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。