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);<span style="font-family: Arial, Helvetica, sans-serif;">         </span>

<span style="white-space:pre"></span><pre name="code" class="java"><span style="white-space:pre">            </span>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

<span></span><pre name="code" class="java"><span style="white-space:pre">
   </span>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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