlex({
-    Key key,
-    @required Axis direction,
+    required Axis direction,
     MainAxisAlignment mainAxisAlignment = MainAxisAlignment.start,
     MainAxisSize mainAxisSize = MainAxisSize.max,
     CrossAxisAlignment crossAxisAlignment = CrossAxisAlignment.center,
-    TextDirection textDirection,
+    TextDirection? textDirection,
     VerticalDirection verticalDirection = VerticalDirection.down,
-    TextBaseline textBaseline,
+    TextBaseline? textBaseline,
     List<Widget> children = const <Widget>[],
     this.decoration,
+    Key? key,
   }) : super(
           key: key,
           children: children,
@@ -30,7 +30,7 @@ class TabluarFlex extends Flex {
   /// What decoration to paint.
   ///
   /// Commonly a [BoxDecoration].
-  final Decoration decoration;
+  final Decoration? decoration;
 
   @override
   RenderTabluarFlex createRenderObject(BuildContext context) {
@@ -231,15 +231,15 @@ class TabluarRow extends TabluarFlex {
   /// `start` or `end` values for the [mainAxisAlignment], the [textDirection]
   /// must not be null.
   TabluarRow({
-    Key key,
     MainAxisAlignment mainAxisAlignment = MainAxisAlignment.start,
     MainAxisSize mainAxisSize = MainAxisSize.max,
     CrossAxisAlignment crossAxisAlignment = CrossAxisAlignment.center,
-    TextDirection textDirection,
+    TextDirection? textDirection,
     VerticalDirection verticalDirection = VerticalDirection.down,
-    TextBaseline textBaseline,
+    TextBaseline? textBaseline,
     List<Widget> children = const <Widget>[],
-    Decoration decoration,
+    Decoration? decoration,
+    Key? key,
   }) : super(
           children: children,
           key: key,
@@ -429,15 +429,15 @@ class TabluarColumn extends TabluarFlex {
   /// to be necessary to disambiguate `start` or `end` values for the
   /// [crossAxisAlignment], the [textDirection] must not be null.
   TabluarColumn({
-    Key key,
     MainAxisAlignment mainAxisAlignment = MainAxisAlignment.start,
     MainAxisSize mainAxisSize = MainAxisSize.max,
     CrossAxisAlignment crossAxisAlignment = CrossAxisAlignment.center,
-    TextDirection textDirection,
     VerticalDirection verticalDirection = VerticalDirection.down,
-    TextBaseline textBaseline,
     List<Widget> children = const <Widget>[],
-    Decoration decoration,
+    TextDirection? textDirection,
+    TextBaseline? textBaseline,
+    Decoration? decoration,
+    Key? key,
   }) : super(
           children: children,
           key: key,
diff --git a/lib/src/widgets/transitions.dart b/lib/src/widgets/transitions.dart
index da5167b..89b486a 100644
--- a/lib/src/widgets/transitions.dart
+++ b/lib/src/widgets/transitions.dart
@@ -11,11 +11,11 @@ class SizeTransitionWithIntrinsicSize extends SingleChildRenderObjectWidget {
   /// defaults to 0.0, which centers the child along the main axis during the
   /// transition.
   SizeTransitionWithIntrinsicSize({
-    Key key,
     this.axis = Axis.vertical,
-    @required this.sizeFactor,
+    required this.sizeFactor,
     double axisAlignment = 0.0,
-    Widget child,
+    Widget? child,
+    Key? key,
   }) : super(
             key: key,
             child: SizeTransition(
diff --git a/lib/src/widgets/wrap.dart b/lib/src/widgets/wrap.dart
index e481e36..15c5dfd 100644
--- a/lib/src/widgets/wrap.dart
+++ b/lib/src/widgets/wrap.dart
@@ -5,14 +5,14 @@ import '../rendering/wrap.dart';
 
 class WrapWithMainAxisCount extends Wrap {
   WrapWithMainAxisCount({
-    Key key,
+    Key? key,
     Axis direction = Axis.horizontal,
     WrapAlignment alignment = WrapAlignment.start,
     double spacing = 0.0,
     WrapAlignment runAlignment = WrapAlignment.start,
     double runSpacing = 0.0,
     WrapCrossAlignment crossAxisAlignment = WrapCrossAlignment.start,
-    TextDirection textDirection,
+    TextDirection? textDirection,
     VerticalDirection verticalDirection = VerticalDirection.down,
     List<Widget> children = const <Widget>[],
     this.minMainAxisCount,
@@ -29,8 +29,8 @@ class WrapWithMainAxisCount extends Wrap {
             verticalDirection: verticalDirection,
             children: children);
 
-  final int minMainAxisCount;
-  final int maxMainAxisCount;
+  final int? minMainAxisCount;
+  final int? maxMainAxisCount;
 
