Android 线程通信（Handler + Message + Looper） 4

参考：

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

android.os.HandlerThread

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HandlerThread 是 Android 系统封装好的已绑定 Looper 对象的线程

下面学习 HandlerThread 类的组成和使用

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主要内容：

1. 构造器
2. run()
3. 线程终止
4. 示例程序

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构造器

构造函数有两个：

public HandlerThread(String name)public HandlerThread(String name, int priority)

参数 name 表示线程名，参数 priority 表示线程优先级

优先级设置

默认优先级大小为 0

public HandlerThread(String name) {    super(name);    mPriority = Process.THREAD_PRIORITY_DEFAULT;}public static final int THREAD_PRIORITY_DEFAULT = 0;

在 Android 系统中的线程，优先级取值范围为 [-20, 19]（-20 表示最高优先级，19 表示最低优先级）

/** * Set the priority of a thread, based on Linux priorities. *  * @param tid The identifier of the thread/process to change. * @param priority A Linux priority level, from -20 for highest scheduling * priority to 19 for lowest scheduling priority. *  * @throws IllegalArgumentException Throws IllegalArgumentException if * <var>tid</var> does not exist. * @throws SecurityException Throws SecurityException if your process does * not have permission to modify the given thread, or to use the given * priority. */public static final native void setThreadPriority(int tid, int priority)        throws IllegalArgumentException, SecurityException;

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run()

HandlerThread 类已经在 run() 方法执行了 Looper.prepare() 方法和 Looper.loop() 方法：

@Overridepublic void run() {    mTid = Process.myTid();    Looper.prepare();    synchronized (this) {        mLooper = Looper.myLooper();        notifyAll();    }    Process.setThreadPriority(mPriority);    onLooperPrepared();    Looper.loop();    mTid = -1;}

如果想要在中间执行一些操作，可以重写 onLooperPrepared() 方法：

/** * Call back method that can be explicitly overridden if needed to execute some * setup before Looper loops. */protected void onLooperPrepared() {}

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线程终止

由于 Looper.loop() 是一个无限循环，其退出条件是消息队列已终止，必须及时调用方法 quit()

/** * Quits the handler thread's looper. * <p> * Causes the handler thread's looper to terminate without processing any * more messages in the message queue. * </p><p> * Any attempt to post messages to the queue after the looper is asked to quit will fail. * For example, the {@link Handler#sendMessage(Message)} method will return false. * </p><p class="note"> * Using this method may be unsafe because some messages may not be delivered * before the looper terminates.  Consider using {@link #quitSafely} instead to ensure * that all pending work is completed in an orderly manner. * </p> * * @return True if the looper looper has been asked to quit or false if the * thread had not yet started running. * * @see #quitSafely */public boolean quit() {    Looper looper = getLooper();    if (looper != null) {        looper.quit();        return true;    }    return false;}

调用 quit() 方法会立即终止 Looper 对象，舍弃消息队列中的消息

如果想要先完成剩余的消息再退出，可以使用方法 quitSafely()

/** * Quits the handler thread's looper safely. * <p> * Causes the handler thread's looper to terminate as soon as all remaining messages * in the message queue that are already due to be delivered have been handled. * Pending delayed messages with due times in the future will not be delivered. * </p><p> * Any attempt to post messages to the queue after the looper is asked to quit will fail. * For example, the {@link Handler#sendMessage(Message)} method will return false. * </p><p> * If the thread has not been started or has finished (that is if * {@link #getLooper} returns null), then false is returned. * Otherwise the looper is asked to quit and true is returned. * </p> * * @return True if the looper looper has been asked to quit or false if the * thread had not yet started running. */public boolean quitSafely() {    Looper looper = getLooper();    if (looper != null) {        looper.quitSafely();        return true;    }    return false;}

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示例程序

public class HandlerThreadActivity extends AppCompatActivity {    private MyThread thread = new MyThread("zj");    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_handler_thread);        thread.start();        try {            Thread.sleep(100);        } catch (InterruptedException e) {            e.printStackTrace();        }        Message msg = Message.obtain(thread.handler, 0, 1000);        msg.sendToTarget();        try {            Thread.sleep(100);        } catch (InterruptedException e) {            e.printStackTrace();        }        thread.quit();    }    class MyThread extends HandlerThread {        private static final String TAG = "MyThread";        public Handler handler;        public MyThread(String name) {            super(name);        }        @Override        protected void onLooperPrepared() {            super.onLooperPrepared();            handler = new Handler() {                @Override                public void handleMessage(Message msg) {                    super.handleMessage(msg);                    Log.e(TAG, "handleMessage: " + msg.arg1);                }            };        }    }}

Note：创建子类继承 HandlerThread 后，仍旧需要调用 start() 方法