How to load appointments On-Demand in Flutter Calendar?

List<String> _subjectCollection = <String>[];
List<Color> _colorCollection = <Color>[];
MeetingDataSource _events = MeetingDataSource(<_Meeting>[]);
 
@override
void initState() {
  _addAppointment();
  super.initState();
}

Widget loadMoreWidget(
    BuildContext context, LoadMoreCallback loadMoreAppointments) {
  return FutureBuilder<void>(
    initialData: 'loading',
    future: loadMoreAppointments(),
    builder: (context, snapShot) {
      return Container(
          alignment: Alignment.center, child: CircularProgressIndicator());
    },
  );
}

  child: SfCalendar(
  view: CalendarView.day,
  allowedViews: [
    CalendarView.day,
    CalendarView.week,
    CalendarView.workWeek,
    CalendarView.month,
    CalendarView.timelineDay,
    CalendarView.timelineWeek,
    CalendarView.timelineWorkWeek,
    CalendarView.timelineMonth,
    CalendarView.schedule,
  ],
  dataSource: _events,
  loadMoreWidgetBuilder: loadMoreWidget,
)

 
class MeetingDataSource extends CalendarDataSource {
  MeetingDataSource(this.source);
 
  List<_Meeting> source;
 
  @override
  List<dynamic> get appointments => source;
 
  @override
  DateTime getStartTime(int index) {
    return source[index].from;
  }
 
  @override
  DateTime getEndTime(int index) {
    return source[index].to;
  }
 
  @override
  bool isAllDay(int index) {
    return source[index].isAllDay;
  }
 
  @override
  String getSubject(int index) {
    return source[index].eventName;
  }
 
  @override
  Color getColor(int index) {
    return source[index].background;
  }
 
  @override
  Future<void> handleLoadMore(DateTime startDate, DateTime endDate) async {
    await Future<void>.delayed(const Duration(seconds: 1));
    List<_Meeting> meetings = <_Meeting>[];
    DateTime date = DateTime(startDate.year, startDate.month, startDate.day);
    DateTime appEndDate =
        DateTime(endDate.year, endDate.month, endDate.day, 23, 59, 59);
    while (date.isBefore(appEndDate)) {
      final List<_Meeting>? data = _dataCollection[date];
      if (data == null) {
        date = date.add(const Duration(days: 1));
        continue;
      }
 
      for (final _Meeting meeting in data) {
        if (appointments.contains(meeting)) {
          continue;
        }
 
        meetings.add(meeting);
      }
      date = date.add(const Duration(days: 1));
    }
 
    appointments.addAll(meetings);
    notifyListeners(CalendarDataSourceAction.add, meetings);;
  }
}