android主线程和子线程之间消息传递详解

从主线程发送消息到子线程（准确地说应该是非UI线程）

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{
 
 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
  }
 }
}

从非UI线程传递消息到UI线程(界面主线程），因为主界面已经有MessageQueue，所以可以直接获取消息处理消息。而上面由主线程向非UI线程中处理消息的时候，非UI线程需要先添加消息队列，然后处理消息循环。

public class ThreadHandlerActivity extends Activity {
 
 private static final int MSG_SUCCESS = 0;//获取图片成功的标识
 private static final int MSG_FAILURE = 1;//获取图片失败的标识
 private ImageView mImageView;
 private Button mButton;
 private Thread mThread;
 private Handler mHandler = new Handler() {
  public void handleMessage (Message msg) {//此方法在ui线程运行
   switch(msg.what) {
   case MSG_SUCCESS:
    mImageView.setImageBitmap((Bitmap) msg.obj);//imageview显示从网络获取到的loGo
    Toast.makeText(getApplication(), getApplication().getString(R.string.get_pic_success), Toast.LENGTH_LONG).show();
    break;
   case MSG_FAILURE:
    Toast.makeText(getApplication(), getApplication().getString(R.string.get_pic_failure), Toast.LENGTH_LONG).show();
    break;
   }
  }
 };
 @Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
  mImageView= (ImageView) findViewById(R.id.imageView);//显示图片的ImageView
  mButton = (Button) findViewById(R.id.button);
  mButton.setOnClickListener(new OnClickListener() {
   @Override
   public void onClick(View v) {
    if(mThread == null) {
     mThread = new Thread(runnable);
     mThread.start();//线程启动
    }
    else {
     Toast.makeText(getApplication(), getApplication().getString(R.string.thread_started), Toast.LENGTH_LONG).show();
    }
   }
  });
 }
 Runnable runnable = new Runnable() {
  @Override
  public void run() {//run()在新的线程中运行
   HttpClient hc = new DefaultHttpClient();
   HttpGet hg = new HttpGet("/file/imgs/upload/202206/06/v45vx5vmkzr.gif");//获取csdn的logo
   final Bitmap bm;
   try {
    HttpResponse hr = hc.execute(hg);
    bm = BitmapFactory.decodeStream(hr.getEntity().getContent());
   } catch (Exception e) {
    mHandler.obtainMessage(MSG_FAILURE).sendToTarget();//获取图片失败
    return;
   }
   mHandler.obtainMessage(MSG_SUCCESS,bm).sendToTarget();//获取图片成功，向ui线程发送MSG_SUCCESS标识和bitmap对象
//   mImageView.setImageBitmap(bm); //出错！不能在非ui线程操作ui元素
//   mImageView.post(new Runnable() {//另外一种更简洁的发送消息给ui线程的方法。
//    
//    @Override
//    public void run() {//run()方法会在ui线程执行
//     mImageView.setImageBitmap(bm);
//    }
//   });
  }
 };

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