✨ Feature Request: Add freezed as an alternative to built_value in Dart/Dio & Dart generators

Background

Currently, the Dart and Dart-Dio generators in OpenAPI Generator use built_value
for model generation. While built_value is a solid library, it has some limitations and additional boilerplate requirements that make it less convenient for many Dart/Flutter developers.

The Flutter ecosystem has widely adopted freezed
as the preferred code generation library for immutable data classes, pattern matching, union types, and null-safety.

Supporting freezed alongside or as an alternative to built_value would significantly improve the developer experience and adoption of OpenAPI Generator in Dart/Flutter projects.

Why Freezed?

✅ Better integration with Flutter/Dart ecosystem: Freezed is now the standard for immutable classes in Dart/Flutter.

✅ Less boilerplate: Provides a cleaner API with annotations and code generation.

✅ Equatable built-in: Automatic == and hashCode generation without extra packages.

✅ JSON serialization: Seamless integration with json_serializable.

✅ Union/sealed classes: Helpful for handling API response states (success, error, etc.).

✅ Null-safety first: Fully compatible with Dart’s null safety.

Proposed Solution

Add a generator flag (similar to --use-built-value) to enable freezed models, e.g.:

openapi-generator-cli generate
-i openapi.yaml
-g dart-dio
-o ./lib/api
--additional-properties=useFreezed=true

Generated models should:

Use @freezed annotations.

Use @JsonSerializable() integration for JSON parsing.

Include part 'model.freezed.dart'; and part 'model.g.dart';.

Replace built_value style factories with Freezed constructors.

Example (instead of built_value style):

part 'user.freezed.dart';
part 'user.g.dart';

@freezed
class User with _$User {
  const factory User({
    required String id,
    String? name,
    String? email,
  }) = _User;

  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
}```

References

freezed package: https://pub.dev/packages/freezed



Would you accept a PR that adds freezed support as an option alongside built_value?

Start with the Dart and Dart-Dio generator entry points and the existing --use-built-value option. Compare current generated models with the proposed Freezed example, then verify that the CLI option, annotations, JSON serialization parts, constructors, and generated output meet the requested behavior.