Android消息处理机制（源码分析）

public class LooperThread extends Thread{
	public Handler mhHandler;

	@Override
	public void run() {
		// 1. 调用Looper
		Looper.prepare();
		
		// ... 其他处理，例如实例化handler
		Handler handler = new Handler();
		
		// 2. 进入消息循环
		Looper.loop();
	}
	
}

    public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }
    
    private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();	// 得到空的message
        m.callback = r;	// 将runnable设置为message的callback
        return m;
    }
    
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
    }

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;	// 将Message的target设置为自己，然后加到消息队列中
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

Handler处理消息

    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

    private static void handleCallback(Message message) {
        message.callback.run();
    }
    
    /**
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }

    /**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }
    /** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
    */
    public Message() {
    }

    /**
     * arg1 and arg2 are lower-cost alternatives to using
     * {@link #setData(Bundle) setData()} if you only need to store a
     * few integer values.
     */
    public int arg1; 

    /**
     * arg1 and arg2 are lower-cost alternatives to using
     * {@link #setData(Bundle) setData()} if you only need to store a
     * few integer values.
     */
    public int arg2;
    
    /**
     * Sets a Bundle of arbitrary data values. Use arg1 and arg1 members 
     * as a lower cost way to send a few simple integer values, if you can.
     * @see #getData() 
     * @see #peekData()
     */
    public void setData(Bundle data) {
        this.data = data;
    }

示例代码

public class MainActivity extends Activity {
	TextView mTextView;
	MyHandler mHandler = new MyHandler();
	

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		mTextView = (TextView)findViewById(R.id.test1);
		new Thread(new UpdateTitleTask(mHandler)).start();
	}

	private class MyHandler extends Handler {

		@Override
		public void handleMessage(Message msg) {
			Bundle bundle = msg.getData();
			mTextView.setText(bundle.getString("title", ""));
		}
		
	}
}

public class UpdateTitleTask implements Runnable{
	private Handler handler;
	
	public UpdateTitleTask(Handler handler) {
		this.handler = handler;
	}
	
	private Message prepareMsg() {
		Message msg = Message.obtain();
		Bundle bundle = new Bundle();
		bundle.putString("title", "From Update Task");;
		msg.setData(bundle);
		return msg;
	}
	
	@Override
	public void run() {
		try {
			Thread.sleep(2000);
			Message msg = prepareMsg();
			handler.sendMessage(msg);
		} catch (InterruptedException e) {
			
		}
	}

}

参考文献