如何知道检索数据是否完成-实时数据库

Question

我正在尝试从Firebase Realtime数据库中获取数据，并且数据检索工作正常，但是存在一些延迟。我想知道数据是否返回，并且代码中是否存在导致我遇到延迟的东西。

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数据库：

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代码：

   private void Get_Rank ( )
     {
         for ( int ind = 0 ; ind < Child_One.size ( ) ; ind++ )
         {
             DatabaseReference databaseReference = FirebaseDatabase.getInstance ( ).getReference ( );
             Query lastQuery = databaseReference.child ( "voting" ).child ( Child_One.get ( ind ) );
             lastQuery.addListenerForSingleValueEvent ( new ValueEventListener ( )
             {
                 double total20 = 0.0, total60 = 0.0;
                 double count20 = 0, count60 = 0;

                 @Override
                 public void onDataChange ( DataSnapshot dataSnapshot )
                 {
                     for ( DataSnapshot data : dataSnapshot.getChildren ( ) )
                     {
                         if ( data.child ( "type" ).getValue ( ).toString ( ).equals ( "AUD" ) || data.child ( "type" ).getValue (
 ).toString ( ).equals ( "CEO" ) )
                         {

                             total20 += Double.parseDouble ( data.child ( "rating" ).getValue ( ).toString ( ) );
                             count20 += 1;
                         }
                         else
                         {
                             total60 += Double.parseDouble ( data.child ( "rating" ).getValue ( ).toString ( ) );
                             count60 += 1;
                         }
                     }

                     double rate20 = ( total20 / count20 ) * 0.400;
                     double rate60 = ( total60 / count60 ) * 0.600;

                     double final_rate = rate20 + rate60;

                     Final_Rate.add ( final_rate );
                 }


                 @Override
                 public void onCancelled ( DatabaseError databaseError )
                 {

                     Toast.makeText ( getApplicationContext () , "Error" , Toast.LENGTH_SHORT ).show ( );
                 }


             } );
         }

         System.err.println ( String.valueOf ( Final_Rate.size ( ) ) );
         System.err.println ( String.valueOf ( Final_Rate.toString ( ) ) );
         System.err.println ( String.valueOf ( Final_Rate.toString ( ) ) );
         System.err.println ( String.valueOf ( Final_Rate.toString ( ) ) );
     }

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打印日志如下所示（单击3按钮后）：

W/System.err: 0
    []
W/System.err: []
    []
W/System.err: 0
    []
    []
    []
W/System.err: 2
    [3.86, 2.4000000000000004]
    [3.86, 2.4000000000000004]
    [3.86, 2.4000000000000004]

全部。