Android 线程通信（Handler + Message + Looper） 0

参考：

进程和线程

Android异步消息处理机制完全解析，带你从源码的角度彻底理解

Android 异步消息处理机制 让你深入理解 Looper、Handler、Message三者关系

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在 Android 开发中，可以使用 Handler + Looper + Message 的组合进行线程通信

当前运行环境：Android 7.1.1 API Level 25

本次学习暂不涉及进程操作

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主要内容：

1. 简单使用例子
2. Message
3. Looper
4. Handler
5. Android 封装的 Handler 操作
6. HandlerThread

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简单使用例子

首先创建子类 MyHandler 继承 Handler，并实现函数 handleMessage

class MyHandler extends Handler {    @Override    public void handleMessage(Message msg) {        super.handleMessage(msg);        Log.e(TAG, "handleMessage: " + Thread.currentThread().getName());    }}

在 MainActivity 实例化 MyHandler，创建线程，然后在线程中使用 MyHandler 对象发送消息：

final MyHandler handler = new MyHandler();new Thread(new Runnable() {    @Override    public void run() {        Log.e(TAG, "run: " + Thread.currentThread().getName());        handler.sendEmptyMessage(1);    }}).start();

运行程序，结果如下所示：

Handler 在线程中发送消息，最后在 UI 线程中得到消息，实现了线程之间的通信

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Message

Looper

Handler

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Android 封装的 Handler 操作

Handler + Message + Looper 的异步消息通信机制如下图所示（应该是郭神自己画的吧）

Android 也封装了几个 Handler 操作:

• Activity.runOnUiThread(Runnable)
• View.post(Runnable)
• View.postDelayed(Runnable, long)

查看其实现：

// we must have a handler before the FragmentController is constructedfinal Handler mHandler = new Handler();/** * Runs the specified action on the UI thread. If the current thread is the UI * thread, then the action is executed immediately. If the current thread is * not the UI thread, the action is posted to the event queue of the UI thread. * * @param action the action to run on the UI thread */public final void runOnUiThread(Runnable action) {    if (Thread.currentThread() != mUiThread) {        mHandler.post(action);    } else {        action.run();    }}

其实 Activity.runOnUiThread(Runnable) 是调用了自带的 Handler 的 post 方法，其作用是让 Runnable 对象运行在主线程

/** * <p>Causes the Runnable to be added to the message queue. * The runnable will be run on the user interface thread.</p> * * @param action The Runnable that will be executed. * * @return Returns true if the Runnable was successfully placed in to the *         message queue.  Returns false on failure, usually because the *         looper processing the message queue is exiting. * * @see #postDelayed * @see #removeCallbacks */public boolean post(Runnable action) {    final AttachInfo attachInfo = mAttachInfo;    if (attachInfo != null) {        return attachInfo.mHandler.post(action);    }    ...}

同样也是调用 Handler 对象实现的

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HandlerThread