freezed how to assign my own JsonConverter on top level model?

Question

I have freezed model (simplified):

part 'initial_data_model.freezed.dart';
part 'initial_data_model.g.dart';

@freezed
class InitialDataModel with _$InitialDataModel {
  const factory InitialDataModel() = Data;

  const factory InitialDataModel.loading() = Loading;

  const factory InitialDataModel.error([String? message]) = Error;

  factory InitialDataModel.fromJson(Map<String, dynamic> json) => _$InitialDataModelFromJson(json);
}

documentation says how to assign custom converters on fields but not on model itself

I got json from backend and somewhere in api_provider I do
return InitialDataModel.fromJson(json);
I have no control on json structure, there aren't "runtimeType" and other stupid redundant things

when I want to create a model from json I call fromJson I'm having this

flutter: CheckedFromJsonException
Could not create `InitialDataModel`.
There is a problem with "runtimeType".
Invalid union type "null"!

ok, again
I have api_provider

final apiProvider = Provider<_ApiProvider>((ref) => _ApiProvider(ref.read));

class _ApiProvider {
  final Reader read;

  _ApiProvider(this.read);

  Future<InitialDataModel> fetchInitialData() async {
    final result = await read(repositoryProvider).send('/initial_data');
    return result.when(
      (json) => InitialDataModel.fromJson(json),
      error: (e) => InitialDataModel.error(e),
    );
  }
}

you may see I'm trying to create InitialDataModel from json

this line throws an error I mentioned above

I don't understand how to create InitialDataModel from json, now in my example it's just empty model, there are no fields

(json) => InitialDataModel.fromJson(json),
json here is Map, it shows an error even if I pass simple empty map {} instead of real json object

Answer 1

The easiest solution is to use the correct constructor instead of _$InitialDataModelFromJson. Example:

@freezed
class InitialDataModel with _$InitialDataModel {
  const factory InitialDataModel() = Data;

  ...

  factory InitialDataModel.fromJson(Map<String, dynamic> json) => Data.fromJson(json);
}

The drawback of course is that you can only use the fromJson when you're sure you have the correct json, which isn't great. I actually wouldn't recommend this way because it leaves to the caller the burden of checking the validity and calling the correct constructor.

---

Another solution, maybe the best, is to follow the documentation and create a custom converter, even though this would require you to have two separated classes.

---

Otherwise you could chose a different approach and separate the data class from the union, so you'll have a union used just for the state of the request and a data class for the success response:

@freezed
class InitialDataModel with _$InitialDataModel {
  factory InitialDataModel(/* here go your attributes */) = Data;

  factory InitialDataModel.fromJson(Map<String, dynamic> json) => _$InitialDataModelFromJson(json);
}

@freezed
class Status with _$Status {
  const factory Status.success(InitialDataModel model) = Data;
  const factory Status.loading() = Loading;
  const factory Status.error([String? message]) = Error;
}

and then

[...]
    return result.when(
      (json) => Status.success(InitialDataModel.fromJson(json)),
      error: (e) => Status.error(e),
    );
[...]