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);<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: //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
<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。
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