????? DataSnapshot ? ??

Firebase ??? ?????? ?? ? addListenerForSingleValueEvent() ???? ???? ?????? ???? ???? ?? ??????. ??? ?? ?? ???? ??? ??? ?? ????? ? ? ??? ???.

??? ?? ??

Firebase ??? ??????? ??? ??? addListenerForSingleValueEvent()? ??? ? ?? ???? ?? ???? ?? ?? ????. ?? ???? ???? onDataChange() ???? ?????.

???? ??? ??

private String getUserName(String uid) {
    databaseReference.child(String.format("users/%s/name", uid))
            .addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            // How to return this value?
            dataSnapshot.getValue(String.class);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {}
    });
}

? ???? getValue() ???? onDataChange() ???? ???? ?? ?????. ???? ?? ??? ? ???? null ?? ?????.

?? ?? ??

?? ??:

???? ??? ? ???? ???? ?? ??? ?? ?? ?????? ????. ??? ? ????.

public interface MyCallback {
    void onCallback(String value);
}

public void readData(MyCallback myCallback) {
    databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            String value = dataSnapshot.getValue(String.class);
            myCallback.onCallback(value);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {}
    });
}

?? ????? ?? ?????? ????? ???? ???? ????.

readData(new MyCallback() {
    @Override
    public void onCallback(String value) {
        // Use the returned value here
    }
});

Future ??:

??? ??? ?? ??? ????? Future? ?????.

public Future<String> readData() {
    final Future<String> future = new CompletableFuture<>();
    databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            future.complete(dataSnapshot.getValue(String.class));
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {}
    });
    return future;
}

In the ???? ???? ?? ?? ??? ???? ??? ????? ?? ??? ?? ??? ?????.

??

????? DataSnapshot ?? ????? ?? ??? ???? ???. ??? ???? ??. ???? Future? ???? ???? ?? ?? ?? ????? ?? ???? ?? ?? ???? ???? ?? ? ????.

? ??? ???? ????? Firebase ??? ?????? DataSnapshot ?? ???? ??? ??????? ?? ?????. ??? ??? PHP ??? ????? ?? ?? ??? ?????!