java - Android + Firebase: synchronous for into an asynchronous function

Question

I'm making a function (Java - Android) which should return me a list of objects filled with data from Firebase. My problem is that I need to send the result to the listener just after I loop through all the items in a for, take a look at the code:

final DatabaseReference beaconMessageRef = dataSnapshot.getRef().getRoot().child("region_messages/" + regionKey);
beaconMessageRef.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {

        if(!dataSnapshot.exists())
            FirebaseHelper.this.messageByBeaconListener.onSuccess(null);

        for (DataSnapshot regionMessageDS : dataSnapshot.getChildren()) {

            RegionMessage regionMessage = regionMessageDS.getValue(RegionMessage.class);
            String msgKey = regionMessageDS.getKey();

            final DatabaseReference messageRef = regionMessageDS.getRef().getRoot().child("messages/"+msgKey);
            messageRef.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {

                    if(dataSnapshot.exists()){
                        Message msg = dataSnapshot.getValue(Message.class);
                        FirebaseHelper.this.addMessageByBeacon(msg);
                    }
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {
                    FirebaseHelper.this.messageByBeaconListener.onFail(databaseError);
                }
            });
        }

        List<Message> msgs = FirebaseHelper.this.beaconMessageList;
        FirebaseHelper.this.beaconMessageList = null;
        FirebaseHelper.this.messageByBeaconListener.onSuccess(msgs);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        FirebaseHelper.this.messageByBeaconListener.onFail(databaseError);
    }
});

I need to execute this line: FirebaseHelper.this.messageByBeaconListener.onSuccess(msgs); just after I loop through all the items in the for.

How can I achieve that?

Answer 1

Waitting for answers