Android 嵌套Firebase查询

我试图在另一个firebase查询（如嵌套for循环）中执行firebase查询，但外部查询在内部查询开始之前完全运行。我如何做到这一点？我是一个android和firebase的初学者

外部查询

 private void retrieveMarkerDetails(final String key) {
    databaseReference = mDatabase.getReference();

    databaseReference.child("Parking Lots").child(key).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            parking_lot_details = dataSnapshot.getValue(Parking_Lot_Details.class);



            Marker marker;

            if (markers.size() != 0) {
                marker = markers.get(key);
                String title = parking_lot_details.getLotName();




                //Calculate Remaining space
                spaceAvailable();

Log.i（“可用剩余空间”，String.valueOf（parking\u lot\u details.getAvailableSpace（））

内部查询 这是应该运行的内部查询

private void spaceAvailable() {

    spaceAvailable = new int[1];


    //Strip email of dots and use it to access requests, since the requests use the emails
    //of lot owners as keys
    String lot_email= parking_lot_details.getEmail().replace(".","");

    Log.i("lot_email",lot_email);



    databaseReference.child("Requests").child(lot_email).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            if (dataSnapshot.exists()) {
                long requestsMade = dataSnapshot.getChildrenCount();


                Log.i("Requests ", dataSnapshot.toString());
                Log.i("RequestsMade", Long.toString(requestsMade));
                spaceAvailable[0] = (int) (Integer.parseInt(parking_lot_details.getCapacity()) - requestsMade);

                parking_lot_details.setAvailableSpace(String.valueOf(spaceAvailable[0]));

                Log.i("Space Available ", String.valueOf(spaceAvailable[0]));
                Log.i("Space Available in met ", parking_lot_details.getAvailableSpace());


            }else{

                //if the email does not appear under requests then we assume that no requests have
                //been made and the available space is equal to the lots capacity
                parking_lot_details.setAvailableSpace(parking_lot_details.getCapacity());
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });



}

数据库

日志

---

为了使其正常工作，请更改第二个查询，如下所示：

databaseReference.child("Requests").child(lot_email).addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {

        if (dataSnapshot.exists()) {
            long requestsMade = dataSnapshot.getChildrenCount();
            Log.d("TAG", dataSnapshot.getKey() + " / No. of childrens: " + requestsMade);

            for(DataSnapshot ds : dataSnapshot.getChildren()) {
                long childRequestsMade = ds.getChildrenCount();
                String lot_email = ds.child("lot_email").getValue(String.class);
                Log.d("TAG", lot_email);
            }
        }else{

            //if the email does not appear under requests then we assume that no requests have
            //been made and the available space is equal to the lots capacity
            parking_lot_details.setAvailableSpace(parking_lot_details.getCapacity());
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

---

要显示这些子项中的值，需要使用for循环并使用

getChildren（）

方法。在本例中，它将只打印

lot\u email

，因此您说第一个查询首先运行，第二个查询第二次运行？是的，第一个查询在第二个查询之前完全运行，因此我在哪一行得到空值？请添加整个错误，以便看得更清楚。更新了代码这行代码，工作正常吗<代码>Log.i（“批次电子邮件”，批次电子邮件）返回用户电子邮件？感谢您的帮助，但我想获取ChildrenCount（）而不是getChildren（），您仍然可以获取它。请看我更新的答案。我为两个

dataSnapshot

对象都添加了。它有效吗？什么是

null

？在哪一行获取

null

？所有电子邮件都会被记录，这意味着代码会运行外部查询，直到耗尽所有节点，然后再运行内部查询，在外部查询完成更改后也会运行

addValueEventListener

和

addListenerForSingleValueEvent

。现在应该可以工作了。对吗？

databaseReference.child("Requests").child(lot_email).addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {

        if (dataSnapshot.exists()) {
            long requestsMade = dataSnapshot.getChildrenCount();
            Log.d("TAG", dataSnapshot.getKey() + " / No. of childrens: " + requestsMade);

            for(DataSnapshot ds : dataSnapshot.getChildren()) {
                long childRequestsMade = ds.getChildrenCount();
                String lot_email = ds.child("lot_email").getValue(String.class);
                Log.d("TAG", lot_email);
            }
        }else{

            //if the email does not appear under requests then we assume that no requests have
            //been made and the available space is equal to the lots capacity
            parking_lot_details.setAvailableSpace(parking_lot_details.getCapacity());
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});