Firebase查询帮助我

Question

final int[] ndomande = {14};

//Database
mFirebaseInstance = FirebaseDatabase.getInstance();

final DatabaseReference Refndom = mFirebaseInstance.getReference("Dati");
final Query queryn = Refndom.orderByChild("numero_domande");

queryn.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        //Toast.makeText(PlayTest.this, "FIN QUI ",Toast.LENGTH_SHORT).show();

        for (DataSnapshot messageSnapshot : dataSnapshot.getChildren()) {
           // Toast.makeText(PlayTest.this, "QUI ",Toast.LENGTH_SHORT).show();

             long numero_domande = (long) messageSnapshot.getValue();
             ndomande[0] = (int) numero_domande;

            Toast.makeText(PlayTest.this, "quante domande ci sono totali: "+ numero_domande,Toast.LENGTH_SHORT).show();
        }

    }
    @Override
    public void onCancelled(DatabaseError databaseError) {
        Toast.makeText(PlayTest.this, "???", Toast.LENGTH_SHORT).show();
    }
});

//PROBLEM ON NDOMANDE 

Toast.makeText(PlayTest.this, "x: "+ ndomande[0] ,Toast.LENGTH_SHORT).show();
Random random = new Random();
final int x = random.nextInt(ndomande[0]);
 Toast.makeText(PlayTest.this, "x: "+ndomande[0] ,Toast.LENGTH_SHORT).show();

Answer 1

要解决此问题，请将final int[]ndomande={14}；数组的声明移到ondatachange（）方法中，否则它将始终为null。

public void onDataChange(DataSnapshot dataSnapshot) {
    //Toast.makeText(PlayTest.this, "FIN QUI ",Toast.LENGTH_SHORT).show();
    final int[] ndomande = {14};

    for (DataSnapshot messageSnapshot : dataSnapshot.getChildren()) {
        //Toast.makeText(PlayTest.this, "QUI ",Toast.LENGTH_SHORT).show();

        long numero_domande = (long) messageSnapshot.getValue();
        ndomande[0] = (int) numero_domande;

        Toast.makeText(PlayTest.this, "quante domande ci sono totali: "+ numero_domande,Toast.LENGTH_SHORT).show();
    }
}

这是由于ondatachange（）方法的异步行为造成的，甚至在试图从该数组获取值之前就调用了该方法。在方法内部移动声明并在那里使用它，就可以解决问题。

如果您想在该方法之外使用该数组的值，请看一下我在这篇文章中的回答，如何在onDataChange（）方法之外获得异步值。