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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