接上文。

角色綜述（回顧）:

(1)UI thread通常就是main thread，而Android啟動(dòng)程序時(shí)會(huì)替它建立一個(gè)MessageQueue。

(2)需要一個(gè)Looper對(duì)象，來管理MessageQueue。

(3)可以構(gòu)造Handler對(duì)象來push新消息到Message Queue里；或者接收Looper所送來的消息。

(4)線程A的Handler對(duì)象可以傳遞給別的線程，讓別的線程能送訊息來給線程A。

(5)線程A的Message Queue里的消息，只有線程A所屬的對(duì)象可以處理。

 例二、子線程傳遞消息給主線程

public class Activity2extends Activityimplements OnClickListener{

      Buttonbutton =null;

      TextViewtext =null;

      MyHandlermHandler =null;

      Threadthread ;

      protected void onCreate(Bundle savedInstanceState) {

             super.onCreate(savedInstanceState);

             setContentView(R.layout.activity1);        

             button = (Button)findViewById(R.id.btn);

             button.setOnClickListener(this);

             text = (TextView)findViewById(R.id.content);

      }

      public void onClick(View v) {

             switch (v.getId()) {

             case R.id.btn:

                    thread =new MyThread();

                    thread.start();

                    break;

             }            

      }     

      private class MyHandlerextends Handler{             

             public MyHandler(Looper looper){

                    super(looper);

             }

             public void handleMessage(Message msg) {

                    text.setText(msg.obj.toString());

             }            

      }

      private class MyThreadextends Thread{

             public void run() {

                    Looper curLooper = Looper.myLooper();

                    Looper mainLooper = Looper.getMainLooper();

                    String msg ;

                    if(curLooper==null){

                           mHandler =new MyHandler(mainLooper);

                           msg ="curLooper is null";

                    }else{

                           mHandler =new MyHandler(curLooper);

                           msg ="This is curLooper";

                    }

                    mHandler.removeMessages(0);

                    Message m =mHandler.obtainMessage(1, 1, 1, msg);

                    mHandler.sendMessage(m);

             }            

      }

}

說明：

Android會(huì)自動(dòng)替主線程建立Message Queue。子線程沒有建立Message Queue。所以mHandler屬于主線程。mHandler.sendMessage(m);將m消息存入主線程的Message Queue。mainLooper看到Message Queue里有訊息，主線程執(zhí)行到mHandler的handleMessage()來處理消息。