add snippets to demonstrate fcm registration tokens best practices (firebase#426)

Copying the snippets from firebase/quickstart-android#1453 to this repo so that they can be added to the docs.

Having these snippets on the other repo makes it harder to maintain the samples. Having them on this repo instead should be safer.

Author: thatfiredev

messaging/app/build.gradle

@@ -11,6 +11,7 @@ android {
         versionCode 1
         versionName "1.0"
         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+        multiDexEnabled true
     }
     buildTypes {
         release {
@@ -30,8 +31,14 @@ dependencies {
     // for Google Analytics. This is recommended, but not required.
     implementation 'com.google.firebase:firebase-analytics:21.2.0'
 
+    // Used to store FCM Registration Token.
+    // This is recommended, but not required.
+    // See: https://firebase.google.com/docs/cloud-messaging/manage-tokens
+    implementation 'com.google.firebase:firebase-firestore-ktx:24.4.3'
+
     implementation "com.google.android.gms:play-services-auth:20.4.1"
     implementation 'androidx.work:work-runtime-ktx:2.7.1'
+    implementation 'androidx.lifecycle:lifecycle-runtime-ktx:2.5.1'
 }
 
 apply plugin: 'com.google.gms.google-services'

messaging/app/src/main/java/com/google/firebase/example/messaging/kotlin/MainActivity.kt

@@ -1,6 +1,7 @@
 package com.google.firebase.example.messaging.kotlin
 
 import android.Manifest
+import android.content.Context
 import android.content.pm.PackageManager
 import android.os.Build
 import android.os.Bundle
@@ -10,11 +11,19 @@ import androidx.activity.result.contract.ActivityResultContracts
 import androidx.annotation.RequiresApi
 import androidx.appcompat.app.AppCompatActivity
 import androidx.core.content.ContextCompat
+import androidx.lifecycle.lifecycleScope
+import com.google.firebase.Timestamp
 import com.google.firebase.example.messaging.MainActivity
+import com.google.firebase.firestore.FieldValue
+import com.google.firebase.firestore.ktx.firestore
 import com.google.firebase.ktx.Firebase
 import com.google.firebase.messaging.ktx.messaging
 import com.google.firebase.messaging.ktx.remoteMessage
+import java.util.Calendar
+import java.util.Date
 import java.util.concurrent.atomic.AtomicInteger
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.tasks.await
 
 class MainActivity : AppCompatActivity() {
 
@@ -130,4 +139,20 @@ class MainActivity : AppCompatActivity() {
     }
     // [END ask_post_notifications]
 
+    // [START get_store_token]
+    private suspend fun getAndStoreRegToken(): String {
+        val token = Firebase.messaging.token.await()
+        // Add token and timestamp to Firestore for this user
+        val deviceToken = hashMapOf(
+            "token" to token,
+            "timestamp" to FieldValue.serverTimestamp(),
+        )
+
+        // Get user ID from Firebase Auth or your own server
+        Firebase.firestore.collection("fcmTokens").document("myuserid")
+            .set(deviceToken).await()
+        return token
+    }
+    // [END get_store_token]
+
 }

messaging/functions/index.js

@@ -0,0 +1,44 @@
+'use strict';
+
+const functions = require('firebase-functions');
+const admin = require('firebase-admin');
+
+admin.initializeApp();
+
+const EXPIRATION_TIME = 1000 * 60 * 60 * 24 * 30; // 30 days
+
+/**
+ * Scheduled function that runs once a month. It updates the last refresh date for
+ * tokens so that a client can refresh the token if the last time it did so was
+ * before the refresh date.
+ */
+// [START refresh_date_scheduled_function]
+exports.scheduledFunction = functions.pubsub.schedule('0 0 1 * *').onRun((context) => {
+    admin.firestore().doc('refresh/refreshDate').set({ lastRefreshDate : Date.now() });
+});
+// [END refresh_date_scheduled_function]
+
+/**
+ * Scheduled function that runs once a day. It retrieves all stale tokens then
+ * unsubscribes them from 'topic1' then deletes them.
+ *
+ * Note: weather is an example topic here. It is up to the developer to unsubscribe
+ * all topics the token is subscribed to.
+ */
+// [START remove_stale_tokens]
+exports.pruneTokens = functions.pubsub.schedule('every 24 hours').onRun(async (context) => {
+    const staleTokensResult = await admin.firestore().collection('fcmTokens')
+        .where("timestamp", "<", Date.now() - EXPIRATION_TIME)
+        .get();
+
+    const staleTokens = staleTokensResult.docs.map(staleTokenDoc => staleTokenDoc.id);
+
+    await admin.messaging().unsubscribeFromTopic(staleTokens, 'weather');
+
+    const deletePromises = [];
+    for (const staleTokenDoc of staleTokensResult.docs) {
+        deletePromises.push(staleTokenDoc.ref.delete());
+    }
+    await Promise.all(deletePromises);
+});
+// [END remove_stale_tokens]