Android Handler移除Message詳解

問題：

1.removeMessage(what)函數是否只能移除對應what值的Message？

2.對于Delayed發送的Message，能否提前remove？

代碼測試：

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; //子線程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:

//開始發送消息mHandler.sendEmptyMessage(1);break;case R.id.endButton:

//移除主線程Handler的消息mHandler.removeMessages(1);break;default:break;}}}

測試結果：

1. removeMassage(1)無法移除what=0的Message。

2. 在子線程中執行完

mHandler.sendEmptyMessageDelayed(1, 2000);

Log.d("info", "handle sub-thread message...");

之后，即可通過removeMesage(1)來移除消息，mHandler將不能接收到該條消息。

源碼分析：

1.Android如何移除一條Message？

查看源碼可知，Handler.removeMessage(int what)內部調用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;

}

}

}

篩選要移除的Message的條件是：target(handler)，what，object

該函數分兩步來移除Message：

1).移除在前端的符合條件的Message

2).移除后面的符合條件的Message

2.為何延遲發送的Message在延遲時間到達之前就可以被移除？

Handler.sendEmptyMessageDelayed() ---調用---> sendMessageAtTime() -----調用---> enqueueMessage() ----調用MessageQueue.enqueueMessage()

實際進行處理的就是MessageQueue，源碼如下：

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會對需要延遲發送的Message排序，按照需要延遲的時間長短(when)。

即，雖然是延遲發送的消息，其實當你調用發送函數之后，Message就已經被添加到MessageQueue中去了。

感謝閱讀，希望能幫助到大家，謝謝大家對本站的支持！

總結

以上是生活随笔為你收集整理的android message 代码,Android Handler移除Message详解及实例代码的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。