   @override
   RenderWrapWithMainAxisCount createRenderObject(BuildContext context) {
diff --git a/pubspec.yaml b/pubspec.yaml
index 2a651d8..93c33ae 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,10 +1,10 @@
 name: reorderables
 description: Reorderable table, row, column, wrap, sliver list that allow drag and drop of their children.
-version: 0.3.2
+version: 0.4.1
 homepage: https://github.com/hanshengchiu/reorderables
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 #  flutter: ">=1.6.3"
 
 dependencies:

From f6f704c6f4fd609dea19327403e210bdce3f4293 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stefan=20P=C3=BChringer?= <stefan@insight.co>
Date: Tue, 12 Jul 2022 10:30:17 +0200
Subject: [PATCH 2/2] Made haptic feedback customizable

---
 lib/src/widgets/reorderable_flex.dart |  40 ++++-
 lib/src/widgets/reorderable_wrap.dart | 205 ++++++++++++++++++--------
 pubspec.yaml                          |   2 +-
 3 files changed, 180 insertions(+), 67 deletions(-)

diff --git a/lib/src/widgets/reorderable_flex.dart b/lib/src/widgets/reorderable_flex.dart
index da3d245..1e7bf84 100644
--- a/lib/src/widgets/reorderable_flex.dart
+++ b/lib/src/widgets/reorderable_flex.dart
@@ -56,6 +56,9 @@ class ReorderableFlex extends StatefulWidget {
     this.reorderAnimationDuration,
     this.scrollAnimationDuration,
     this.ignorePrimaryScrollController = false,
+    this.hapticFeedbackOnStart = true,
+    this.onReorderGhostStep,
+    this.onReorderStarted,
   })  : assert(
           children.every((Widget w) => w.key != null),
           'All children of this widget must have a key.',
@@ -89,6 +92,8 @@ class ReorderableFlex extends StatefulWidget {
   /// children.
   final ReorderCallback onReorder;
   final NoReorderCallback? onNoReorder;
+  final ReorderCallback? onReorderGhostStep;
+  final ReorderStartedCallback? onReorderStarted;
 
   final BuildItemsContainer? buildItemsContainer;
   final BuildDraggableFeedback? buildDraggableFeedback;
@@ -102,6 +107,8 @@ class ReorderableFlex extends StatefulWidget {
   final Duration? scrollAnimationDuration;
   final bool ignorePrimaryScrollController;
 
+  final bool hapticFeedbackOnStart;
+
   @override
   _ReorderableFlexState createState() => _ReorderableFlexState();
 }
@@ -148,6 +155,9 @@ class _ReorderableFlexState extends State<ReorderableFlex> {
               const Duration(milliseconds: 200),
           scrollAnimationDuration: widget.scrollAnimationDuration ??
               const Duration(milliseconds: 200),
+          onReorderGhostStep: widget.onReorderGhostStep,
+          hapticFeedbackOnStart: widget.hapticFeedbackOnStart,
+          onReorderStarted: widget.onReorderStarted,
         );
       },
     );
@@ -186,6 +196,9 @@ class _ReorderableFlexContent extends StatefulWidget {
     required this.padding,
     this.reorderAnimationDuration = const Duration(milliseconds: 200),
     this.scrollAnimationDuration = const Duration(milliseconds: 200),
+    this.onReorderGhostStep,
+    this.hapticFeedbackOnStart = true,
+    this.onReorderStarted,
   });
 
   final Widget? header;
@@ -205,6 +218,9 @@ class _ReorderableFlexContent extends StatefulWidget {
   final double draggingWidgetOpacity;
   final Duration reorderAnimationDuration;
   final Duration scrollAnimationDuration;
+  final bool hapticFeedbackOnStart;
+  final ReorderCallback? onReorderGhostStep;
+  final ReorderStartedCallback? onReorderStarted;
 
   @override
   _ReorderableFlexContentState createState() => _ReorderableFlexContentState();
@@ -344,6 +360,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
         return;
       }
 
+      widget.onReorderGhostStep?.call(_currentIndex, _nextIndex);
       _currentIndex = _nextIndex;
       _ghostController.reverse(from: 1.0);
       _entranceController.forward(from: 0.0);
@@ -439,6 +456,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
         _currentIndex = index;
         _entranceController.value = 1.0;
         _draggingFeedbackSize = keyIndexGlobalKey.currentContext?.size;
+        widget.onReorderStarted?.call(index);
       });
     }
 
@@ -515,8 +533,9 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
         }
         semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] =
             moveAfter;
-        semanticsActions[CustomSemanticsAction(
-            label: localizations.reorderItemToEnd)] = moveToEnd;
+        semanticsActions[
+                CustomSemanticsAction(label: localizations.reorderItemToEnd)] =
+            moveToEnd;
       }
 
