线程 同步 ConditionVariable-------特别使用在mediaplayer和camera多次调用中.

转自: http://blog.csdn.net/shaojie519/article/details/6684326

ConditionVariable类位于android.os.ConditionVariable，它可以帮助Android线程同步。在SDK上的介绍ConditionVariable不同于标准Java位于java.lang.Object wait() 和 notify() ，这个类可以等待自己，这就意味着 open(), close() 和 block() 可能会假死 ，如果使用ConditionVariable类的open()在调用 block() 之前， block() 将不会阻塞，相反将会返回立即。

该类一共有4个方法

boolean  block(long timeout)
阻止当前线程知道条件是open，或直到超时，这里参数long timeout为超时设置，Android123提示大家如果你们从事过Win32开发，这个方法类似DWORD WaitForSingleObject(HANDLE hHandle,DWORD dwMilliseconds); 函数。

void  block()
阻止当前线程知道条件 open ，是上面的无超时等待重载版本。

void  close()
重置条件为 close状态。

void  open()
Open条件，释放所有线程的阻塞.

ConditionVariable在创建时还有一种构造方法是 public ConditionVariable (boolean state) ，如果为true，默认时为opened，如果为false则是closed. ，默认public ConditionVariable()为close()

给出api中一个demo：

/*  * Copyright (C) 2007 The Android Open Source Project  *  * Licensed under the Apache License, Version 2.0 (the "License");  * you may not use this file except in compliance with the License.  * You may obtain a copy of the License at  *  *      http://www.apache.org/licenses/LICENSE-2.0  *  * Unless required by applicable law or agreed to in writing, software  * distributed under the License is distributed on an "AS IS" BASIS,  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  * See the License for the specific language governing permissions and  * limitations under the License.  */    package com.example.android.apis.app;    // Need the following import to get access to the app resources, since this  // class is in a sub-package.  import com.example.android.apis.R;    import android.app.Notification;  import android.app.NotificationManager;  import android.app.PendingIntent;  import android.app.Service;  import android.content.Intent;  import android.os.Binder;  import android.os.ConditionVariable;  import android.os.IBinder;  import android.os.Parcel;  import android.os.RemoteException;  import android.widget.RemoteViews;    /**  * This is an example of service that will update its status bar balloon   * every 5 seconds for a minute.  *   */  public class NotifyingService extends Service {            // Use a layout id for a unique identifier      private static int MOOD_NOTIFICATIONS = R.layout.status_bar_notifications;        // variable which controls the notification thread       private ConditionVariable mCondition;//它可以帮助Android线程同步         @Override      public void onCreate() {          mNM = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);            // Start up the thread running the service.  Note that we create a          // separate thread because the service normally runs in the process's          // main thread, which we don't want to block.          Thread notifyingThread = new Thread(null, mTask, "NotifyingService");          mCondition = new ConditionVariable(false);          notifyingThread.start();      }        @Override      public void onDestroy() {          // Cancel the persistent notification.          mNM.cancel(MOOD_NOTIFICATIONS);          // Stop the thread from generating further notifications          mCondition.open();       }        private Runnable mTask = new Runnable() {          public void run() {              for (int i = 0; i < 4; ++i) {                  showNotification(R.drawable.stat_happy,                          R.string.status_bar_notifications_happy_message);                  if (mCondition.block(5 * 1000))                       break;                  showNotification(R.drawable.stat_neutral,                          R.string.status_bar_notifications_ok_message);                  if (mCondition.block(5 * 1000))                       break;                  showNotification(R.drawable.stat_sad,                          R.string.status_bar_notifications_sad_message);                  if (mCondition.block(5 * 1000))                       break;              }              // Done with our work...  stop the service!              NotifyingService.this.stopSelf();          }      };        @Override      public IBinder onBind(Intent intent) {          return mBinder;      }            private void showNotification(int moodId, int textId) {          // In this sample, we'll use the same text for the ticker and the expanded notification          CharSequence text = getText(textId);            // Set the icon, scrolling text and timestamp.          // Note that in this example, we pass null for tickerText.  We update the icon enough that          // it is distracting to show the ticker text every time it changes.  We strongly suggest          // that you do this as well.  (Think of of the "New hardware found" or "Network connection          // changed" messages that always pop up)          Notification notification = new Notification(moodId, null, System.currentTimeMillis());            // The PendingIntent to launch our activity if the user selects this notification          PendingIntent contentIntent = PendingIntent.getActivity(this, 0,                  new Intent(this, NotifyingController.class), 0);            // Set the info for the views that show in the notification panel.          notification.setLatestEventInfo(this, getText(R.string.status_bar_notifications_mood_title),                         text, contentIntent);            // Send the notification.          // We use a layout id because it is a unique number.  We use it later to cancel.          mNM.notify(MOOD_NOTIFICATIONS, notification);      }        // This is the object that receives interactions from clients.  See      // RemoteService for a more complete example.      private final IBinder mBinder = new Binder() {          @Override          protected boolean onTransact(int code, Parcel data, Parcel reply,                  int flags) throws RemoteException {              return super.onTransact(code, data, reply, flags);          }      };        private NotificationManager mNM;  }  

