有主线程发送message给子线程

　　通常我们在处理耗时任务时候都会通过新建线程来处理，当任务处理完后通过Handler将结果发送回主线程。比如下面示例：

 package com.example.testlistener;

 import java.util.Timer;
 import java.util.TimerTask;

 import android.app.Activity;
 import android.content.res.Configuration;
 import android.os.Bundle;
 import android.os.Handler;
 import android.os.Message;
 import android.view.View;
 import android.view.View.OnClickListener;
 import android.view.View.OnLongClickListener;
 import android.widget.Button;
 import android.widget.EditText;
 import android.widget.Toast;

 public class MainActivity extends Activity {
     EditText txt;
     final Handler hander = new Handler(){
         public void handleMessage(android.os.Message msg) {
             if(msg.what == 0x1234){
                 Toast.makeText(MainActivity.this, msg.arg1 + "", 1000).show();
             }
         };
     };
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);

         new Timer().schedule(new TimerTask() {
             int i = 0;
             @Override
             public void run() {
                 i++;
                 Message msg = new Message();
                 msg.what = 0x1234;
                 msg.arg1 = i;
                 hander.sendMessage(msg);
             }
         }, 0, 2000);
     }

 }

　　那么，我们能不能通过Handler从主线程发送消息给子线程呢？答案是肯定的，需要用到Looper.prepare()和Looper.loop()。如下面的代码：

 public class MainActivity extends Activity {
     EditText txt;
     Button bt;
     TestThread testThread;
     int j = 0;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         txt = (EditText) findViewById(R.id.txt);
         bt = (Button) findViewById(R.id.bn);
         testThread = new TestThread();
         testThread.start();
         bt.setOnClickListener(new OnClickListener() {

             @Override
             public void onClick(View arg0) {
                 j++;
                 Message msg = new Message();
                 msg.what = 0x1000;
                 msg.arg1 = j;
                 testThread.thander.sendMessage(msg);
             }
         });

     }

     //自定义子线程
     class TestThread extends Thread{
         public Handler thander;
         @Override
         public void run() {
             //创建一个Looper对象
             Looper.prepare();
             thander = new Handler(){
                 @Override
                 public void handleMessage(Message msg) {
                     if(msg.what == 0x1000){
                         System.out.println("j = " + msg.arg1);
                         //txt.setText(msg.arg1 + "");
                     }
                 }
             };
             //启动Looper
             Looper.loop();
         }
     }

 }