       // We pass toWrap with a GlobalKey into the Draggable so that when a list
@@ -634,6 +653,7 @@ class _ReorderableFlexContentState extends State<_ReorderableFlexContent>
                 // had been dragged to.
                 onDraggableCanceled: (Velocity velocity, Offset offset) =>
                     onDragEnded(),
+                hapticFeedbackOnStart: widget.hapticFeedbackOnStart,
               )
             : Draggable<Key>(
                 maxSimultaneousDrags: 1,
@@ -974,6 +994,9 @@ class ReorderableRow extends ReorderableFlex {
     Duration? reorderAnimationDuration,
     Duration? scrollAnimationDuration,
     bool ignorePrimaryScrollController = false,
+    bool hapticFeedbackOnStart = true,
+    ReorderCallback? onReorderGhostStep,
+    ReorderStartedCallback? onReorderStarted,
   }) : super(
             key: key,
             header: header,
@@ -1003,7 +1026,10 @@ class ReorderableRow extends ReorderableFlex {
             draggingWidgetOpacity: draggingWidgetOpacity,
             reorderAnimationDuration: reorderAnimationDuration,
             scrollAnimationDuration: scrollAnimationDuration,
-            ignorePrimaryScrollController: ignorePrimaryScrollController);
+            ignorePrimaryScrollController: ignorePrimaryScrollController,
+            hapticFeedbackOnStart: hapticFeedbackOnStart,
+            onReorderGhostStep: onReorderGhostStep,
+            onReorderStarted: onReorderStarted);
 }
 
 /// Reorderable (drag and drop) version of [Column], a widget that displays its
@@ -1053,6 +1079,9 @@ class ReorderableColumn extends ReorderableFlex {
     Duration? reorderAnimationDuration,
     Duration? scrollAnimationDuration,
     bool ignorePrimaryScrollController = false,
+    bool hapticFeedbackOnStart = true,
+    ReorderCallback? onReorderGhostStep,
+    ReorderStartedCallback? onReorderStarted,
   }) : super(
             key: key,
             header: header,
@@ -1081,5 +1110,8 @@ class ReorderableColumn extends ReorderableFlex {
             draggingWidgetOpacity: draggingWidgetOpacity,
             reorderAnimationDuration: reorderAnimationDuration,
             scrollAnimationDuration: scrollAnimationDuration,
-            ignorePrimaryScrollController: ignorePrimaryScrollController);
+            ignorePrimaryScrollController: ignorePrimaryScrollController,
+            hapticFeedbackOnStart: hapticFeedbackOnStart,
+            onReorderGhostStep: onReorderGhostStep,
+            onReorderStarted: onReorderStarted);
 }
diff --git a/lib/src/widgets/reorderable_wrap.dart b/lib/src/widgets/reorderable_wrap.dart
index c7dbb2d..43db99a 100644
--- a/lib/src/widgets/reorderable_wrap.dart
+++ b/lib/src/widgets/reorderable_wrap.dart
@@ -60,6 +60,8 @@ class ReorderableWrap extends StatefulWidget {
     this.scrollAnimationDuration = const Duration(milliseconds: 200),
     this.ignorePrimaryScrollController = false,
     this.enabledDragging = true,
+    this.hapticFeedbackOnStart = true,
+    this.onReorderGhostStep,
     Key? key,
   }) :
 //        assert(
@@ -98,6 +100,7 @@ class ReorderableWrap extends StatefulWidget {
   /// children.
   final ReorderCallback onReorder;
   final NoReorderCallback? onNoReorder;
+  final ReorderCallback? onReorderGhostStep;
 
   /// Called when the draggable starts being dragged.
   final ReorderStartedCallback? onReorderStarted;
@@ -241,6 +244,8 @@ class ReorderableWrap extends StatefulWidget {
 
   final bool enabledDragging;
 
+  final bool hapticFeedbackOnStart;
+
   @override
   _ReorderableWrapState createState() => _ReorderableWrapState();
 }
@@ -256,7 +261,8 @@ class ReorderableWrap extends StatefulWidget {
 // insert Draggables into the Overlay above itself.
 class _ReorderableWrapState extends State<ReorderableWrap> {
   // We use an inner overlay so that the dragging list item doesn't draw outside of the list itself.
-  final GlobalKey _overlayKey = GlobalKey(debugLabel: '$ReorderableWrap overlay key');
+  final GlobalKey _overlayKey =
+      GlobalKey(debugLabel: '$ReorderableWrap overlay key');
 
   // This entry contains the scrolling list itself.
   late PassthroughOverlayEntry _listOverlayEntry;
@@ -293,6 +299,8 @@ class _ReorderableWrapState extends State<ReorderableWrap> {
           reorderAnimationDuration: widget.reorderAnimationDuration,
           scrollAnimationDuration: widget.scrollAnimationDuration,
           enableDragging: widget.enabledDragging,
+          hapticFeedbackOnStart: widget.hapticFeedbackOnStart,
+          onReorderGhostStep: widget.onReorderGhostStep,
         );
       },
     );
@@ -300,10 +308,11 @@ class _ReorderableWrapState extends State<ReorderableWrap> {
 
