Android Handler移除Message详解及实例代码

2019-12-12 03:35:54

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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);<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。

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