android主線程和子線程之間消息傳遞詳解

從主線程發送消息到子線程（准確地說應該是非UI線程）

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();//新建並啟動CustomThread實例
  
  findViewById(R.id.send_btn).setOnClickListener(new OnClickListener() {
   
   @Override
   public void onClick(View v) {//點擊界面時發送消息
    String str = "hello";
    Log.d("Test", "MainThread is ready to send msg:" + str);
    mHandler.obtainMessage(MSG_HELLO, str).sendToTarget();//發送消息到CustomThread實例
    
   }
  });
  
 }
 
 
 
 
 
 class CustomThread extends Thread {
  @Override
  public void run() {
   //建立消息循環的步驟
   Looper.prepare();//1、初始化Looper
   mHandler = new Handler(){//2、綁定handler到CustomThread實例的Looper對象
    public void handleMessage (Message msg) {//3、定義處理消息的方法
     switch(msg.what) {
     case MSG_HELLO:
      Log.d("Test", "CustomThread receive msg:" + (String) msg.obj);
     }
    }
   };
   Looper.loop();//4、啟動消息循環
  }
 }
}

從非UI線程傳遞消息到UI線程(界面主線程），因為主界面已經有MessageQueue，所以可以直接獲取消息處理消息。而上面由主線程向非UI線程中處理消息的時候，非UI線程需要先添加消息隊列，然後處理消息循環。

public class ThreadHandlerActivity extends Activity {
 /** Called when the activity is first created. */
 
 private static final int MSG_SUCCESS = 0;//獲取圖片成功的標識
 private static final int MSG_FAILURE = 1;//獲取圖片失敗的標識
 
 private ImageView mImageView;
 private Button mButton;
 
 private Thread mThread;
 
 private Handler mHandler = new Handler() {
  public void handleMessage (Message msg) {//此方法在ui線程運行
   switch(msg.what) {
   case MSG_SUCCESS:
    mImageView.setImageBitmap((Bitmap) msg.obj);//imageview顯示從網絡獲取到的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);//顯示圖片的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();//線程啟動
    }
    else {
     Toast.makeText(getApplication(), getApplication().getString(R.string.thread_started), Toast.LENGTH_LONG).show();
    }
   }
  });
 }
 
 Runnable runnable = new Runnable() {
  
  @Override
  public void run() {//run()在新的線程中運行
   HttpClient hc = new DefaultHttpClient();
   HttpGet hg = new HttpGet("http://csdnimg.cn/www/images/csdnindex_logo.gif");//獲取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();//獲取圖片失敗
    return;
   }
   mHandler.obtainMessage(MSG_SUCCESS,bm).sendToTarget();//獲取圖片成功，向ui線程發送MSG_SUCCESS標識和bitmap對象

//   mImageView.setImageBitmap(bm); //出錯！不能在非ui線程操作ui元素

//   mImageView.post(new Runnable() {//另外一種更簡潔的發送消息給ui線程的方法。
//    
//    @Override
//    public void run() {//run()方法會在ui線程執行
//     mImageView.setImageBitmap(bm);
//    }
//   });
  }
 };

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