   @override
   Widget build(BuildContext context) {
-    final PassthroughOverlay passthroughOverlay =
-        PassthroughOverlay(key: _overlayKey, initialEntries: <PassthroughOverlayEntry>[
-      _listOverlayEntry,
-    ]);
+    final PassthroughOverlay passthroughOverlay = PassthroughOverlay(
+        key: _overlayKey,
+        initialEntries: <PassthroughOverlayEntry>[
+          _listOverlayEntry,
+        ]);
     return widget.ignorePrimaryScrollController
         ? PrimaryScrollController.none(child: passthroughOverlay)
         : passthroughOverlay;
@@ -339,6 +348,8 @@ class _ReorderableWrapContent extends StatefulWidget {
     this.reorderAnimationDuration = const Duration(milliseconds: 200),
     this.scrollAnimationDuration = const Duration(milliseconds: 200),
     this.enableDragging = true,
+    this.hapticFeedbackOnStart = true,
+    this.onReorderGhostStep,
   });
 
   final List<Widget>? header;
@@ -367,6 +378,8 @@ class _ReorderableWrapContent extends StatefulWidget {
   final Duration reorderAnimationDuration;
   final Duration scrollAnimationDuration;
   final bool enableDragging;
+  final bool hapticFeedbackOnStart;
+  final ReorderCallback? onReorderGhostStep;
 
   @override
   _ReorderableWrapContentState createState() => _ReorderableWrapContentState();
@@ -461,8 +474,10 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
     super.initState();
     _reorderAnimationDuration = widget.reorderAnimationDuration;
     _scrollAnimationDuration = widget.scrollAnimationDuration;
-    _entranceController = AnimationController(value: 1.0, vsync: this, duration: _reorderAnimationDuration);
-    _ghostController = AnimationController(value: 0, vsync: this, duration: _reorderAnimationDuration);
+    _entranceController = AnimationController(
+        value: 1.0, vsync: this, duration: _reorderAnimationDuration);
+    _ghostController = AnimationController(
+        value: 0, vsync: this, duration: _reorderAnimationDuration);
     _entranceController.addStatusListener(_onEntranceStatusChanged);
 //    _childKeys = List.filled(widget.children.length, null);
     _childContexts = List.filled(widget.children.length, null);
@@ -479,7 +494,9 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 
   @override
   void didChangeDependencies() {
-    _scrollController = widget.controller ?? PrimaryScrollController.of(context) ?? ScrollController();
+    _scrollController = widget.controller ??
+        PrimaryScrollController.of(context) ??
+        ScrollController();
     super.didChangeDependencies();
   }
 
@@ -499,7 +516,7 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
         // && _dragStartIndex == _ghostIndex
         return;
       }
-
+      widget.onReorderGhostStep?.call(_currentDisplayIndex, _nextDisplayIndex);
       _currentDisplayIndex = _nextDisplayIndex;
       _ghostController.reverse(from: 1.0);
       _entranceController.forward(from: 0.0);
@@ -519,11 +536,14 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
   void _scrollTo(BuildContext context) {
     if (_scrolling) return;
     final RenderObject contextObject = context.findRenderObject()!;
-    final RenderAbstractViewport viewport = RenderAbstractViewport.of(contextObject)!;
+    final RenderAbstractViewport viewport =
+        RenderAbstractViewport.of(contextObject)!;
     // If and only if the current scroll offset falls in-between the offsets
     // necessary to reveal the selected context at the top or bottom of the
     // screen, then it is already on-screen.
-    final double margin = widget.direction == Axis.horizontal ? _dropAreaSize.width : _dropAreaSize.height;
+    final double margin = widget.direction == Axis.horizontal
+        ? _dropAreaSize.width
+        : _dropAreaSize.height;
     final double scrollOffset = _scrollController.offset;
     final double topOffset = max(
       _scrollController.position.minScrollExtent,
@@ -533,7 +553,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
       _scrollController.position.maxScrollExtent,
       viewport.getOffsetToReveal(contextObject, 1.0).offset + margin,
     );
-    final bool onScreen = scrollOffset <= topOffset && scrollOffset >= bottomOffset;
+    final bool onScreen =
+        scrollOffset <= topOffset && scrollOffset >= bottomOffset;
     // If the context is off screen, then we request a scroll to make it visible.
     if (!onScreen) {
       _scrolling = true;
@@ -632,19 +653,22 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 
         if (_wrapKey.currentContext != null) {
           RenderWrapWithMainAxisCount wrapRenderObject =
-              _wrapKey.currentContext!.findRenderObject() as RenderWrapWithMainAxisCount;
+              _wrapKey.currentContext!.findRenderObject()
+                  as RenderWrapWithMainAxisCount;
           _wrapChildRunIndexes = wrapRenderObject.childRunIndexes;
           for (int i = 0; i < _childRunIndexes.length; i++) {
-            _nextChildRunIndexes[i] = _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
+            _nextChildRunIndexes[i] =
+                _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
           }
         } else {
           if (widget.minMainAxisCount != null &&
               widget.maxMainAxisCount != null &&
               widget.minMainAxisCount == widget.maxMainAxisCount) {
-            _wrapChildRunIndexes =
-                List.generate(widget.children.length, (int index) => index ~/ widget.minMainAxisCount!);
+            _wrapChildRunIndexes = List.generate(widget.children.length,
+                (int index) => index ~/ widget.minMainAxisCount!);
             for (int i = 0; i < _childRunIndexes.length; i++) {
-              _nextChildRunIndexes[i] = _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
+              _nextChildRunIndexes[i] =
+                  _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
             }
           }
         }
@@ -692,7 +716,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 
     Widget wrapWithSemantics() {
       // First, determine which semantics actions apply.
-      final Map<CustomSemanticsAction, VoidCallback> semanticsActions = <CustomSemanticsAction, VoidCallback>{};
+      final Map<CustomSemanticsAction, VoidCallback> semanticsActions =
+          <CustomSemanticsAction, VoidCallback>{};
 
