Flutter流构建器不会更新小部件

Question

我正在使用TableCalender和Cloud Firestore创建日历屏幕。

我想在设置selectedEvent的位置将一个值分配给selectedEvent之后设置_buildEventList()，但是由于首先调用_buildEventList()，所以它将为空。 但是，设置完值后，我将调用setState(){}。为什么屏幕无法更新？

final _firestore = Firestore.instance;
FirebaseUser loggedInUser;

// Example holidays
final Map<DateTime, List> _holidays = {
  DateTime(2019, 1, 1): ['New Year\'s Day'],
  DateTime(2019, 1, 6): ['Epiphany'],
  DateTime(2019, 2, 14): ['Valentine\'s Day'],
  DateTime(2019, 4, 21): ['Easter Sunday'],
  DateTime(2019, 4, 22): ['Easter Monday'],
};

class CalenderScreen extends StatefulWidget {
  @override
  _CalenderScreenState createState() => _CalenderScreenState();
}

class _CalenderScreenState extends State<CalenderScreen>
    with TickerProviderStateMixin {
  DateTime _selectedDay;
  Map<DateTime, List> _events = {};
  Map<DateTime, List> _visibleEvents;
  Map<DateTime, List> _visibleHolidays;
  List _selectedEvents;
  AnimationController _controller;
  final _auth = FirebaseAuth.instance;
  Widget streamBuilder;
  Widget buildEvents;

  @override
  void initState() {
    print("calender");
    super.initState();
    getCurrentUser();
    _selectedDay = DateTime.now();
    _selectedEvents = _events[_selectedDay] ?? [];
    _visibleEvents = _events;
    _visibleHolidays = _holidays;

    _controller = AnimationController(
      vsync: this,
      duration: const Duration(milliseconds: 400),
    );

    _controller.forward();
  }

  void getCurrentUser() async {
    try {
      final user = await _auth.currentUser();
      if (user != null) {
        loggedInUser = user;
        setStreamBuilder();
      }
    } catch (e) {
      print(e);
    }
  }

  void setStreamBuilder() {
    setState(() {
      streamBuilder = StreamBuilder<QuerySnapshot>(
        stream: _firestore
            .collection('users')
            .document(loggedInUser.uid)
            .collection('history')
            .snapshots(),
        builder: (context, snapshot) {
          if (snapshot.data == null)
            return Center(child: CircularProgressIndicator());
          final historys = snapshot.data.documents;
          _events = {};
          for (var history in historys) {
            DateTime timeStamp = history.data['date'].toDate();
            DateTime currentDate =
                DateTime(timeStamp.year, timeStamp.month, timeStamp.day);
            if (_events.containsKey(currentDate)) {
              _events[currentDate].add(history);
            } else {
              _events[currentDate] = [history];
            }
          }
          print(_events);
          DateTime now = DateTime.now();
          _selectedDay = DateTime(now.year, now.month, now.day);
　　　　　　//here i set the _selectedEvents
          _selectedEvents = _events[_selectedDay] ?? [];
          _visibleEvents = _events;
          return _buildTableCalendar();
        },
      );
    });
  }

  void _onDaySelected(DateTime day, List events) {
    setState(() {
      _selectedDay = day;
      _selectedEvents = events;
    });
  }

  void _onVisibleDaysChanged(
      DateTime first, DateTime last, CalendarFormat format) {
    setState(() {
      _visibleEvents = Map.fromEntries(
        _events.entries.where(
          (entry) =>
              entry.key.isAfter(first.subtract(const Duration(days: 1))) &&
              entry.key.isBefore(last.add(const Duration(days: 1))),
        ),
      );

      _visibleHolidays = Map.fromEntries(
        _holidays.entries.where(
          (entry) =>
              entry.key.isAfter(first.subtract(const Duration(days: 1))) &&
              entry.key.isBefore(last.add(const Duration(days: 1))),
        ),
      );
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Color(0xFF232D3D),
      body: Column(
        mainAxisSize: MainAxisSize.max,
        children: <Widget>[
          streamBuilder != null
              ? streamBuilder
              : Center(
                  child: CircularProgressIndicator(),
                ),
          const SizedBox(height: 8.0),
          _buildEventList()
        ],
      ),
      floatingActionButton: FloatingPenButton(),
    );
  }

  Widget _buildTableCalendar() {
    return TableCalendar(
      ~~
    );
  }

  Widget _buildEventList() {
    print("bulid event");
    return Expanded(
      child: ListView(
        children: _selectedEvents
            .map(
              (event) => HistoryCard(
                history: HistoryData(
                    ~~
              ),
            )
            .toList(),
      ),
    );
  }
}