The default value of an optional parameter must be constant - list

Question

So Im creating this Event Tracker app and I have two screens which are the map and the events list.
I am trying to get the place list to be equal to my places in my App state. Bare in mind that placeList is a modifiable list that I need to add places to this list.
However I am getting a "The default value of an optional parameter must be constant" whenever I initialize this.places=PlaceMapState.placeList and I cant change it to a constant since i need it to update my list of places in the PlaceMapState class and I cant remove it from AppState since I am using it in the PlaceList class to get the places as a list.
I also dont want to remove the AppState entirely because it also contains the map.
Please any solution to this???
Here is my class where I use this list:
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);
}
Here is the class of placeList where I use places to get a list:
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(),
),
),
],
);
}
}

Answer 1

A common workaround to requiring constant values for default arguments is to instead use a sentinel argument as the default that can be const. Typically that sentinel argument can be null:
class AppState {
AppState({
List<Place>? places,
...
}) : places = places ?? PlaceMapState.placeList;
List<Place> places;
...
}