       // Create the appropriate semantics actions.
       void moveToStart() => reorder(index, 0);
@@ -702,17 +727,20 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
       // before index+2, which is after the space at index+1.
       void moveAfter() => reorder(index, index + 2);
 
-      final MaterialLocalizations localizations = MaterialLocalizations.of(context);
+      final MaterialLocalizations localizations =
+          MaterialLocalizations.of(context);
 
       if (index > 0) {
-        semanticsActions[CustomSemanticsAction(label: localizations.reorderItemToStart)] = moveToStart;
+        semanticsActions[CustomSemanticsAction(
+            label: localizations.reorderItemToStart)] = moveToStart;
         String reorderItemBefore = localizations.reorderItemUp;
         if (widget.direction == Axis.horizontal) {
           reorderItemBefore = Directionality.of(context) == TextDirection.ltr
               ? localizations.reorderItemLeft
               : localizations.reorderItemRight;
         }
-        semanticsActions[CustomSemanticsAction(label: reorderItemBefore)] = moveBefore;
+        semanticsActions[CustomSemanticsAction(label: reorderItemBefore)] =
+            moveBefore;
       }
 
       // If the item can move to after its current position in the list.
@@ -723,8 +751,11 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
               ? localizations.reorderItemRight
               : localizations.reorderItemLeft;
         }
-        semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] = moveAfter;
-        semanticsActions[CustomSemanticsAction(label: localizations.reorderItemToEnd)] = moveToEnd;
+        semanticsActions[CustomSemanticsAction(label: reorderItemAfter)] =
+            moveAfter;
+        semanticsActions[
+                CustomSemanticsAction(label: localizations.reorderItemToEnd)] =
+            moveToEnd;
       }
 
       // We pass toWrap with a GlobalKey into the Draggable so that when a list
@@ -775,7 +806,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
       Widget feedbackBuilder = Builder(builder: (BuildContext context) {
 //          RenderRepaintBoundary renderObject = _contentKey.currentContext.findRenderObject();
 //          BoxConstraints contentSizeConstraints = BoxConstraints.loose(renderObject.size);
-        BoxConstraints contentSizeConstraints = BoxConstraints.loose(_draggingFeedbackSize!); //renderObject.constraints
+        BoxConstraints contentSizeConstraints = BoxConstraints.loose(
+            _draggingFeedbackSize!); //renderObject.constraints
 //          debugPrint('feedbackBuilder: contentConstraints:$contentSizeConstraints');
         return (widget.buildDraggableFeedback ?? defaultBuildDraggableFeedback)(
             context, contentSizeConstraints, toWrap);
@@ -801,7 +833,9 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
                 feedback: feedbackBuilder,
                 // Wrap toWrapWithSemantics with a widget that supports HitTestBehavior
                 // to make sure the whole toWrapWithSemantics responds to pointer events, i.e. dragging
-                child: MetaData(child: toWrapWithSemantics, behavior: HitTestBehavior.opaque),
+                child: MetaData(
+                    child: toWrapWithSemantics,
+                    behavior: HitTestBehavior.opaque),
                 //toWrapWithSemantics,//_dragging == toWrap.key ? const SizedBox() : toWrapWithSemantics,
                 childWhenDragging: IgnorePointer(
                     ignoring: true,
@@ -818,7 +852,9 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
                 // When the drag does not end inside a DragTarget widget, the
                 // drag fails, but we still reorder the widget to the last position it
                 // had been dragged to.
-                onDraggableCanceled: (Velocity velocity, Offset offset) => onDragEnded(),
+                onDraggableCanceled: (Velocity velocity, Offset offset) =>
+                    onDragEnded(),
+                hapticFeedbackOnStart: widget.hapticFeedbackOnStart,
               )
             : Draggable<int>(
                 maxSimultaneousDrags: widget.enableDragging ? 1 : 0,
@@ -826,7 +862,9 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
                 //toWrap.key,
                 ignoringFeedbackSemantics: false,
                 feedback: feedbackBuilder,
-                child: MetaData(child: toWrapWithSemantics, behavior: HitTestBehavior.opaque),
+                child: MetaData(
+                    child: toWrapWithSemantics,
+                    behavior: HitTestBehavior.opaque),
                 childWhenDragging: IgnorePointer(
                   ignoring: true,
                   child: Opacity(
@@ -837,7 +875,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
                 dragAnchor: DragAnchor.child,
                 onDragStarted: onDragStarted,
                 onDragCompleted: onDragEnded,
-                onDraggableCanceled: (Velocity velocity, Offset offset) => onDragEnded(),
+                onDraggableCanceled: (Velocity velocity, Offset offset) =>
+                    onDragEnded(),
               );
       }
 
