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();
        }
    }
    
}