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，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）。
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。
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。
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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object PhotoManager { ... private val handler: Handler = Handler(Looper.getMainLooper()) ... }
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java
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private PhotoManager() { ... // Defines a Handler object that's attached to the UI thread handler = new Handler(Looper.getMainLooper()) { ...
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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object PhotoManager { private val handler: Handler = object : Handler(Looper.getMainLooper()) { /* * handleMessage() defines the operations to perform when * the Handler receives a new Message to process. */ override fun handleMessage(inputMessage: Message) { // Gets the image task from the incoming Message object. val photoTask = inputMessage.obj as PhotoTask ... } } ... }
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java
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/* * handleMessage() defines the operations to perform when * the Handler receives a new Message to process. */ @Override public void handleMessage(Message inputMessage) { // Gets the image task from the incoming Message object. PhotoTask photoTask = (PhotoTask) inputMessage.obj; ... } ... } }
下一节将介绍如何通知Handler移动数据。
要把运行在后台线程的任务中的数据传送到UI线程上中，请首先将后台任务中数据的引用以及UI对象的引用存储下来。接下来，将要传送的数据和状态代码传递给实例化Handler的对象，在此对象中，将含有状态和要发送数据的Message通过Handler对象发送到其handleMessage（）中。因为Handler在UI线程上运行，所以它可以将数据传送到UI对象。
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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const val DECODE_STATE_COMPLETED: Int = ... // A class that decodes photo files into Bitmaps class PhotoDecodeRunnable( private val photoTask: PhotoTask, // Gets the downloaded byte array private var imageBuffer: ByteArray = photoTask.getByteBuffer() ) : Runnable { ... // Runs the code for this task override fun run() { ... // Tries to decode the image buffer BitmapFactory.decodeByteArray( imageBuffer, 0, imageBuffer.size, bitmapOptions )?.also { returnBitmap -> ... // Sets the ImageView Bitmap photoTask.image = returnBitmap } // Reports a status of "completed" photoTask.handleDecodeState(DECODE_STATE_COMPLETED) ... } ... }
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java
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// A class that decodes photo files into Bitmaps class PhotoDecodeRunnable implements Runnable { ... PhotoDecodeRunnable(PhotoTask downloadTask) { photoTask = downloadTask; } ... // Gets the downloaded byte array byte[] imageBuffer = photoTask.getByteBuffer(); ... // Runs the code for this task public void run() { ... // Tries to decode the image buffer returnBitmap = BitmapFactory.decodeByteArray( imageBuffer, 0, imageBuffer.length, bitmapOptions ); ... // Sets the ImageView Bitmap photoTask.setImage(returnBitmap); // Reports a status of "completed" photoTask.handleDecodeState(DECODE_STATE_COMPLETED); ... } ... } ...
PhotoTask还包含一个显示Bitmap的ImageView的句柄。但即使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。
所以，下一步是将数据发送到PhotoTask对象。
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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// Gets a handle to the object that creates the thread pools class PhotoTask() { ... private val photoManager: PhotoManager = PhotoManager.getInstance() ... fun handleDecodeState(state: Int) { // Converts the decode state to the overall state. val outState: Int = when(state) { PhotoDecodeRunnable.DECODE_STATE_COMPLETED -> PhotoManager.TASK_COMPLETE ... } ... // Calls the generalized state method handleState(outState) } ... // Passes the state to PhotoManager private fun handleState(state: Int) { /* * Passes a handle to this task and the * current state to the class that created * the thread pools */ PhotoManager.handleState(this, state) } ... }
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java
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public class PhotoTask { ... // Gets a handle to the object that creates the thread pools photoManager = PhotoManager.getInstance(); ... public void handleDecodeState(int state) { int outState; // Converts the decode state to the overall state. switch(state) { case PhotoDecodeRunnable.DECODE_STATE_COMPLETED: outState = PhotoManager.TASK_COMPLETE; break; ... } ... // Calls the generalized state method handleState(outState); } ... // Passes the state to PhotoManager void handleState(int state) { /* * Passes a handle to this task and the * current state to the class that created * the thread pools */ photoManager.handleState(this, state); } ... }
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码和要传送数据的Message，并将其发送给Handler：
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kotlin
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object PhotoManager { ... // Handle status messages from tasks fun handleState(photoTask: PhotoTask, state: Int) { when(state) { ... TASK_COMPLETE -> { // The task finished downloading and decoding the image /* * Creates a message for the Handler * with the state and the task object */ handler.obtainMessage(state, photoTask)?.apply { sendToTarget() } } ... } ... }
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java
public class PhotoManager { ... // Handle status messages from tasks public void handleState(PhotoTask photoTask, int state) { switch (state) { ... // The task finished downloading and decoding the image case TASK_COMPLETE: /* * Creates a message for the Handler * with the state and the task object */ Message completeMessage = handler.obtainMessage(state, photoTask); completeMessage.sendToTarget(); break; ... } ... }
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： -
kotlin
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object PhotoManager { ... private val handler: Handler = object : Handler(Looper.getMainLooper()) { override fun handleMessage(inputMessage: Message) { // Gets the image task from the incoming Message object. val photoTask = inputMessage.obj as PhotoTask // Gets the ImageView for this task val localView: PhotoView = photoTask.getPhotoView() ... when (inputMessage.what) { ... TASK_COMPLETE -> localView.setImageBitmap(photoTask.image) ... else -> super.handleMessage(inputMessage) } ... } ... } ... ... } ... }
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java
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private PhotoManager() { ... handler = new Handler(Looper.getMainLooper()) { @Override public void handleMessage(Message inputMessage) { // Gets the task from the incoming Message object. PhotoTask photoTask = (PhotoTask) inputMessage.obj; // Gets the ImageView for this task PhotoView localView = photoTask.getPhotoView(); ... switch (inputMessage.what) { ... // The decoding is done case TASK_COMPLETE: /* * Moves the Bitmap from the task * to the View */ localView.setImageBitmap(photoTask.getImage()); break; ... default: /* * Pass along other messages from the UI */ super.handleMessage(inputMessage); } ... } ... } ... } ... }
关于多线程的更多内容，请参阅进程和线程——概述。