Android Handler移除Message详解及实例代码

Android Handler移除Message详解

问题：

1.removeMessage(what)函数是否只能移除对应what值的Message？

2.对于Delayed发送的Message，能否提前remove？

代码测试：

package javine.k.testhandler; 
 
import android.app.Activity; 
import android.os.Bundle; 
import android.os.Handler; 
import android.os.HandlerThread; 
import android.os.Message; 
import android.util.Log; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 
 
public class TestHandlerActivity extends Activity implements onClickListener { 
 
  private Button startBtn; 
  private Button endBtn; 
  public Handler threadHandler; //子线程Handler 
 
  private Handler mHandler = new Handler() { 
    public void handleMessage(android.os.Message msg) { 
      threadHandler.sendEmptyMessageDelayed(1, 2000); 
      Log.d("info", "handle main-thread message..."); 
    }; 
  }; 
 
  @Override 
  protected void onCreate(Bundle savedInstanceState) { 
    // TODO Auto-generated method stub 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.main); 
 
    startBtn = (Button) findViewById(R.id.startButton); 
    endBtn = (Button) findViewById(R.id.endButton); 
    startBtn.setonClickListener(this); 
    endBtn.setonClickListener(this); 
 
    new Thread(new Runnable() { 
      @Override 
      public void run() { 
 HandlerThread handlerThread = new HandlerThread("handler"); 
 handlerThread.start(); 
 threadHandler = new Handler(handlerThread.getLooper()) { 
   @Override 
   public void handleMessage(Message msg) { 
     //mHandler.sendEmptyMessageDelayed(0, 2000);   

     mHandler.sendEmptyMessageDelayed(1, 2000); 



Log.d("info", "handle sub-thread message...");}};}}).start();}@Overridepublic void onClick(View v) 
{// TODO Auto-generated method stubswitch (v.getId()) {case R.id.startButton: 
//开始发送消息mHandler.sendEmptyMessage(1);break;case R.id.endButton:
//移除主线程Handler的消息mHandler.removeMessages(1);break;default:break;}}}

测试结果：
        1. removeMassage（1）无法移除what=0的Message。
2. 在子线程中执行完

 
   mHandler.sendEmptyMessageDelayed(1, 2000); 



Log.d("info", "handle sub-thread message...");

  之后，即可通过removeMesage(1)来移除消息，mHandler将不能接收到该条消息。
源码分析：
1.Android如何移除一条Message？
查看源码可知，Handler.removeMessage(int what)内部调用MessageQueue.removeMessage(this, what, null)
查看MessageQueue的removeMessage方法如下：

void removeMessages(Handler h, int what, Object object) { 
    if (h == null) { 
      return; 
    } 
 
    synchronized (this) { 
      Message p = mMessages; 
 
      // Remove all messages at front. 
      while (p != null && p.target == h && p.what == what 
  && (object == null || p.obj == object)) { 
 Message n = p.next; 
 mMessages = n; 
 p.recycle(); 
 p = n; 
      } 
 
      // Remove all messages after front. 
      while (p != null) { 
 Message n = p.next; 
 if (n != null) { 
   if (n.target == h && n.what == what 
     && (object == null || n.obj == object)) { 
     Message nn = n.next; 
     n.recycle(); 
     p.next = nn; 
     continue; 
   } 
 } 
 p = n; 
      } 
    } 
  } 


筛选要移除的Message的条件是：target（handler），what，object
该函数分两步来移除Message：
1）.移除在前端的符合条件的Message
2）.移除后面的符合条件的Message
2.为何延迟发送的Message在延迟时间到达之前就可以被移除？
Handler.sendEmptyMessageDelayed() ---调用---> sendMessageAtTime() -----调用---> enqueueMessage() ----调用MessageQueue.enqueueMessage()
实际进行处理的就是MessageQueue，源码如下：

boolean enqueueMessage(Message msg, long when) { 
    if (msg.isInUse()) { 
      throw new AndroidRuntimeException(msg + " This message is already in use."); 
    } 
    if (msg.target == null) { 
      throw new AndroidRuntimeException("Message must have a target."); 
    } 
 
    synchronized (this) { 
      if (mQuitting) { 
 RuntimeException e = new RuntimeException( 
     msg.target + " sending message to a Handler on a dead thread"); 
 Log.w("MessageQueue", e.getMessage(), e); 
 return false; 
      } 
 
      msg.when = when; 
      Message p = mMessages; 
      boolean needWake; 
      if (p == null || when == 0 || when < p.when) { 
 // New head, wake up the event queue if blocked. 
 msg.next = p; 
 mMessages = msg; 
 needWake = mBlocked; 
      } else { 
 // Inserted within the middle of the queue. Usually we don't have to wake 
 // up the event queue unless there is a barrier at the head of the queue 
 // and the message is the earliest asynchronous message in the queue. 
 needWake = mBlocked && p.target == null && msg.isAsynchronous(); 
 Message prev; 
 for (;;) { 
   prev = p; 
   p = p.next; 
   if (p == null || when < p.when) { 
     break; 
   } 
   if (needWake && p.isAsynchronous()) { 
     needWake = false; 
   } 
 } 
 msg.next = p; // invariant: p == prev.next 
 prev.next = msg; 
      } 
 
      // We can assume mPtr != 0 because mQuitting is false. 
      if (needWake) { 
 nativeWake(mPtr); 
      } 
    } 
    return true; 
  } 


由上可知：MessageQueue会对需要延迟发送的Message排序，按照需要延迟的时间长短（when）。


即，虽然是延迟发送的消息，其实当你调用发送函数之后，Message就已经被添加到MessageQueue中去了。
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