[Tut] Chat Pagination with Cloud Firestore

[UsamaKarim]

I tried to create pagination with firestore while taking inspiration from FilledStack's video about Flutter and Firestore real-time Pagination. The approach is not perfect but it's working fine for me.

class MessagesService with ChangeNotifier {
  late int currentIndex;

  MessagesService();

  final databaseService = sl.get<DatabaseService>();

  ///constants
  static const _serverTime = 'metadata.createdAt';
  // static const _createdAt = 'createdAt';
  static const _resultLimit = 10;

  bool _hasMoreItems = true;
  bool _isLoading = false;
  bool get hasMoreItems => _hasMoreItems;
  bool get isLoading => _isLoading;

  void _setLoading(bool value) {
    _isLoading = value;
    // notifyListeners();
  }

  ///
  DocumentSnapshot? _lastDocument;

  final List<List<types.Message>> _allMessage = [];

  final _streamController = StreamController<List<types.Message>>.broadcast();

  Stream<List<types.Message>> messageStream() {
    requestData();
    return _streamController.stream;
  }

  void requestData() {
    if (_isLoading) return;
    _setLoading(true);
    Query<types.Message> query;

    /// First Query to get `_resultLimit` messages
    query = databaseService
        .messages()
        // .where(_serverTime, isGreaterThanOrEqualTo: timeStamp)
        .orderBy(_serverTime, descending: true)
        .limit(_resultLimit);

    currentIndex = _allMessage.length;

    /// If User scrolls to upwards
    if (_lastDocument != null) {
      if (_hasMoreItems) {
        query = query.startAfterDocument(_lastDocument!);
        query.snapshots().listen((snapshot) => _createList(snapshot, false));

        return;
      }
    }
    query.snapshots().listen((snapshot) => _createList(snapshot, true));
  }

  void _createList(QuerySnapshot<types.Message> snapshot, bool isNewMessage) {
    _setLoading(true);
    if (isNewMessage) {
// I'm using a hack here, this hack fixes a problem when user scroll upwards and a new mesasge came, the message doesn't appear. 
      currentIndex = 0;
    }

    print('Current Index $currentIndex');
    print('All Message Length ${_allMessage.length}');
    final tempList = snapshot.docs.map((e) => e.data()).toList();

    final pageExists = currentIndex < _allMessage.length;
    if (pageExists) {
      _allMessage[currentIndex] = tempList;
    } else {
      _allMessage.add(tempList);
    }
    final foldedList = _allMessage.fold<List<types.Message>>(<types.Message>[],
        (initialValue, element) => initialValue..addAll(element));
    _streamController.add(foldedList);

    if (currentIndex == _allMessage.length - 1) {
      _lastDocument = snapshot.docs.last;
    }

    // Determine if there's more Messages to request
    _hasMoreItems = tempList.length == _resultLimit;
    _setLoading(false);
  }

  @override
  void dispose() {
    super.dispose();
    _streamController.close();
  }
}