Decompose different user subfeatures into mixins with freezed.

user_subclasses_with_mixin_components.dart

import 'package:freezed_annotation/freezed_annotation.dart';

part 'user.freezed.dart';
abstract class IUserBase {
  int? get id;
}

enum UserStatus {
  normal,
  pro,
  banned,
}

abstract class IUserStatus {
  UserStatus get status;
}

extension ProExtension on IUserBase {
  bool get isPro => this is IUserStatus &&
    (this as IUserStatus).status == UserStatus.pro;
}

mixin ProMixin implements IUserStatus {
  bool get isPro => status == UserStatus.pro;
}

@freezed
class UserBase with _$UserBase implements IUserBase {
  const factory UserBase({
    required int? id,
  }) = _UserBase;
}

@freezed
class User with _$User implements IUserBase, IUserStatus {
  @With<ProMixin>()
  const factory User({
    required int? id,
    required UserStatus status,
  }) = _User;
}

void main() {
  /// Suppose we have a widespread widget depending on user subclass,
  /// which can implement different interfaces, based on those we might want to
  /// show userStatus badge.
  /// Then we might write something like this:
  /// ```dart
  /// Widget build(BuildContext context) {
  ///    return Column(
  ///      children: [
  ///        // ...
  ///        if (x is IUserStatus) StatusBadge(isPro: x.isPro),
  ///        // ...
  ///      ],
  ///    );
  ///  }
  /// ```
}