Android Firebase - 不通过dataSnapshot.getChildren进行迭代

Question

我在迭代dataSnapshot时遇到了麻烦。当我调试代码时，我可以看到dataSnapshot的大小为2，它包含正确的键/值对。但似乎它似乎并没有将键/值对看作是孩子，我猜。

//IMPLEMENT FETCH DATA AND FILL ARRAYLIST
    private void fetchData(DataSnapshot dataSnapshot)
    {
        spacecrafts.clear();
        //for (DataSnapshot ds : dataSnapshot.getChildren())
        for (DataSnapshot ds : dataSnapshot.getChildren())
        {
            Spacecraft spacecraft = new Spacecraft();
            try
            {
                spacecraft.setLevel(Integer.parseInt(ds.getKey()));
            }
            catch (NumberFormatException ex)
            {
                System.out.println("Could not parse " + ex);
            }

        long v = (long)ds.getValue();
            spacecraft.setNumCorrect(v);

            //Spacecraft spacecraft=ds.getValue(Spacecraft.class);
            spacecrafts.add(spacecraft);
        }

        Collections.sort(spacecrafts, new Comparator<Spacecraft>(){
            @Override
            public int compare(Spacecraft spacecraft, Spacecraft t1) {
                return Integer.valueOf(spacecraft.getLevel()).compareTo(t1.getLevel()); // To compare integer values
            }


        });
    }
    //RETRIEVE
    public ArrayList<Spacecraft> retrieve()
    {

        String myUserId = acct.getId();

        DatabaseReference LevelsRef = db.child("levels/uid/").child(myUserId);
        Query queryRef = LevelsRef;

        queryRef.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                //fetchData(dataSnapshot);
                fetchData(dataSnapshot);
                adapter.notifyDataSetChanged();
            }
            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {
                fetchData(dataSnapshot);
            }
            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
            }
            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) {
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        });
        return spacecrafts;
    }

因此，当代码到达for (DataSnapshot ds : dataSnapshot.getChildren())时，它就直接进入排序代码，并且航天器arraylist为空。所以dataSnapshot.getChildren是空的。

我尝试删除for循环并只处理第一个＆＃34;记录＆＃34;在dataSnapshot中它处理正常。有谁知道为什么会发生这种情况以及我能做些什么呢？

编辑：解决方案

public ArrayList<Spacecraft> retrieve()
    {

        String myUserId = acct.getId();

        //DatabaseReference LevelsRef = db.child("levels/uid/" + myUserId + "/");
        DatabaseReference LevelsRef = db.child("levels/uid/");
        Query queryRef = LevelsRef.orderByChild(myUserId);

        queryRef.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                //fetchData(dataSnapshot);
                fetchData(dataSnapshot);
                adapter.notifyDataSetChanged();
            }
            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {
                fetchData(dataSnapshot);
            }
            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
            }
            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) {
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        });
        return spacecrafts;
    }