Dart Generic Workaround

main.dart

class FirestoreServiceMock {}

abstract class IModel {
  String _uid;

  Map<String, dynamic> toJson();

  fromJson(Map<String, dynamic> json);

  String get uid => _uid;

  set uid(String value) {
    _uid = value;
  }
}

class UserModel implements IModel {
  @override
  String uid;

  String _name;
  String _email;

  @override
  Map<String, dynamic> toJson() => {"name": _name, "email": _email};

  @override
  fromJson(Map<String, dynamic> json) {
    uid = json["uid"];
    _name = json["name"];
    _email = json["email"];
  }

  String get email => _email;

  set email(String value) {
    _email = value;
  }

  String get name => _name;

  set name(String value) {
    _name = value;
  }

  @override
  String _uid;
}

typedef S ObjectCreator<S>();

class BaseService<T extends IModel> {
  BaseService(this.creator);
  final ObjectCreator<T> creator;

  T getById(String id) {
    final t = creator();

    final fakeData = {
      'uid': '5',
      'name': 'Maria',
      'email': '[email protected]',
    };

    t.fromJson(fakeData);
    return t;
  }
}

void main() {
  var b = BaseService<UserModel>(() => UserModel());
  var user = b.getById('1');
  print('Id: ${user.uid} Usuario: ${user.name} Email: ${user.email}');
  //
}