android 主线程和子线程之间的消息传递

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);
//                }
//            });
        }
    };
    

}