如何知道活動是否位於堆棧頂部

Question

如何知道活動是否堆棧頂部？我想過使用onResume/onPause，但這並不完全一樣，因爲一旦應用程序轉到後臺，它會失敗。 事實是，我發送了一個接收到的所有活動的廣播接收器（我有一個由所有活動擴展並註冊到廣播的BaseActivity）。因此，只有位於堆棧頂部的活動必須對廣播作出反應。如果我使用isResumed（），它會一直工作，但是當應用程序轉到後臺時。任何想法？

在此先感謝！

Answer 1

 in base activity you register the broadcast Receiver and in receiver function you call one abstract function which one is implemented by all child activities. 
     The activity which is on top will automatically receive that function call. 
     Edit sample code: 

     public abstract class BaseActivity extends AppCompatActivity { 
      private static final String NOTIFICATION_ARRIVED = "arrived"; 
      public abstract void receivedFunction(Intent intent); 
      private BroadcastReceiver mMessageReceiver = new BroadcastReceiver() { 
       @Override 
       public void onReceive(Context context, Intent intent) { 
        displayToast(" received in Base"); 
        receivedFunction(intent); 
       } 
      }; 

      public void displayToast(String s) { 
       Toast.makeText(this,s,Toast.LENGTH_SHORT).show(); 
      } 

      @Override 
      public void onResume() { 
       super.onResume(); 
       registerReceiver(mMessageReceiver, new IntentFilter(BaseActivity.NOTIFICATION_ARRIVED)); 
      } 

      @Override 
      public void onPause() { 
       super.onPause(); 
       unregisterReceiver(mMessageReceiver); 
      } 
     } 

     public class MainActivity extends BaseActivity { 
     @Override 
      public void receivedFunction(Intent intent) { 
       displayToast(" received in child"); 
      } 
     // do whetever you want . if you ovveride onpause and onResume then call super as well 
     } 
    or any other child 

    public class MainActivity2 extends BaseActivity { 
     @Override 
      public void receivedFunction(Intent intent) { 
       displayToast(" received in child"); 
      } 
     // do whetever you want . if you ovveride onpause and onResume then call super as well 
     } 

// to broadcast 

Intent intent = new Intent(BaseActivity.NOTIFICATION_ARRIVED); 
     sendBroadcast(intent);