可选参数的默认值必须是常量

Posted 2023-02-23

【中文标题】可选参数的默认值必须是常量

【英文标题】：The default value of an optional parameter must be constant

【发布时间】：2020-09-09 09:48:24

【问题描述】：

所以我创建了这个事件跟踪器应用程序，我有两个屏幕，分别是地图和事件列表。 我正在尝试使位置列表与我在应用程序状态中的位置相同。请记住，placeList 是一个可修改的列表，我需要将地点添加到此列表中。

但是，每当我初始化 this.places=PlaceMapState.placeList 时，我都会收到“”，并且我无法将其更改为常量，因为我需要它来更新 PlaceMapState 类中的位置列表，并且我无法从 AppState 中删除它，因为我在 PlaceList 类中使用它来获取位置作为列表。

我也不想完全删除 AppState，因为它还包含地图。

请问有什么解决办法吗？？？

这是我使用此列表的班级：

class AppState 
   AppState(
    this.places = PlaceMapState.placeList,           //// here is the problem!!!!!!!!!!!!!!!!!!!
    this.selectedCategory = PlaceCategory.events,
    this.viewType = PlaceTrackerViewType.map,
   ) : //assert(places != null),
        assert(selectedCategory != null);

   List<Place> places;
   PlaceCategory selectedCategory;
   PlaceTrackerViewType viewType;

  AppState copyWith(
    List<Place> places,
    PlaceCategory selectedCategory,
    PlaceTrackerViewType viewType,
  ) 
    return AppState(

      selectedCategory: selectedCategory ?? this.selectedCategory,
      viewType: viewType ?? this.viewType,
    );
  

  static AppState of(BuildContext context) => AppModel.of<AppState>(context);

  static void update(BuildContext context, AppState newState) 
    AppModel.update<AppState>(context, newState);
  

  static void updateWith(
    BuildContext context, 
    List<Place> places,
    PlaceCategory selectedCategory,
    PlaceTrackerViewType viewType,
  ) 
    update(
      context,
      AppState.of(context).copyWith(
        places: places,
        selectedCategory: selectedCategory,
        viewType: viewType,
      ),
    );
  

  @override
  bool operator ==(Object other) 
    if (identical(this, other)) return true;
    if (other.runtimeType != runtimeType) return false;
    return other is AppState &&
        other.places == places &&
        other.selectedCategory == selectedCategory &&
        other.viewType == viewType;
  

  @override
  int get hashCode => hashValues(places, selectedCategory, viewType);

这里是 placeList 类，我使用地点来获取列表：

class PlaceList extends StatefulWidget 
  const PlaceList(Key key) : super(key: key);

  @override
  PlaceListState createState() => PlaceListState();


class PlaceListState extends State<PlaceList> 
  ScrollController _scrollController = ScrollController();

  void _onCategoryChanged(PlaceCategory newCategory) 
    _scrollController.jumpTo(0.0);
    AppState.updateWith(context, selectedCategory: newCategory);
  

  void _onPlaceChanged(Place value) 
    // Replace the place with the modified version.
    final newPlaces = List<Place>.from(AppState.of(context).places);
    final index = newPlaces.indexWhere((place) => place.id == value.id);
    newPlaces[index] = value;

    AppState.updateWith(context, places: newPlaces);
  

  @override
  Widget build(BuildContext context) 
    return Column(
      children: <Widget>[
        _ListCategoryButtonBar(
          selectedCategory: AppState.of(context).selectedCategory,
          onCategoryChanged: (value) => _onCategoryChanged(value),
        ),
        Expanded(
          child: ListView(
            padding: const EdgeInsets.fromLTRB(16.0, 0.0, 16.0, 8.0),
            controller: _scrollController,
            shrinkWrap: true,
            children: AppState.of(context)
                .places                  //this the places im talking about!!!!!!!!!!!!!!!!!
                .where((place) =>
                    place.category == AppState.of(context).selectedCategory)
                .map((place) => _PlaceListTile(
                      place: place,
                      onPlaceChanged: (value) => _onPlaceChanged(value),
                    ))
                .toList(),
          ),
        ),
      ],
    );
  

【参考方案1】：

要求默认参数的常量值的常见解决方法是使用 sentinel 参数作为默认值，可以为const。通常，该哨兵参数可以是null：

class AppState 
  AppState(
    List<Place>? places,
    ...
  ) : places = places ?? PlaceMapState.placeList;

  List<Place> places;

  ...