state.dart

abstract class State {
  bool isAuthorized({Context context});
  String userId({Context context});
}

class Context {
  State _state = new UnauthorizedState();

  bool get isAuthorized => _state.isAuthorized();

  String get userId => _state.userId();

  void changeStateToAuthorized({String userId}) {
    _state = new AuthorizedState(userId: userId);
  }

  void changeStateToUnAuthorized() {
    _state = new UnauthorizedState();
  }
}

class UnauthorizedState extends State {
  @override
  bool isAuthorized({Context context}) => false;

  @override
  String userId({Context context}) {
    return null;
  }
}

class AuthorizedState extends State {
  String _userId = null;
  AuthorizedState({String userId}) : this._userId = userId;

  @override
  bool isAuthorized({Context context}) => true;

  @override
  String userId({Context context}) {
    return _userId;
  }
}


main(List<String> arguments) {
  final userContext = new Context();
  print(userContext.isAuthorized);
  print(userContext.userId);
  userContext.changeStateToAuthorized(userId: "admin");
  print(userContext.isAuthorized);
  print(userContext.userId);
  userContext.changeStateToUnAuthorized();
  print(userContext.isAuthorized);
  print(userContext.userId);
}