Android Handler线程间通信机制分析

Looper: 循环处理消息队列

//用法：
  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();
        }
    }

//第一步，在TLS上创建一个Looper
//TLS，thread local storage线程本地存储空间，也就是这个存储空间是和线程相关的，一个线程内有一个内部存储空间
public static final void prepare() {  
        if (sThreadLocal.get() != null) {  
            throw new RuntimeException("Only one Looper may be created per thread");  
        }  
        sThreadLocal.set(new Looper());  
    }

  /**
     *  Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
// 从当前本地线程中得到loop对象，从中取出message队列，遍历其中的message并发送到其target
// 注意msg.target.dispatchMessage(msg);
 public static final void loop() {
        Looper me = myLooper();
        MessageQueue queue = me.mQueue;
        while (true) {
            Message msg = queue.next(); // might block
            //if (!me.mRun) {
            //    break;
            //}
            if (msg != null) {
                if (msg.target == null) {
                    // No target is a magic identifier for the quit message.
                    return;
                }
                if (me.mLogging!= null) me.mLogging.println(
                        ">>>>> Dispatching to " + msg.target + " "
                        + msg.callback + ": " + msg.what
                        );
                msg.target.dispatchMessage(msg);
                if (me.mLogging!= null) me.mLogging.println(
                        "<<<<< Finished to    " + msg.target + " "
                        + msg.callback);
                msg.recycle();
            }
        }
    }

// 再看看Handler中的该方法
/**
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }
    
    /**
     * 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);
        }
    }

参考：http://blog.csdn.net/Innost/article/details/6055793