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# 将操作发送到多个线程——与UI线程通信
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前一篇文章[将操作发送到多个线程——在线程池中的线程中运行代码](./将操作发送到多个线程——在线程池中的线程中运行代码.md)，向您展示如何在`ThreadPoolExecutor`管理的线程上启动任务。本文将向您展示如何将任务中的数据发送给在用户界面（`UI`）线程上运行的对象。这允许您的任务执行后台工作，然后将结果返回到`UI`元素（如`bitmap`）。
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每个应用程序都有自己的特殊线程来运行`UI`对象（例如`View`对象），这个线程称为`UI`线程。只有在`UI`线程上运行的对象才能访问该线程上的其他对象。由于您在线程池中的线程中运行的任务未在`UI`线程上运行，因此它们无权访问`UI`中的对象。要将数据从后台线程发送到`UI`线程，请使用在`UI`线程上运行的`Handler`。
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## 在UI线程定义一个Handler
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`Handler`是`Android`系统`Framework`层的一部分，其作用是管理线程。 `Handler`对象接收消息并运行代码来处理消息。通常，您为新线程创建一个`Handler`，但您也可以创建一个连接到现有线程的`Handler`。将`Handler`连接到`UI`线程时，处理消息的代码会在`UI`线程上运行。
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在构造创建线程池的类的过程中实例化`Handler`对象，并将对象存储在全局变量中。通过使用`Handler（Looper）`构造函数将其实例化，将其连接到`UI`线程。此构造函数使用`Looper`对象，这是`Android`系统的线程管理框架的另一部分。当您基于特定的`Looper`实例实例化`Handler`时，`Handler`在与`Looper`相同的线程上运行。例如：
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- kotlin
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- ```kotlin
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object PhotoManager {
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...
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private val handler: Handler = Handler(Looper.getMainLooper())
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...
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}
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```
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- java
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- ```java
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private PhotoManager() {
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...
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// Defines a Handler object that's attached to the UI thread
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handler = new Handler(Looper.getMainLooper()) {
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...
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```
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在`Handler`中，重写`handleMessage()`方法，`Android`系统会在接收到新`message`的时候调用`handleMessage()`方法，一个线程的所有`Handler`都会接收到相同的消息。比如：
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- kotlin
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- ```kotlin
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object PhotoManager {
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private val handler: Handler = object : Handler(Looper.getMainLooper()) {
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/*
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* handleMessage() defines the operations to perform when
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* the Handler receives a new Message to process.
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*/
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override fun handleMessage(inputMessage: Message) {
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// Gets the image task from the incoming Message object.
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val photoTask = inputMessage.obj as PhotoTask
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...
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}
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}
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...
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}
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```
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- java
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- ```java
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/*
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* handleMessage() defines the operations to perform when
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* the Handler receives a new Message to process.
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*/
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@Override
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public void handleMessage(Message inputMessage) {
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// Gets the image task from the incoming Message object.
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PhotoTask photoTask = (PhotoTask) inputMessage.obj;
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...
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}
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...
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}
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}
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```
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下一节将介绍如何通知`Handler`移动数据。
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## 从task中将数据移动到UI线程
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要把运行在后台线程的任务对象中的数据移动到`UI`线程上的对象中，请首先在数据和任务对象中存储`UI`对象。接下来，将任务对象和状态代码传递给实例化`Handler`的对象。在此对象中，将包含状态和任务对象的`Message`发送到处理程序。因为`Handler`在`UI`线程上运行，所以它可以将数据移动到UI对象。
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### 在任务对象中存储数据
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例如，这是一个在后台线程上运行的`Runnable`，它解码`Bitmap`并将其存储在其父对象`PhotoTask`中， `Runnable`还存储状态代码`DECODE_STATE_COMPLETED`。
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- kotlin
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- ```kotlin
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const val DECODE_STATE_COMPLETED: Int = ...
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// A class that decodes photo files into Bitmaps
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class PhotoDecodeRunnable(
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private val photoTask: PhotoTask,
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// Gets the downloaded byte array
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private var imageBuffer: ByteArray = photoTask.getByteBuffer()
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) : Runnable {
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...
