https://stackoverflow.com/questions/50951045/how-can-i-populate-a-stream-with-a-value-from-a-future example

bloc.dart

class A {
  Subject<int> _stateSubject;
  Stream<int> _state;
  Stream<int> get state {
    return _state;
  }

  final Stream<DocumentSnapshot> _visitorCount =
      Firestore.instance.document('ServerData/serverStatus').snapshots();
  A() {
    _stateSubject = new BehaviorSubject(seedValue: 0);
    _state = _stateSubject.asBroadcastStream();

    Stream<int> fireStoreStream = Firestore.instance
        .document('ServerData/serverStatus')
        .snapshots()
        .map((DocumentSnapshot doc) =>
            doc['activeUsers'] as int); // your firestore query here
    fireStoreStream.listen((int value) {
      _stateSubject.add(value); // push your new value
    });
  }
}