Problems with array in addChildEventListener (Firebase)

Question

Integer scores[] = new Integer[10];
int count = 0;

I have some data that I want to put into array. I put it in onChildAdded, but if I need to get the data from the array out of the onChildAdded, console shows "null".

If I will try to get the data of array into onChildAdded, it will be success

data.child("scores").addChildEventListener(new ChildEventListener() {
                    @Override
                    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                        count++;
                        scores[count] = dataSnapshot.getValue(Integer.class);
                    }

                    @Override
                    public void onChildChanged(DataSnapshot dataSnapshot, String s) {

                    }

                    @Override
                    public void onChildRemoved(DataSnapshot dataSnapshot) {

                    }

                    @Override
                    public void onChildMoved(DataSnapshot dataSnapshot, String s) {

                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                });

int i = scores[1];

IMPORTANT MOMENT

For example, If I will use operation FOR

for (int i = 0; i < 10; ++i) {
        scores[i] = i;
    }

    int i = scores[3];

i will not be null

Has the `onChildAdded` method already been called when you get the data out of the array? — JensV, Mar 19 '19 at 13:41

score 2 · Accepted Answer · answered Mar 19 '19 at 14:06

2

Firebase APIs are asynchronous, meaning that each one of those methods: onChildAdded(), onChildChanged() etc, return immediately after it's invoked, and the callback from the Task it returns, will be called some time later. There are no guarantees about how long it will take. So it may take from a few hundred milliseconds to a few seconds before that data is available. Because that method returns immediately, the scores array has not have been populated from the callback yet and that's why is empty.

Basically, you're trying to use a value synchronously from an API that's asynchronous. That's not a good idea. You should handle the APIs asynchronously as intended.

A quick solve for this problem would be to move the following lines of code:

for (int i = 0; i < 10; ++i) {
    scores[i] = i;
}
int i = scores[3];

Inside the onChildAdded() method, where the data is available.

If you need to loop your score array outside the callback, I recommend you see the last part of my anwser from this post in which I have explained how it can be done using a custom callback. You can also take a look at this video for a better understanding.

answered Mar 19 '19 at 14:06

Alex Mamo

130,605
17
163
193

Can you help me with solution of problem, pls – Shyngys Rakhad Mar 19 '19 at 15:08
Sorry, I am beginner in Java and It is a little problematic for me – Shyngys Rakhad Mar 19 '19 at 15:11
The first solution is very simple, have you tryed it? The second is a little bit complicated but you should make your own attempt given the information in the answer and ask another question if something else comes up. – Alex Mamo Mar 19 '19 at 15:25
I tried to solve it, but I could not use it in my code, can you help? – Shyngys Rakhad Mar 19 '19 at 15:38
Please see my comments above. – Alex Mamo Mar 19 '19 at 15:42
There one important moment. If I will use callback in onChildAdded, it will be work as much as children have – Shyngys Rakhad Mar 19 '19 at 15:43
What I need. I need to do function, when Firebase completely add children to array. It is what I need – Shyngys Rakhad Mar 19 '19 at 16:01
So, is there the function, which defines that it is the last round of adding child in onChildAdded? – Shyngys Rakhad Mar 19 '19 at 16:03

Problems with array in addChildEventListener (Firebase)

1 Answers1