我有一个问题，将我的 flutter 应用程序迁移到 null_safety

Question

我将我的应用程序迁移到空安全。 我有一些错误，除了最后一个错误之外，我能够修复所有错误。

此页面是日历页面，发生错误的部分是我在构建日历时。 我正在使用 table_calendar 插件。

我加了“？” 在为 _getEventsFroDay() 定义函数类型之后。 我不知道这是否是执行此操作的正确方法。

1. 参数类型 'List? Function(DateTime)' 不能分配给参数类型 'List Function(DateTime)?'。

这是函数：

List<dynamic>? _getEventsForDay(DateTime day) {
    // kEvents is a linkedHashMap
    for (int i = 0; i < eventDoc.length; i++ ) {
      DateTime eventDate = eventDoc[i].eventDate;
      DateTime eventDateUTC = eventDate.toUtc();
      if (day.year == eventDate.year && day.day == eventDate.day && day.month == eventDate.month) {
        List<dynamic> eventList = [];
        eventList.add(eventDoc[i].agencyId);
        eventList.add(eventDoc[i].agentId);
        eventList.add(eventDoc[i].eventDate);
        eventList.add(eventDoc[i].eventDescription);
        eventList.add(eventDoc[i].eventDuration);
        eventList.add(eventDoc[i].eventName);
        eventList.add(eventDoc[i].eventStartTime);
        //print('kEvents: $kEvents');
        return kEvents.putIfAbsent(eventDateUTC, () => eventList);
      }
    }
  }

This is where the error is occuring:
Widget _buildTableCalendar() {
    return TableCalendar(
      eventLoader: _getEventsForDay, <<<< ERROR HERE

这是删除“？”后的错误消息从列表？ _getEventsForDay(DateTime day) {.

这是完整的 .dart 文件：

// Example holidays
final Map<DateTime, List> _holidays = {
  DateTime(2020, 1, 1): ['New Year\'s Day'],
  DateTime(2020, 1, 6): ['Epiphany'],
  DateTime(2020, 2, 14): ['Valentine\'s Day'],
  DateTime(2020, 4, 21): ['Easter Sunday'],
  DateTime(2020, 4, 22): ['Easter Monday'],
};

final kNow = DateTime.now();
final kFirstDay = DateTime(kNow.year, kNow.month - 3, kNow.day);
final kLastDay = DateTime(kNow.year, kNow.month + 3, kNow.day);

final eventsRef = FirebaseFirestore.instance.collection('agency').doc(globals.agencyId).collection('event');
final _db = FirebaseFirestore.instance;
final _firestoreService = FirestoreService();
bool showSpinner = false;
DateTime? _selectedDay;
DateTime _focusedDay = DateTime.now();
LinkedHashMap<DateTime, List<Event>> kEvents = LinkedHashMap<DateTime, List<Event>>();

class AppointmentCalendarScreen extends StatefulWidget {
  AppointmentCalendarScreen({Key? key, this.title}) : super(key: key);

  final String? title;

  @override
  _AppointmentCalendarScreenState createState() => _AppointmentCalendarScreenState();
}

class _AppointmentCalendarScreenState extends State<AppointmentCalendarScreen> with TickerProviderStateMixin {
  late final ValueNotifier<List<Event>> _selectedEvents;
  Map<DateTime, List>? _selectedEventsMap;
  late StreamController<Map<DateTime, List>> _streamController;
  late var eventDoc;

  @override
  void initState() {
    super.initState();

    _streamController = StreamController();
    _selectedDay = _focusedDay;
    _selectedEvents = ValueNotifier(_getEventsForDay(_selectedDay!));
  }

