Using Crimson with a generic class

[simc]

Discussed in #8

^{Originally posted by Reprevise December 31, 2022}
I have a class like this:

class ApiResponse<T> extends Equatable {
  final List<CustomError>? errors;
  final T? data;
  const ApiResponse({
    this.errors,
    this.data,
  });

  T get dataOrThrow => data ?? (throw error!);

  CustomError? get error {
    // ...
  }

  factory ApiResponse.fromJson(
    Map<String, dynamic> map,
    T Function(Map<String, dynamic> json) create,
  ) {
    return ApiResponse<T>(
      errors: map['errors'] != null
          ? List<CustomError>.from(
              map['errors']?.map((x) => CustomError.fromJson(x)),
            )
          : null,
      data: map['data'] != null ? create(map['data']) : null,
    );
  }

  factory ApiResponse.fromJsonList(
    Map<String, dynamic> map,
    T Function(List<Map<String, dynamic>>) create, {
    List<dynamic> Function(Map<String, dynamic> json)? list,
  }) {
    T? getData() {
      if (map['data'] == null) return null;
      final items = List<Map?>.from(
        list?.call(map['data'] as Map<String, dynamic>) ??
            map['data'] as List<dynamic>,
      );
      return create(
        items.whereNotNull().map((e) => e as Map<String, dynamic>).toList(),
      );
    }

    return ApiResponse<T>(
      errors: map['errors'] != null
          ? List<CustomError>.from(
              map['errors'].map((x) => CustomError.fromJson(x)),
            )
          : null,
      data: getData(),
    );
  }

  @override
  List<Object?> get props => [errors, data];
}

I use it as so:

return ApiResponse.fromJsonList(
  response.data as Map<String, dynamic>,
  (json) => json.map(MyModel.fromJson).toList(),
  list: (json) => List<Map?>.from(json['items']),
).data!;

How can I do this with Crimson?

[simc]

This is not possible to support currently unfortuantely. The reason is that Crimson needs to know all possible types of T at compile time to generate optimized code.

Do you think something like this would be helpful:

// define possible types of T

@json
typedef StringApiResponse = ApiResponse<String>;

@json
typedef ListApiResponse = ApiResponse<List<String>>;

void main() {
  // use one of the generated decoders
  Crimson(bytes).readStringApiResponse();
}

Another option would be waiting for sealed class support in dart and requiring generic types to be subtypes of sealed classes. That would be problematic with String for example however.

What do you think @Reprevise?

[Reprevise]

For my case specifically, it's not like I'm giving it a type and expecting it to parse it on its own as I'm providing it the method that it can parse it with as you can see with my create callback passed into my fromJson factory method. So how it works with my usecase is that it would parse the errors field and then use the method returned from the create callback to parse the data field. The issue that I'm facing is that Crimson only parses things via bytes (which I presume is where it gets its performance from).

[simc]

Is there a reason you want to write the fromJson method yourself? It's not doing anything special and could be created by either Crimson or json_serializable.

[Reprevise]

If you're talking about ApiResponse's fromJson method, then yeah, it can be generated.