另一篇文章---------------

转自:http://blog.csdn.net/pocoyoshamoo/article/details/9156089

在Android开发中，如果需要线程同步，可以使用Java系统库的wait()和notify()...但是这些代码的编写比较麻烦。Android提供了一个ConditionVariable类方便线程同步操作。

  ConditionVariable有三个方法，分别是block()、open()、close()。

  void block()
  阻塞当前线程，直到条件为open
  void block(long timeout)
  阻塞当前线程，直到条件为open或超时
  void open()
  释放所有阻塞的线程
  void close()
  将条件重置为close

  ConditionVariable 在创建时还有一种构造方法是 public ConditionVariable (boolean state) ，如果为true，默认时为opened，如果为false则是closed. ，默认public ConditionVariable()为closed.

  自己简单写了一个Demo，效果有点牵强...Demo的效果是，TextView每过3秒，就会输出一个字符s。如果点击按钮可以马上输出。

[java] view plaincopy

1. public class MainActivity extends Activity implements OnClickListener {  
2.     private Button btn_output;  
3.     private TextView textView;  
4.     private MyHandler mHandler;  
5.     private boolean isStart;  
6.     private String str = "";  
7.     private ConditionVariable mConditionVariable;  
8.     private final int REFRESHTEXT = 1;  
9.     @Override  
10.     protected void onCreate(Bundle savedInstanceState) {  
11.         super.onCreate(savedInstanceState);  
12.         setContentView(R.layout.activity_main);  
13.         btn_output = (Button) findViewById(R.id.btn);  
14.         btn_output.setOnClickListener(this);  
15.         textView = (TextView) findViewById(R.id.txt);  
16.         mHandler = new MyHandler();  
17.         mConditionVariable = new ConditionVariable();  
18.         isStart = true;  
19.         new Thread(new Runnable() {  
20.             @Override  
21.             public void run() {  
22.                 // TODO Auto-generated method stub  
23.                 while(isStart) {  
24.                     //延时等待3秒  
25.                     mConditionVariable.block(3000);  
26.                     //如果是点击了按钮，则先将条件重置，否则block会失效  
27.                     mConditionVariable.close();  
28.                     //线程唤醒后通知主线程更新TextView的文本  
29.                     mHandler.sendEmptyMessage(REFRESHTEXT);  
30.                 }  
31.             }  
32.         }).start();  
33.     }  
34.   
35.     private class MyHandler extends Handler {  
36.         public void handleMessage(Message msg) {  
37.             switch(msg.what) {  
38.             case REFRESHTEXT:  
39.                 textView.setText(str += 's');  
40.                 break;  
41.             }  
42.         }  
43.     }  
44.       
45.     @Override  
46.     public void onClick(View v) {  
47.         // TODO Auto-generated method stub  
48.         mConditionVariable.open();  
49.     }  
50.   
51.     @Override  
52.     protected void onDestroy() {  
53.         // TODO Auto-generated method stub  
54.         super.onDestroy();  
55.         isStart = false;  
56.     }  
57. }