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// Runs the code for this task
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override fun run() {
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...
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// Tries to decode the image buffer
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BitmapFactory.decodeByteArray(
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imageBuffer,
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0,
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imageBuffer.size,
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bitmapOptions
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)?.also { returnBitmap ->
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...
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// Sets the ImageView Bitmap
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photoTask.image = returnBitmap
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}
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// Reports a status of "completed"
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photoTask.handleDecodeState(DECODE_STATE_COMPLETED)
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...
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}
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...
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}
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```
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- java
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- ```java
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// A class that decodes photo files into Bitmaps
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class PhotoDecodeRunnable implements Runnable {
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...
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PhotoDecodeRunnable(PhotoTask downloadTask) {
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photoTask = downloadTask;
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}
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...
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// Gets the downloaded byte array
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byte[] imageBuffer = photoTask.getByteBuffer();
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...
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// Runs the code for this task
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public void run() {
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...
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// Tries to decode the image buffer
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returnBitmap = BitmapFactory.decodeByteArray(
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imageBuffer,
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0,
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imageBuffer.length,
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bitmapOptions
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);
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...
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// Sets the ImageView Bitmap
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photoTask.setImage(returnBitmap);
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// Reports a status of "completed"
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photoTask.handleDecodeState(DECODE_STATE_COMPLETED);
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...
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}
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...
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}
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...
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```
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`PhotoTask`还包含一个显示`Bitmap`的`ImageView`的句柄。即使`Bitmap`和`ImageView`的引用位于同一对象中，也无法将`Bitmap`分配给`ImageView`，因为您当前没有在`UI`线程上运行。
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所以，下一步是将此状态发送到`PhotoTask`对象。
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### 将状态发送到对象层
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`PhotoTask`是层次结构中的下一个更高级的对象。它维护对已解码数据的引用以及显示数据的`View`对象。它从`PhotoDecodeRunnable`接收状态代码并将其传递给维护线程池的对象并实例化`Handler`：
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- kotlin
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- ```kotlin
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// Gets a handle to the object that creates the thread pools
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class PhotoTask() {
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...
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private val photoManager: PhotoManager = PhotoManager.getInstance()
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...
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fun handleDecodeState(state: Int) {
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// Converts the decode state to the overall state.
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val outState: Int = when(state) {
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PhotoDecodeRunnable.DECODE_STATE_COMPLETED -> PhotoManager.TASK_COMPLETE
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...
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}
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...
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// Calls the generalized state method
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handleState(outState)
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}
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...
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// Passes the state to PhotoManager
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private fun handleState(state: Int) {
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/*
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* Passes a handle to this task and the
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* current state to the class that created
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* the thread pools
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*/
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PhotoManager.handleState(this, state)
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}
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...
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}
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```
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- java
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- ```java
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public class PhotoTask {
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...
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// Gets a handle to the object that creates the thread pools
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photoManager = PhotoManager.getInstance();
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...
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public void handleDecodeState(int state) {
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int outState;
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// Converts the decode state to the overall state.
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switch(state) {
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case PhotoDecodeRunnable.DECODE_STATE_COMPLETED:
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outState = PhotoManager.TASK_COMPLETE;
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break;
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...
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}
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...
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// Calls the generalized state method
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handleState(outState);
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}
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...
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// Passes the state to PhotoManager
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void handleState(int state) {
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/*
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* Passes a handle to this task and the
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* current state to the class that created
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* the thread pools
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*/
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photoManager.handleState(this, state);
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}
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...
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}
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```
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### 将数据移动到UI
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`PhotoManager`对象从`PhotoTask`对象接收状态代码和`PhotoTask`对象的句柄。由于状态为`TASK_COMPLETE`，因此创建一个包含状态和任务对象的`Message`，并将其发送给`Handler`：
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- kotlin
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- ```kotlin
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object PhotoManager {
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...
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// Handle status messages from tasks
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fun handleState(photoTask: PhotoTask, state: Int) {
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when(state) {
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...
