Handler如何移除Message

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Handlerprivate Handler mHandler = new Handler() {public void handleMessage(android.os.Message msg) {threadHandler.sendEmptyMessageDelayed(1, 2000);Log.d("info", "handle main-thread message...");};};@Overrideprotected void onCreate(Bundle savedInstanceState) {// TODO Auto-generated method stubsuper.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() {@Overridepublic void run() {HandlerThread handlerThread = new HandlerThread("handler");handlerThread.start();threadHandler = new Handler(handlerThread.getLooper()) {@Overridepublic 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。