安卓高级应用——多线程二（UI主线程发送消息到非UI线程（子线程））

• UI主线程发送消息给非UI线程
  • 　说明1：无论是UI主线程还是非UI线程，需要接受另外一个线程发送的消息，就必须使用Handler来接收
  •    说明2：UI主线程系统已经默认创建了Looper和MessageQueue。所以在子线程发送消息给UI主线程的时候并不需要再去创建。相反UI线程发送消息给子线程的话，子线程必须先创建Looper（消息泵）和 MessageQueue(消息队列）
• 子线程中创建MessageQueue和Looper

  •  Looper.prepare();//1、初始化Looper和MessageQueue
    
    //以下是Android源码。可以看一下创建过程
    //通过源码可以看出，Looper和MessageQueue是一块被创建的
    //而且MessageQueue和Looper是一对一的关系
     public static void prepare() {
            prepare(true);
        }
    
        private static void prepare(boolean quitAllowed) {
            if (sThreadLocal.get() != null) {
                throw new RuntimeException("Only one Looper may be created per thread");
            }
            sThreadLocal.set(new Looper(quitAllowed));
        }
     private Looper(boolean quitAllowed) {
            mQueue = new MessageQueue(quitAllowed);
            mThread = Thread.currentThread();
     }

     

• 完整的示例

• package com.zhuozhuo;
  
  import android.app.Activity;
  
  import android.os.Bundle;
  
  import android.os.Handler;
  
  import android.os.Looper;
  
  import android.os.Message;
  
  import android.util.Log;
  
  import android.view.View;
  
  import android.view.View.OnClickListener;
  
  
  
  public class LooperThreadActivity extends Activity{
  
      /** Called when the activity is first created. */
      private final int MSG_HELLO = 0;
      private Handler mHandler;
      @Override
  
      public void onCreate(Bundle savedInstanceState) {
  
          super.onCreate(savedInstanceState);
  
          setContentView(R.layout.main);
  
          new CustomThread().start();//新建并启动CustomThread实例
  
          
  
          findViewById(R.id.send_btn).setOnClickListener(new OnClickListener() {
  
              
  
              @Override
  
              public void onClick(View v) {//点击界面时发送消息
  
                  String str = "hello";
  
                  Log.d("Test", "MainThread is ready to send msg:" + str);
  
                  mHandler.obtainMessage(MSG_HELLO, str).sendToTarget();//发送消息到CustomThread实例
  
                  
  
              }
  
          });
  
          
  
      }
  
      class CustomThread extends Thread {
  
          @Override
  
          public void run() {
  
              //建立消息循环的步骤
  
              Looper.prepare();//1、初始化Looper
  
              mHandler = new Handler(){//2、绑定handler到CustomThread实例的Looper对象
  
                  public void handleMessage (Message msg) {//3、定义处理消息的方法
  
                      switch(msg.what) {
  
                      case MSG_HELLO:
  
                          Log.d("Test", "CustomThread receive msg:" + (String) msg.obj);
  
                      }
  
                  }
  
              };
              Looper.loop();//4、启动消息循环
          }
      }
  
  }