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{
/** 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
 }
}
}

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 {
/** Called when the activity is first created. */

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("http://csdnimg.cn/www/images/csdnindex_logo.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);
//    }
//   });
 }
};