HandlerThread，子线程中处理Handler消息

    Handler用来处理异步消息，这个我们都知道，一般来说消息的处理都是在主线程中，比如后台获取数据，然后到主线程中更新。那如果我们需要消息的处理也在子线程中呢？Android系统提供了HandlerThread类，方便的解决了这个问题（当然也可以自己实现Thread和Looper，请参考Android消息机制）。

示例代码

public class MainActivity extends AppCompatActivity {


    private HandlerThread mHandlerThread;
    private Handler mHandler;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        findViewById(R.id.send).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mHandler.sendEmptyMessage(0);
            }
        });

        mHandlerThread = new HandlerThread("test");
        mHandlerThread.start();
        mHandler = new Handler(mHandlerThread.getLooper()) {
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                Log.d("dingfeng", "Thread:" + Thread.currentThread().getName());
            }
        };

    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        mHandlerThread.quit();
    }
}

    很简单，先初始化HandlerThread并启动，然后用HandlerThread中的Looper来实例化Handler，这样Handler处理消息时就是在子线程中进行了。退出时不要忘了quit。

源码解析

public class HandlerThread extends Thread {
    int mPriority;
    int mTid = -1;
    Looper mLooper;

    public HandlerThread(String name) {
        super(name);
        mPriority = Process.THREAD_PRIORITY_DEFAULT;
    }
    
    /**
     * Constructs a HandlerThread.
     * @param name
     * @param priority The priority to run the thread at. The value supplied must be from 
     * {@link android.os.Process} and not from java.lang.Thread.
     */
    public HandlerThread(String name, int priority) {
        super(name);
        mPriority = priority;
    }
    
    /**
     * Call back method that can be explicitly overridden if needed to execute some
     * setup before Looper loops.
     */
    protected void onLooperPrepared() {
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }
    
    /**
     * This method returns the Looper associated with this thread. If this thread not been started
     * or for any reason is isAlive() returns false, this method will return null. If this thread 
     * has been started, this method will block until the looper has been initialized.  
     * @return The looper.
     */
    public Looper getLooper() {
        if (!isAlive()) {
            return null;
        }
        
        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) {
                try {
                    wait();
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }

    /**
     * Quits the handler thread's looper.
     * 

     * Causes the handler thread's looper to terminate without processing any
     * more messages in the message queue.
     * 

     * 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.
     * 

     * 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.
     * 
     *
     * @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;
    }

    /**
     * Quits the handler thread's looper safely.
     * 

     * 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.
     * 

     * 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.
     * 

     * 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.
     * 
     *
     * @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;
    }

    /**
     * Returns the identifier of this thread. See Process.myTid().
     */
    public int getThreadId() {
        return mTid;
    }
}

   Thread的子类，start后执行run方法，创建Looper，创建完成后notifyAll（这是因为在初始化Handler的时候，需要获取HandlerThread的Looper，通过getLooper获取，如果Looper为null时就会wait，Looper必须创建后才能初始化Handler）。最后开启Loop循环，从消息队列读取消息并处理（具体请参考Android消息机制）。