Freezed - creating subclasses - flutter

Question

I have a JSON data structure from an API, which has some fields in it.
I want to create a Freezed model that has a factory to create the correct subclass, depending on one of the fields' type.
Here is my code:
abstract class ItineraryData<T> implements _$ItineraryData<T> {
const ItineraryData._();
const factory ItineraryData({
List<dynamic> summary,
double totalPrice,
double totalSeparatePay,
double totalTax,
List<String> covidAlerts,
}) = _ItineraryData;
const factory ItineraryData.flight({
List<FlightSummary> summary,
double totalPrice,
double totalSeparatePay,
double totalTax,
List<String> covidAlerts,
}) = _FlightItineraryData;
const factory ItineraryData.hotel({
List<HotelSummary> summary,
double totalPrice,
double totalSeparatePay,
double totalTax,
}) = _HotelItineraryData;
const factory ItineraryData.car({
List<CarSummary> summary,
double totalPrice,
double totalSeparatePay,
double totalTax,
List<String> covidAlerts,
}) = _CarItineraryData;
factory ItineraryData.fromJson(Map json) => _$ItineraryDataFromJson(json);
}
When it's this way, accessing a reference of ItineraryData<A> would never have a summary list field (which should always be there, but with different type).
Then I tried to change the unnamed constructor to use <T> in the List type:
const factory ItineraryData({
List<T> summary,
double totalPrice,
double totalSeparatePay,
double totalTax,
List<String> covidAlerts,
}) = _ItineraryData;
But then I get the following error:
Could not generate `fromJson` code for `summary` because of type `T` (type parameter).
None of the provided `TypeHelper` instances support the defined type.
To support type paramaters (generic types) you can:
1) Use `JsonConverter`
https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonConverter-class.html
2) Use `JsonKey` fields `fromJson` and `toJson`
https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/fromJson.html
https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/toJson.html
3) Set `JsonSerializable.genericArgumentFactories` to `true`
https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonSerializable/genericArgumentFactories.html
package:<???>/models/itinerary_data.freezed.dart:253:17
╷
253 │ final List<T> summary;
│ ^^^^^^^
╵
[INFO] Running build completed, took 221ms
I did try adding a converter, like so:
class ItineraryDataConverter
implements JsonConverter<ItineraryData, Map<String, dynamic>> {
const ItineraryDataConverter();
#override
ItineraryData fromJson(Map<String, dynamic> json) {
if (json == null || !json.containsKey('data')) {
return null;
}
json = json['data'];
// type data was already set (e.g. because we serialized it ourselves)
if (!json.keys
.any((k) => ['cabin_class', 'hotel', 'car_type'].contains(k))) {
return ItineraryData.fromJson(json);
}
// you need to find some condition to know which type it is. e.g. check the presence of some field in the json
if (json.containsKey('cabin_class')) {
return _FlightItineraryData.fromJson(json);
} else if (json.containsKey('hotel')) {
return _HotelItineraryData.fromJson(json);
} else if (json.containsKey('car_type')) {
return _CarItineraryData.fromJson(json);
} else {
throw Exception(
'Could not determine the constructor for mapping from JSON');
}
}
#override
Map<String, dynamic> toJson(ItineraryData data) => data.toJson();
}
But I still get the same error.
How can I make sure summary is visible + strongly typed from subclasses?
Or what would a better approach to this be?

Answer 1

Have you try adding custom converter for type T?
const factory ItineraryData({
#CustomConverter() List<T> summary,
...
...
class CustomConverter<T> implements JsonConverter<T, Object> {
const CustomConverter();
#override
T fromJson(Object json) {
// TODO: implement fromJson
throw UnimplementedError();
}
#override
Object toJson(T object) {
// TODO: implement toJson
throw UnimplementedError();
}
}