@@ -865,27 +904,33 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 //      debugPrint('index:$index displayIndex:$displayIndex _nextDisplayIndex:$_nextDisplayIndex _currentDisplayIndex:$_currentDisplayIndex _ghostDisplayIndex:$_ghostDisplayIndex _dragStartIndex:$_dragStartIndex');
 //      debugPrint(' _childRunIndexes:$_childRunIndexes _nextChildRunIndexes:$_nextChildRunIndexes _wrapChildRunIndexes:$_wrapChildRunIndexes');
 
-      List<Widget> _includeMovedAdjacentChildIfNeeded(Widget child, int childDisplayIndex) {
+      List<Widget> _includeMovedAdjacentChildIfNeeded(
+          Widget child, int childDisplayIndex) {
 //        debugPrint(' checking ${_childDisplayIndexToIndex[childDisplayIndex]}($childDisplayIndex)');
         int checkingTargetDisplayIndex = -1;
-        if (_ghostDisplayIndex < _currentDisplayIndex && childDisplayIndex > _ghostDisplayIndex) {
+        if (_ghostDisplayIndex < _currentDisplayIndex &&
+            childDisplayIndex > _ghostDisplayIndex) {
           checkingTargetDisplayIndex = childDisplayIndex - 1;
-        } else if (_ghostDisplayIndex > _currentDisplayIndex && childDisplayIndex < _ghostDisplayIndex) {
+        } else if (_ghostDisplayIndex > _currentDisplayIndex &&
+            childDisplayIndex < _ghostDisplayIndex) {
           checkingTargetDisplayIndex = childDisplayIndex + 1;
         }
         if (checkingTargetDisplayIndex == -1) {
           return [child];
         }
-        int checkingTargetIndex = _childDisplayIndexToIndex[checkingTargetDisplayIndex];
+        int checkingTargetIndex =
+            _childDisplayIndexToIndex[checkingTargetDisplayIndex];
         if (checkingTargetIndex == _dragStartIndex) {
           return [child];
         }
         if (_childRunIndexes[checkingTargetIndex] == -1 ||
-            _childRunIndexes[checkingTargetIndex] == _wrapChildRunIndexes[checkingTargetDisplayIndex]) {
+            _childRunIndexes[checkingTargetIndex] ==
+                _wrapChildRunIndexes[checkingTargetDisplayIndex]) {
           return [child];
         }
 //        debugPrint(' make $checkingTargetIndex($checkingTargetDisplayIndex) disappearing around $index');
-        Widget disappearingPreChild = _makeDisappearingWidget(_wrapChildren[checkingTargetIndex]!);
+        Widget disappearingPreChild =
+            _makeDisappearingWidget(_wrapChildren[checkingTargetIndex]!);
 //        return _buildContainerForMainAxis(
 //          children: _ghostDisplayIndex < _currentDisplayIndex
 //            ? [disappearingPreChild, child]
@@ -902,7 +947,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
       if (_currentDisplayIndex == -1 || displayIndex == _currentDisplayIndex) {
         //we still wrap dragTarget with a container so that widget's depths are the same and it prevents layout alignment issue
         return _buildContainerForMainAxis(
-            children: _includeMovedAdjacentChildIfNeeded(containedDraggable, displayIndex));
+            children: _includeMovedAdjacentChildIfNeeded(
+                containedDraggable, displayIndex));
       }
 
       bool _onWillAccept(int? toAccept, bool isPre) {
@@ -914,7 +960,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
         }
 
         bool movingToAdjacentChild =
-            nextDisplayIndex <= _currentDisplayIndex + 1 && nextDisplayIndex >= _currentDisplayIndex - 1;
+            nextDisplayIndex <= _currentDisplayIndex + 1 &&
+                nextDisplayIndex >= _currentDisplayIndex - 1;
         bool willAccept = _dragStartIndex == toAccept &&
 //          toAccept != toWrap.key &&
             toAccept != index &&
@@ -926,13 +973,15 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
         if (!willAccept) {
           return false;
         }
-        if (!(_childDisplayIndexToIndex[_currentDisplayIndex] != index && _currentDisplayIndex != displayIndex)) {
+        if (!(_childDisplayIndexToIndex[_currentDisplayIndex] != index &&
+            _currentDisplayIndex != displayIndex)) {
           return false;
         }
 
