Android Handler移除Message詳解及實例代碼
Android Handler移除Message詳解
問題:
1.removeMessage(what)函數(shù)是否只能移除對應(yīng)what值的Message?
2.對于Delayed發(fā)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(fā)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(jié)果:
1. removeMassage(1)無法移除what=0的Message。
2. 在子線程中執(zhí)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(nèi)部調(diào)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(shù)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(fā)送的Message在延遲時間到達(dá)之前就可以被移除?
Handler.sendEmptyMessageDelayed() ---調(diào)用---> sendMessageAtTime() -----調(diào)用---> enqueueMessage() ----調(diào)用MessageQueue.enqueueMessage()
實際進(jìn)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(fā)送的Message排序,按照需要延遲的時間長短(when)。
即,雖然是延遲發(fā)送的消息,其實當(dāng)你調(diào)用發(fā)送函數(shù)之后,Message就已經(jīng)被添加到MessageQueue中去了。
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