public class Activity extends android.app.Activity {
	private Handler mHandler = new Handler(){
		@Override        
		        public void handleMessage(Message msg) {
			super.handleMessage(msg);
			// 更新UI
		}
	}
	;
	@Override    
	public void onCreate(Bundle savedInstanceState, PersistableBundle persistentState) {
		super.onCreate(savedInstanceState, persistentState);
		setContentView(R.layout.activity_main);
		new Thread(new Runnable() {
			@Override            
			             public void run() {
				// 执行耗时任务                ...                
				// 任务执行完后，通知Handler更新UI                
				Message message = Message.obtain();
				message.what = 1;
				mHandler.sendMessage(message);
			}
		}
		).start();
	}
}

class LooperThread extends Thread {
	public Handler mHandler;
	public void run() {
		Looper.prepare();
		mHandler = new Handler() {
			public void handleMessage(Message msg) {
				// process incoming messages here
			}
		}
		;
		Looper.loop();
	}
}
 

public final class ActivityThread extends ClientTransactionHandler {
	// ...    
	public static void main(String[] args) {
		// ...        
		Looper.prepareMainLooper();
		// ...        
		Looper.loop();
	}
}

public final class Looper {
	public static void prepare() {
		prepare(true);
	}
	private static void prepare(Boolean quitAllowed) {
		if (sThreadLocal.get() != null) {
			throw new RuntimeException("Only one Looper may be created per thread");
		}
		sThreadLocal.set(new Looper(quitAllowed));
	}
	public static void prepareMainLooper() {
		prepare(false);
		synchronized (Looper.class) {
			if (sMainLooper != null) {
				throw new IllegalStateException("The main Looper has already been prepared.");
			}
			sMainLooper = myLooper();
		}
	}
	private Looper(Boolean quitAllowed) {
		mQueue = new MessageQueue(quitAllowed);
		mThread = Thread.currentThread();
	}
	// ...
}

public final class Looper {
	// ...    
	public static void loop() {
		// 获取TLS存储的Looper对象        
		final Looper me = myLooper();
		if (me == null) {
			throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
		}
		final MessageQueue queue = me.mQueue;
		// 进入loop主循环方法        
		for (;;) {
			Message msg = queue.next();
			// 可能会阻塞，因为next()方法可能会无线循环            
			if (msg == null) {
				// No message indicates that the message queue is quitting.                
				return;
			}
			// This must be in a local variable, in case a UI event sets the logger            
			final Printer logging = me.mLogging;
			if (logging != null) {
				logging.println(">>>>> Dispatching to " + msg.target + " " +                        
				                                msg.callback + ": " + msg.what);
			}
			// ...            
			final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
			final long dispatchEnd;
			try {
				// 获取msg的目标Handler，然后分发Message                
				msg.target.dispatchMessage(msg);
				dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
			}
			finally {
				if (traceTag != 0) {
					Trace.traceEnd(traceTag);
				}
			}
			// ...            
			msg.recycleUnchecked();
		}
	}
}

public class Handler {
	// ...    
	public Handler() {
		this(null, false);
	}
	public Handler(Callback callback, Boolean async) {
		// ...        
		// 必须先执行Looper.prepare()，才能获取Looper对象，否则为null        
		mLooper = Looper.myLooper();
		if (mLooper == null) {
			throw new RuntimeException(                
			      "Can't create handler inside thread " + Thread.currentThread()                        
			      + " that has not called Looper.prepare()");
		}
		mQueue = mLooper.mQueue;
		// 消息队列，来自Looper对象        
		mCallback = callback;
		// 回调方法        
		mAsynchronous = async;
		// 设置消息是否为异步处理方式
	}
}

public final Boolean post(Runnable r){
	return sendMessageDelayed(getPostMessage(r), 0);
}
public final Boolean postAtTime(Runnable r, long uptimeMillis){
	return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}
public final Boolean postAtTime(Runnable r, Object token, long uptimeMillis){
	return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}
public final Boolean postDelayed(Runnable r, long delayMillis){
	return sendMessageDelayed(getPostMessage(r), delayMillis);
}
private static Message getPostMessage(Runnable r) {
	Message m = Message.obtain();
	m.callback = r;
	return m;
}

public final Boolean sendMessage(Message msg){
	return sendMessageDelayed(msg, 0);
}
public final Boolean sendEmptyMessage(int what){
	return sendEmptyMessageDelayed(what, 0);
}
public final Boolean sendEmptyMessageDelayed(int what, long delayMillis) {
	Message msg = Message.obtain();
	msg.what = what;
	return sendMessageDelayed(msg, delayMillis);
}
public final Boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
	Message msg = Message.obtain();
	msg.what = what;
	return sendMessageAtTime(msg, uptimeMillis);
}
public final Boolean sendMessageDelayed(Message msg, long delayMillis){
	if (delayMillis < 0) {
		delayMillis = 0;
	}
	return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
sendMessageAtTime()
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;
	if (mAsynchronous) {
		msg.setAsynchronous(true);
	}
	return queue.enqueueMessage(msg, uptimeMillis);
}

public class Handler {
	// ...    
	public void dispatchMessage(Message msg) {
		if (msg.callback != null) {
			// 当Message存在回调方法，调用该回调方法            
			handleCallback(msg);
		} else {
			if (mCallback != null) {
				// 当Handler存在Callback成员变量时，回调其handleMessage()方法                
				if (mCallback.handleMessage(msg)) {
					return;
				}
			}
			// Handler自身的回调方法            
			handleMessage(msg);
		}
	}
	private static void handleCallback(Message message) {
		message.callback.run();
	}
}