深入剖析Android消息机制原理

class LooperThread extends Thread 
{ 
  public Handler mHandler; 
  public void run() 
  { 
    Looper.prepare(); 
    //其它需要处理的操作 
    Looper.loop(); 
  } 
} 

/** 
 * 
 * @author coolszy 
 * 
 */ 
 
public class MessageService extends Service 
 
{ 
  private static final String TAG = "MessageService"; 
  private static final int KUKA = 0; 
  private Looper looper; 
  private ServiceHandler handler; 
  /** 
   * 由于处理消息是在Handler的handleMessage()方法中，因此我们需要自己编写类 
   * 继承自Handler类，然后在handleMessage()中编写我们所需要的功能代码 
   * @author coolszy 
   * 
   */ 
  private final class ServiceHandler extends Handler 
  { 
    public ServiceHandler(Looper looper) 
    { 
      super(looper); 
    } 
 
    @Override 
    public void handleMessage(Message msg) 
    { 
      // 根据what字段判断是哪个消息 
      switch (msg.what) 
      { 
      case KUKA: 
        //获取msg的obj字段。我们可在此编写我们所需要的功能代码 
        Log.i(TAG, "The obj field of msg:" + msg.obj); 
        break; 
      // other cases 
      default: 
        break; 
      } 
      // 如果我们Service已完成任务，则停止Service 
      stopSelf(msg.arg1); 
    } 
  }  
 
  @Override 
  public void onCreate() 
  { 
    Log.i(TAG, "MessageService-->onCreate()"); 
    // 默认情况下Service是运行在主线程中，而服务一般又十分耗费时间，如果 
    // 放在主线程中，将会影响程序与用户的交互，因此把Service 
    // 放在一个单独的线程中执行 
    HandlerThread thread = new HandlerThread("MessageDemoThread", Process.THREAD_PRIORITY_BACKGROUND); 
    thread.start(); 
    // 获取当前线程中的looper对象 
    looper = thread.getLooper(); 
    //创建Handler对象，把looper传递过来使得handler、 
    //looper和messageQueue三者建立联系 
    handler = new ServiceHandler(looper); 
  } 
 
  @Override 
  public int onStartCommand(Intent intent, int flags, int startId) 
  { 
    Log.i(TAG, "MessageService-->onStartCommand()"); 
 
    //从消息池中获取一个Message实例 
    Message msg = handler.obtainMessage(); 
    // arg1保存线程的ID，在handleMessage()方法中 
    // 我们可以通过stopSelf(startId)方法，停止服务 
    msg.arg1 = startId; 
    // msg的标志 
    msg.what = KUKA; 
    // 在这里我创建一个date对象，赋值给obj字段 
    // 在实际中我们可以通过obj传递我们需要处理的对象 
    Date date = new Date(); 
    msg.obj = date; 
    // 把msg添加到MessageQueue中 
    handler.sendMessage(msg); 
    return START_STICKY; 
  } 
 
  @Override 
  public void onDestroy() 
  { 
    Log.i(TAG, "MessageService-->onDestroy()"); 
  } 
  
  @Override 
  public IBinder onBind(Intent intent) 
  { 
    return null; 
  } 
} 

public void run() { 
    mTid = Process.myTid(); 
    Looper.prepare(); 
    synchronized (this) { 
      mLooper = Looper.myLooper(); 
      notifyAll(); 
    } 
    Process.setThreadPriority(mPriority); 
    onLooperPrepared(); 
    Looper.loop(); 
    mTid = -1; 
  } 

 public static final void loop() { 
 
    Looper me = myLooper(); 
    MessageQueue queue = me.mQueue; 
    while (true) { 
      Message msg = queue.next(); // might block 
      //if (!me.mRun) { 
      //  break; 
      //} 
      if (msg != null) { 
        if (msg.target == null) { 
          // No target is a magic identifier for the quit message. 
          return; 
        } 
        if (me.mLogging!= null) me.mLogging.println( 
            ">>>>> Dispatching to " + msg.target + " " 
            + msg.callback + ": " + msg.what 
            ); 
        msg.target.dispatchMessage(msg); 
        if (me.mLogging!= null) me.mLogging.println( 
            "<<<<< Finished to  " + msg.target + " " 
            + msg.callback); 
        msg.recycle(); 
      } 
    } 
  } 

final boolean enqueueMessage(Message msg, long when) { 
    if (msg.when != 0) { 
      throw new AndroidRuntimeException(msg 
          + " This message is already in use."); 
    } 
    if (msg.target == null && !mQuitAllowed) { 
      throw new RuntimeException("Main thread not allowed to quit"); 
    } 
    synchronized (this) { 
      if (mQuiting) { 
        RuntimeException e = new RuntimeException( 
          msg.target + " sending message to a Handler on a dead thread"); 
        Log.w("MessageQueue", e.getMessage(), e); 
        return false; 
      } else if (msg.target == null) { 
        mQuiting = true; 
      } 
      msg.when = when; 
      //Log.d("MessageQueue", "Enqueing: " + msg); 
      Message p = mMessages; 
      if (p == null || when == 0 || when < p.when) { 
        msg.next = p; 
        mMessages = msg; 
        this.notify(); 
      } else { 
        Message prev = null; 
        while (p != null && p.when <= when) { 
          prev = p; 
          p = p.next; 
        } 
        msg.next = prev.next; 
        prev.next = msg; 
        this.notify(); 
      } 
    } 
    return true; 
  } 

public void dispatchMessage(Message msg) { 
    if (msg.callback != null) { 
      handleCallback(msg); 
    } else { 
      if (mCallback != null) { 
        if (mCallback.handleMessage(msg)) { 
          return; 
        } 
      } 
      handleMessage(msg); 
    } 
  }