         if (_wrapKey.currentContext != null) {
           RenderWrapWithMainAxisCount wrapRenderObject =
-              _wrapKey.currentContext!.findRenderObject() as RenderWrapWithMainAxisCount;
+              _wrapKey.currentContext!.findRenderObject()
+                  as RenderWrapWithMainAxisCount;
           _wrapChildRunIndexes = wrapRenderObject.childRunIndexes;
 //          for (int i=0; i<_childRunIndexes.length; i++) {
 //            _childRunIndexes[i] = _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
@@ -941,8 +990,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
           if (widget.minMainAxisCount != null &&
               widget.maxMainAxisCount != null &&
               widget.minMainAxisCount == widget.maxMainAxisCount) {
-            _wrapChildRunIndexes =
-                List.generate(widget.children.length, (int index) => index ~/ widget.minMainAxisCount!);
+            _wrapChildRunIndexes = List.generate(widget.children.length,
+                (int index) => index ~/ widget.minMainAxisCount!);
 //            for (int i=0; i<_childRunIndexes.length; i++) {
 //              _childRunIndexes[i] = _wrapChildRunIndexes[_childIndexToDisplayIndex[i]];
 //            }
@@ -960,13 +1009,17 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
       }
 
       Widget preDragTarget = DragTarget<int>(
-        builder: (BuildContext context, List<int?> acceptedCandidates, List<dynamic> rejectedCandidates) => SizedBox(),
+        builder: (BuildContext context, List<int?> acceptedCandidates,
+                List<dynamic> rejectedCandidates) =>
+            SizedBox(),
         onWillAccept: (int? toAccept) => _onWillAccept(toAccept, true),
         onAccept: (int accepted) {},
         onLeave: (Object? leaving) {},
       );
       Widget nextDragTarget = DragTarget<int>(
-        builder: (BuildContext context, List<int?> acceptedCandidates, List<dynamic> rejectedCandidates) => SizedBox(),
+        builder: (BuildContext context, List<int?> acceptedCandidates,
+                List<dynamic> rejectedCandidates) =>
+            SizedBox(),
         onWillAccept: (int? toAccept) => _onWillAccept(toAccept, false),
         onAccept: (int accepted) {},
         onLeave: (Object? leaving) {},
@@ -981,14 +1034,22 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
           Positioned(
               left: 0,
               top: 0,
-              width: widget.direction == Axis.horizontal ? _childSizes[index].width / 2 : _childSizes[index].width,
-              height: widget.direction == Axis.vertical ? _childSizes[index].height / 2 : _childSizes[index].height,
+              width: widget.direction == Axis.horizontal
+                  ? _childSizes[index].width / 2
+                  : _childSizes[index].width,
+              height: widget.direction == Axis.vertical
+                  ? _childSizes[index].height / 2
+                  : _childSizes[index].height,
               child: preDragTarget),
           Positioned(
               right: 0,
               bottom: 0,
-              width: widget.direction == Axis.horizontal ? _childSizes[index].width / 2 : _childSizes[index].width,
-              height: widget.direction == Axis.vertical ? _childSizes[index].height / 2 : _childSizes[index].height,
+              width: widget.direction == Axis.horizontal
+                  ? _childSizes[index].width / 2
+                  : _childSizes[index].width,
+              height: widget.direction == Axis.vertical
+                  ? _childSizes[index].height / 2
+                  : _childSizes[index].height,
               child: nextDragTarget),
         ],
       );
@@ -1039,7 +1100,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
           : Opacity(opacity: 0.2, child: _draggingWidget);
 //      Widget spacing = SizedBox.fromSize(size: _dropAreaSize, child: Container(color: Colors.red));
 
-      if (_childRunIndexes[index] != -1 && _childRunIndexes[index] != _wrapChildRunIndexes[displayIndex]) {
+      if (_childRunIndexes[index] != -1 &&
+          _childRunIndexes[index] != _wrapChildRunIndexes[displayIndex]) {
         dragTarget = _makeAppearingWidget(dragTarget);
       }
 
@@ -1048,15 +1110,20 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
         if (_ghostDisplayIndex < _currentDisplayIndex) {
           //ghost is on the left of current, so shift it to the right
           return _buildContainerForMainAxis(
-              children: [ghostSpacing] + _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex));
+              children: [ghostSpacing] +
+                  _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex));
         } else if (_ghostDisplayIndex > _currentDisplayIndex) {
           return _buildContainerForMainAxis(
-              children: _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex) + [ghostSpacing]);
+              children:
+                  _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex) +
+                      [ghostSpacing]);
         }
       }
 
       //we still wrap dragTarget with a container so that widget's depths are the same and it prevent's layout alignment issue
-      return _buildContainerForMainAxis(children: _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex));
+      return _buildContainerForMainAxis(
+          children:
+              _includeMovedAdjacentChildIfNeeded(dragTarget, displayIndex));
 