  @override
  void dispose() {
    _selectedEvents.dispose();
    _streamController.close();

    super.dispose();
  }

  List<Event> _getEventsForDay(DateTime day) {
    // kEvents is a linkedHashMap
    for (int i = 0; i < eventDoc.length; i++ ) {
      DateTime eventDate = eventDoc[i].eventDate;
      DateTime eventDateUTC = eventDate.toUtc();
      if (day.year == eventDate.year && day.day == eventDate.day && day.month == eventDate.month) {
        List<Event> eventList = [];
        eventList.add(eventDoc[i].agencyId);
        eventList.add(eventDoc[i].agentId);
        eventList.add(eventDoc[i].eventDate);
        eventList.add(eventDoc[i].eventDescription);
        eventList.add(eventDoc[i].eventDuration);
        eventList.add(eventDoc[i].eventName);
        eventList.add(eventDoc[i].eventStartTime);
        //print('kEvents: $kEvents');
        return (kEvents.putIfAbsent(eventDateUTC, () => eventList))??[];

      }
    }
  }

  void _onDaySelected(DateTime selectedDay, DateTime focusedDay) {
    if (!isSameDay(_selectedDay, selectedDay)) {
      setState(() {
        _selectedDay = selectedDay;
        _focusedDay = focusedDay;
      });

      _selectedEvents.value = _getEventsForDay(selectedDay);
    }
  }

  void _onVisibleDaysChanged(DateTime first, DateTime last,
      CalendarFormat format) {
  }

  void _onCalendarCreated(DateTime first, DateTime last,
      CalendarFormat format) {
  }

  @override
  Widget build(BuildContext context) {
    final eventProvider = Provider.of<EventProvider>(context);
    FirebaseFirestore _db = FirebaseFirestore.instance;

    return Scaffold(
      appBar: AppBar(
        automaticallyImplyLeading: false,
        title: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Image.asset('assets/images/Appbar_logo.png',
                fit: BoxFit.cover, height: 56),
          ],
        ),
      ),
      backgroundColor: Colors.white,
      resizeToAvoidBottomInset: false,
      body: Column(
        mainAxisSize: MainAxisSize.max,
        children: <Widget>[
          StreamBuilder(
            stream: _db.collection('agency').doc(globals.agencyId).collection('event')
              .where('eventDate', isGreaterThanOrEqualTo: kFirstDay)
              .where('eventDate', isLessThanOrEqualTo: kLastDay)
                    .snapshots().map((snapshot) => snapshot.docs
              .map((document) => Event.fromFirestore(document.data()))
              .toList()),
            builder: (context, AsyncSnapshot <List<Event>> eventsSnapShot) {
              if (eventsSnapShot.hasData) {

                return _buildTableCalendar();
              } else {
                return CircularProgressIndicator();
              }
            },
          ),
          const SizedBox(height: 8.0),
          //_buildButtons(),
          ElevatedButton(
            onPressed: () async {
              setState(() {
                showSpinner = true;
              });
              try {
                globals.newAgency = true;

                Navigator.of(context).pushReplacement(MaterialPageRoute(
                    builder: (context) => AddEventScreen()));

                setState(() {
                  showSpinner = false;
                });
              } catch (e) {
                // todo: add better error handling
                print(e);
              }
            },
            child: Text('Add Event'),
          ),
          const SizedBox(height: 8.0),
          Expanded(child: _buildEventList()),
        ],
      ),
    );
  }

  // Simple TableCalendar configuration (using Styles)
  Widget _buildTableCalendar() {
    return TableCalendar(
      firstDay: kFirstDay,
      lastDay: kLastDay,
      focusedDay: _focusedDay,
      selectedDayPredicate: (day) => isSameDay(_selectedDay, day),
      locale: 'en_US',
      eventLoader: (day) {
        return _getEventsForDay(day);
      },
      startingDayOfWeek: StartingDayOfWeek.sunday,
      calendarStyle: CalendarStyle(
        isTodayHighlighted: true,
        selectedDecoration: BoxDecoration(color: Colors.deepOrange[400]),
        todayDecoration: BoxDecoration(color: Colors.deepOrange[200]),
        markerDecoration: BoxDecoration(color: Colors.deepPurpleAccent),
        outsideDaysVisible: false,
      ),
      headerStyle: HeaderStyle(
        formatButtonTextStyle:
        TextStyle().copyWith(color: Colors.white, fontSize: 15.0),
        formatButtonDecoration: BoxDecoration(
          color: Colors.deepOrange[400],
          borderRadius: BorderRadius.circular(16.0),
        ),
      ),
      onDaySelected: (selectedDay, focusedDay) {
        setState(() {
          _selectedDay = selectedDay;
          _focusedDay = focusedDay; // update `_focusedDay` here as well
        });
      },
      onPageChanged: (focusedDay) {
        _focusedDay = focusedDay;
      },
    );
  }

  Widget _buildHolidaysMarker() {
    return Icon(
      Icons.add_box,
      size: 20.0,
      color: Colors.blueGrey[800],
    );
  }

  Widget _buildEventList() {
    final _db = FirebaseFirestore.instance;

    return Container(
      child: StreamBuilder<QuerySnapshot>(
        //stream: FirestoreService().getEventStream(_focusedDay),
        stream: _db.collection('agency').doc(globals.agencyId).collection(
            'event').where('eventDate', isEqualTo: _focusedDay).snapshots(),
        builder: (context, snapshot) {
          if (!snapshot.hasData) {
            return Center(
                child: const Text(
                  'Loading...',
                  style: TextStyle(
                      fontSize: 20, fontWeight: FontWeight.bold),
                ));
          } else {
            var doc = snapshot.data!.docs;
            return new ListView.builder(
                itemCount: doc.length,
                itemBuilder: (BuildContext context, int index) {
                  Event _event = Event.fromFirestore(
                      doc[index].data() as Map<String, dynamic>);
                  return ListTile(
                    isThreeLine: true,
                    title: Text(
                      '${_event.eventName ?? 'n/a'}',
                      style: TextStyle(
                          fontWeight: FontWeight.w900,
                          color: Colors.blueAccent),
                    ),
                    subtitle: Text.rich(TextSpan(
                        text:
                        '${DateFormat('EE MM-dd-yyyy').format(_event.eventDate!) ?? 'n/a'}\n'
                        '${DateFormat('h:mm a').format(_event.eventStartTime!) ?? 'n/a'}, '
                        'Duration: ${_event.eventDuration ?? 'n/a'} minutes',
                        children: <TextSpan>[
                          TextSpan(
                            text:
                            '\n${_event.eventDescription ?? 'n/a'}',
                            style: TextStyle(
                                fontWeight: FontWeight.w900,
                                color: Colors.blueGrey),
                          )
                        ])),
                    //trailing: Text('MLS#: ${_event.mlsNumber ?? 'n/a'}'),
                    onTap: () {
                      globals.newTrxn = false;
                      /*
                    Navigator.of(context).push(MaterialPageRoute(
                        builder: (context) =>
                            AddEventScreen(
                                doc[index].data())));

                     */
                    },
                  );
                }
            );
          }
        },

      ),
    );
  }

  Map<DateTime, List>? convertToMap(List<Event> item) {
    Map<DateTime, List>? result;

    for (int i = 0; i < item.length; i++) {
      Event data = item[i];
      //get the date and convert it to a DateTime variable
      //DateTime currentDate = DateFormat('MM-dd-yyyy - kk:mm').format(data.eventDate);
      DateTime currentDate = DateTime(data.eventDate!.year, data.eventDate!.month, data.eventDate!.day, data.eventDate!.hour, data.eventDate!.minute);

      List eventNames = [];
      //add the event name to the the eventNames list for the current date.
      //search for another event with the same date and populate the eventNames List.
      for (int j = 0; j < item.length; j++) {
        //create temp calendarItemData object.
        Event temp = item[j];
        //establish that the temp date is equal to the current date
        if (data.eventDate == temp.eventDate) {
          //add the event name to the event List.
          eventNames.add(temp.eventName);
        } //else continue
      }

      //add the date and the event to the map if the date is not contained in the map
      if (result == null) {
        result = {
          currentDate: eventNames
        };
      } else {
        result[currentDate] = eventNames;
      }
      return result;
    }
  }
}

Answer 1

如果您的列表是Event类型，那么您应该返回List<Event>? 而不是List<dynamic>? 来自_getEventsForDay函数。 希望这能解决您的问题。

编辑：

我的错。 我已经阅读了文档，它说，您可以将 eventloader 的值指定为 null 或返回类型应为非空列表的函数。

那么，在您的情况下，您要返回一个列表？ 这意味着函数返回类型可以为空。 这就是为什么你会出错。

List<Event> _getEventsForDay(DateTime day) {
    // kEvents is a linkedHashMap
    for (int i = 0; i < eventDoc.length; i++ ) {
      DateTime eventDate = eventDoc[i].eventDate;
      DateTime eventDateUTC = eventDate.toUtc();
      if (day.year == eventDate.year && day.day == eventDate.day && day.month == eventDate.month) {
        List<dynamic> eventList = [];
        eventList.add(eventDoc[i].agencyId);
        eventList.add(eventDoc[i].agentId);
        eventList.add(eventDoc[i].eventDate);
        eventList.add(eventDoc[i].eventDescription);
        eventList.add(eventDoc[i].eventDuration);
        eventList.add(eventDoc[i].eventName);
        eventList.add(eventDoc[i].eventStartTime);
        //print('kEvents: $kEvents');
        return kEvents.putIfAbsent(eventDateUTC, () => eventList);
      }
    }
  }

第二个问题：

现在这个错误告诉kEvents.putIfAbsent(eventDateUTC, () => eventList); 可能返回空值。 这是不可接受的，因为我们的函数现在不会返回任何null值。 在这种情况下，我们可以像这样使用空检查运算符。

return (kEvents.putIfAbsent(eventDateUTC, () => eventList))??[];

希望这能解决您的问题。

Answer 2

  List<Event> _getEventsForDay(DateTime day) {
    // kEvents is a linkedHashMap
    for (int i = 0; i < eventDoc.length; i++ ) {
      DateTime eventDate = eventDoc[i].eventDate;
      DateTime eventDateUTC = eventDate.toUtc();
      if (day.year == eventDate.year && day.day == eventDate.day && day.month == eventDate.month) {
        List<dynamic> eventList = [];
        eventList.add(eventDoc[i].agencyId);
        eventList.add(eventDoc[i].agentId);
        eventList.add(eventDoc[i].eventDate);
        eventList.add(eventDoc[i].eventDescription);
        eventList.add(eventDoc[i].eventDuration);
        eventList.add(eventDoc[i].eventName);
        eventList.add(eventDoc[i].eventStartTime);
        //print('kEvents: $kEvents');
        return kEvents.putIfAbsent(eventDateUTC, () => eventList);
      }
    }
  }

由于您的for内部有一个if子句，并且eventDoc.length可以为空，因此您的函数可能永远不会到达return语句：

return kEvents.putIfAbsent(eventDateUTC, () => eventList);

所以 IDE 告诉的是你需要在函数的末尾添加一个return ，这样它就可以返回null，如果eventDoc.length < 1或者if子句永远不会被满足。

要解决您的问题，请添加：

return [];

在for循环之后，在函数的末尾：

List<Event> _getEventsForDay(DateTime day) {
    // kEvents is a linkedHashMap
    for (int i = 0; i < eventDoc.length; i++ ) {
      DateTime eventDate = eventDoc[i].eventDate;
      DateTime eventDateUTC = eventDate.toUtc();
      if (day.year == eventDate.year && day.day == eventDate.day && day.month == eventDate.month) {
        List<dynamic> eventList = [];
        eventList.add(eventDoc[i].agencyId);
        eventList.add(eventDoc[i].agentId);
        eventList.add(eventDoc[i].eventDate);
        eventList.add(eventDoc[i].eventDescription);
        eventList.add(eventDoc[i].eventDuration);
        eventList.add(eventDoc[i].eventName);
        eventList.add(eventDoc[i].eventStartTime);
        //print('kEvents: $kEvents');
        return kEvents.putIfAbsent(eventDateUTC, () => eventList);
      }
    }

    return [];
  }