Handler的创建需要绑定一个Looper对象和Looper对象的MessageQueen，默认调用mLooper = Looper.myLooper()，会返回当前线程中的Looper对象。

/**

* Return the Looper object associated with the current thread.  Returns

* null if the calling thread is not associated with a Looper.

*/

public static @Nullable Looper myLooper() {

return sThreadLocal.get();

}

Handler发送消息到MessageQueen中，Looper对象取出MessageQueen中的meesage，发送给Handler处理。

for (;;) {

Message msg = queue.next(); // might block

msg.target.dispatchMessage(msg);

}

创建looper sample

*  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();

*      }