 //      if (shiftedIndex == _currentDisplayIndex) {
 //        Widget entranceSpacing = SizeTransitionWithIntrinsicSize(
@@ -1102,7 +1169,8 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 //    _childSizes = List.filled(widget.children.length, Size(0, 0));
     List<E> _resizeListMember<E>(List<E> listVar, E initValue) {
       if (listVar.length < widget.children.length) {
-        return listVar + List.filled(widget.children.length - listVar.length, initValue);
+        return listVar +
+            List.filled(widget.children.length - listVar.length, initValue);
       } else if (listVar.length > widget.children.length) {
         return listVar.sublist(0, widget.children.length);
       }
@@ -1113,10 +1181,15 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
     _childContexts = _resizeListMember(_childContexts, null);
     _childSizes = _resizeListMember(_childSizes, Size(0, 0));
 
-    _childDisplayIndexToIndex = List.generate(widget.children.length, (int index) => index);
-    _childIndexToDisplayIndex = List.generate(widget.children.length, (int index) => index);
-    if (_dragStartIndex >= 0 && _currentDisplayIndex >= 0 && _dragStartIndex != _currentDisplayIndex) {
-      _childDisplayIndexToIndex.insert(_currentDisplayIndex, _childDisplayIndexToIndex.removeAt(_dragStartIndex));
+    _childDisplayIndexToIndex =
+        List.generate(widget.children.length, (int index) => index);
+    _childIndexToDisplayIndex =
+        List.generate(widget.children.length, (int index) => index);
+    if (_dragStartIndex >= 0 &&
+        _currentDisplayIndex >= 0 &&
+        _dragStartIndex != _currentDisplayIndex) {
+      _childDisplayIndexToIndex.insert(_currentDisplayIndex,
+          _childDisplayIndexToIndex.removeAt(_dragStartIndex));
     }
     int index = 0;
     _childDisplayIndexToIndex.forEach((int element) {
@@ -1136,9 +1209,12 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
     for (int i = 0; i < widget.children.length; i++) {
       wrappedChildren.add(_wrap(widget.children[i], i));
     }
-    if (_dragStartIndex >= 0 && _currentDisplayIndex >= 0 && _dragStartIndex != _currentDisplayIndex) {
+    if (_dragStartIndex >= 0 &&
+        _currentDisplayIndex >= 0 &&
+        _dragStartIndex != _currentDisplayIndex) {
       //we are dragging an widget
-      wrappedChildren.insert(_currentDisplayIndex, wrappedChildren.removeAt(_dragStartIndex));
+      wrappedChildren.insert(
+          _currentDisplayIndex, wrappedChildren.removeAt(_dragStartIndex));
     }
     if (widget.header != null) {
       wrappedChildren.insertAll(0, widget.header!);
@@ -1149,20 +1225,23 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
 
     if (widget.controller != null) {
       return Padding(
-        child: (widget.buildItemsContainer ?? defaultBuildItemsContainer)(context, widget.direction, wrappedChildren),
+        child: (widget.buildItemsContainer ?? defaultBuildItemsContainer)(
+            context, widget.direction, wrappedChildren),
         padding: widget.padding,
       );
     }
     return SingleChildScrollView(
 //      key: _contentKey,
       scrollDirection: widget.scrollDirection,
-      child: (widget.buildItemsContainer ?? defaultBuildItemsContainer)(context, widget.direction, wrappedChildren),
+      child: (widget.buildItemsContainer ?? defaultBuildItemsContainer)(
+          context, widget.direction, wrappedChildren),
       padding: widget.padding,
       controller: _scrollController,
     );
   }
 
-  Widget defaultBuildItemsContainer(BuildContext context, Axis direction, List<Widget> children) {
+  Widget defaultBuildItemsContainer(
+      BuildContext context, Axis direction, List<Widget> children) {
     return WrapWithMainAxisCount(
       key: _wrapKey,
       direction: direction,
@@ -1179,12 +1258,14 @@ class _ReorderableWrapContentState extends State<_ReorderableWrapContent>
     );
   }
 
-  Widget defaultBuildDraggableFeedback(BuildContext context, BoxConstraints constraints, Widget child) {
+  Widget defaultBuildDraggableFeedback(
+      BuildContext context, BoxConstraints constraints, Widget child) {
     return Transform(
       transform: new Matrix4.rotationZ(0),
       alignment: FractionalOffset.topLeft,
       child: Material(
-        child: Card(child: ConstrainedBox(constraints: constraints, child: child)),
+        child:
+            Card(child: ConstrainedBox(constraints: constraints, child: child)),
         elevation: 6.0,
         color: Colors.transparent,
         borderRadius: BorderRadius.zero,
diff --git a/pubspec.yaml b/pubspec.yaml
index 93c33ae..ee11223 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@ version: 0.4.1
 homepage: https://github.com/hanshengchiu/reorderables
 
 environment:
-  sdk: ">=2.12.0 <3.0.0"
+  sdk: ">=2.14.0 <3.0.0"
 #  flutter: ">=1.6.3"
 
 dependencies: