我有 3 个从迁移到 null-safety 的问题

Question

我今天迁移到 null-safety，但有 3 个问题我不知道如何解决。下面，我提供了错误和每个错误所指的代码。我不知道如何解决这些问题，它们是唯一剩下的错误。

error • The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type at lib\Services\firestore_service.dart:29:16 • (body_might_complete_normally)

Future<Void> saveNewAgency(Agency agency) async {
    await _db.collection('agency').doc(globals.agencyId).set(agency.toMap());
  }

      error • The property 'docs' can't be unconditionally accessed because the receiver can be 'null' at lib\screens\agent_dash_board.dart:57:43 •(unchecked_use_of_nullable_value)
I fixed similar errors to this one with the code at itemCount: (snapshot.data! as QuerySnapshot).docs.length,  in the code snippet. 
    
    return new ListView.builder(
                          itemCount: (snapshot.data! as QuerySnapshot).docs.length,
                          itemBuilder: (BuildContext context, int index) {
                            Trxns trxns = Trxns.fromFirestore(
                                snapshot.data.docs[index].data());

  error • The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type at lib\screens\appointment_calendar.dart:434:31 • (body_might_complete_normally)

Map<DateTime, List<Event?>> 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 as Map<DateTime, List<Event?>>;
    }
  }

Answer 1

Future<Void> saveNewAgency(Agency agency) async {
  await _db.collection('agency').doc(globals.agencyId).set(agency.toMap());
}

尝试返回 Future<void> 我不确定您从哪里获得带有大写 V 的 Void。

---

    return new ListView.builder(
                          itemCount: (snapshot.data! as QuerySnapshot).docs.length,
                          itemBuilder: (BuildContext context, int index) {
                            Trxns trxns = Trxns.fromFirestore(
                                snapshot.data.docs[index].data());

下面讨论的解决方案是这样做：

Trxns trxns = Trxns.fromFirestore((snapshot.data! as QuerySnapshot).docs[index]!.data() as Map<String, dynamic>);

---

Map<DateTime, List<Event?>> 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 as Map<DateTime, List<Event?>>;
    }
  }

我认为你的意思是把 return 语句放在 for 循环之后，而不是放在里面。