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TASK_COMPLETE -> { // The task finished downloading and decoding the image
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/*
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* Creates a message for the Handler
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* with the state and the task object
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*/
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handler.obtainMessage(state, photoTask)?.apply {
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sendToTarget()
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}
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}
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...
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}
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...
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}
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```
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- java
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- ```java
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public class PhotoManager {
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...
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// Handle status messages from tasks
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public void handleState(PhotoTask photoTask, int state) {
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switch (state) {
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...
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// The task finished downloading and decoding the image
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case TASK_COMPLETE:
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/*
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* Creates a message for the Handler
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* with the state and the task object
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*/
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Message completeMessage =
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handler.obtainMessage(state, photoTask);
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completeMessage.sendToTarget();
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break;
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...
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}
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...
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}
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```
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最后，`Handler.handleMessage（）`检查每个传入消息的状态代码。如果状态代码为`TASK_COMPLETE`，则任务结束，`Message`中的`PhotoTask`对象包含`Bitmap`和`ImageView`。因为`Handler.handleMessage（）`在`UI`线程上运行，所以它可以安全地将`Bitmap`移动到`ImageView`：
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- kotlin
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- ```kotlin
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object PhotoManager {
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...
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private val handler: Handler = object : Handler(Looper.getMainLooper()) {
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override fun handleMessage(inputMessage: Message) {
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// Gets the image task from the incoming Message object.
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val photoTask = inputMessage.obj as PhotoTask
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// Gets the ImageView for this task
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val localView: PhotoView = photoTask.getPhotoView()
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...
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when (inputMessage.what) {
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...
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TASK_COMPLETE -> localView.setImageBitmap(photoTask.image)
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...
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else -> super.handleMessage(inputMessage)
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}
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...
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}
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...
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}
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...
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...
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}
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...
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}
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```
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- java
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- ```java
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private PhotoManager() {
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...
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handler = new Handler(Looper.getMainLooper()) {
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@Override
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public void handleMessage(Message inputMessage) {
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// Gets the task from the incoming Message object.
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PhotoTask photoTask = (PhotoTask) inputMessage.obj;
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// Gets the ImageView for this task
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PhotoView localView = photoTask.getPhotoView();
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...
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switch (inputMessage.what) {
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...
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// The decoding is done
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case TASK_COMPLETE:
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/*
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* Moves the Bitmap from the task
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* to the View
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*/
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localView.setImageBitmap(photoTask.getImage());
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break;
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...
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default:
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/*
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* Pass along other messages from the UI
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*/
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super.handleMessage(inputMessage);
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}
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...
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}
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...
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}
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...
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}
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...
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}
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```
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## 更多信息
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关于多线程的更多内容，请参阅[进程和线程——概述](…/BestPractices/Performance/进程和线程——概述.md)。

Diff for: ‎README.md

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@@ -68,7 +68,11 @@ Android Developer Guide翻译项目，主要翻译[Android Developer Guide官方
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- [Sending Operations To Multiple Threads(将操作发送到多个线程)](./BackGroundTasks/SendingOperationsToMultipleThreads)
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- [OverView(概述)](./BackGroundTasks/SendingOperationsToMultipleThreads/将操作发送到多个线程——概述.md)
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- [OverView(概述)](./BackGroundTasks/SendingOperationsToMultipleThreads/将操作发送到多个线程——（1）概述.md)
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- [Specify the code to run on a thread(指定线程中运行的代码)](./BackGroundTasks/SendingOperationsToMultipleThreads/将操作发送到多个线程——（2）指定线程中运行的代码.md)
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- [Create a manager for multiple threads(为多线程创建管理者)](./BackGroundTasks/SendingOperationsToMultipleThreads/将操作发送到多个线程——（3）为多线程创建管理者(manager).md)
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- [Run code on a thread pool thread(在线程池中运行代码)](./BackGroundTasks/SendingOperationsToMultipleThreads/将操作发送到多个线程——（4）在线程池中的线程中运行代码.md)
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- [Communicate with the UI thread(与UI线程进行通信)](./BackGroundTasks/SendingOperationsToMultipleThreads/将操作发送到多个线程——（5）与UI线程通信.md)
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