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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