Как проверить форму при отправке во флаттере с помощью flutter_bloc?

Это мой экран изменения пароля. Я использовал flutter_bloc для реализации шаблона mvvc. Эта страница отлично работает с bloc. Но то, что я пытаюсь достичь, - это проверить форму при отправке формы. Поскольку я был новичком в трепетании, я понятия не имею, как это сделать.

Событие смены пароля

abstract class ChangePasswordEvent extends Equatable {
  const ChangePasswordEvent();
}

class SubmitButtonPressed extends ChangePasswordEvent {
  final String oldPassword;
  final String newPassword;

  const SubmitButtonPressed({@required this.oldPassword, this.newPassword});

  @override
  List<Object> get props => [oldPassword, newPassword];
}

Изменить состояние пароля

abstract class ChangePasswordState extends Equatable {
  const ChangePasswordState();

  @override
  List<Object> get props => [];
}

class ChangePasswordInitial extends ChangePasswordState {}

class ChangePasswordLoading extends ChangePasswordState {}

class ChangePasswordSuccess extends ChangePasswordState {}

class ChangePasswordFailure extends ChangePasswordState {
  final String error;

  const ChangePasswordFailure({@required this.error});

  @override
  List<Object> get props => [error];

  @override
  String toString() => 'ChangePasswordFailure { error: $error }';
}

Блок изменения паролей

class ChangePasswordBloc
    extends Bloc<ChangePasswordEvent, ChangePasswordState> {
  final UserRepository userRepository;

  ChangePasswordBloc({
    @required this.userRepository,
  }) : assert(userRepository != null);

  @override
  ChangePasswordState get initialState => ChangePasswordInitial();

  @override
  Stream<ChangePasswordState> mapEventToState(
      ChangePasswordEvent event) async* {
    if (event is SubmitButtonPressed) {
      yield ChangePasswordLoading();

      try {
        final bool isPasswordChanged = await userRepository.changePassword(
          event.oldPassword,
          event.newPassword,
        );

        if (isPasswordChanged) {
          yield ChangePasswordSuccess();
        }
      } catch (error) {
        yield ChangePasswordFailure(error: error);
      }
    }
  }
}

Страница изменения пароля

class ChangePasswordPage extends StatelessWidget {
  final UserRepository userRepository;

  ChangePasswordPage({Key key, @required this.userRepository})
      : assert(userRepository != null),
        super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Change Password'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(20.0),
        child: BlocProvider(
          create: (context) {
            return ChangePasswordBloc(
              userRepository: userRepository,
            );
          },
          child: ChangePasswordForm(),
        ),
      ),
    );
  }
}

Форма изменения пароля

class ChangePasswordForm extends StatefulWidget {
  @override
  _ChangePasswordFormState createState() => _ChangePasswordFormState();
}

class _ChangePasswordFormState extends State<ChangePasswordForm> {
  final userRepository = UserRepository();
  final _formKey = GlobalKey<FormState>();

  final _oldPassController = TextEditingController();
  final _newPassController = TextEditingController();
  final _confirmPassController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    _onSubmitButtonPressed() {
      BlocProvider.of<ChangePasswordBloc>(context).add(
        SubmitButtonPressed(
          oldPassword: _oldPassController.text,
          newPassword: _newPassController.text,
        ),
      );
    }

    return BlocListener<ChangePasswordBloc, ChangePasswordState>(
      listener: (context, state) {
        if (state is ChangePasswordFailure) {
          Scaffold.of(context).showSnackBar(
            SnackBar(
              content: Text('${state.error}'),
              backgroundColor: Colors.red,
            ),
          );
        }

        if (state is ChangePasswordSuccess) {
          Scaffold.of(context).showSnackBar(
            SnackBar(
              content: Text('Password Changed Successfully'),
              backgroundColor: Colors.green,
            ),
          );
        }
      },
      child: BlocBuilder<ChangePasswordBloc, ChangePasswordState>(
        builder: (context, state) {
          return Form(
            key: _formKey,
            child: Column(
              children: [
                TextFormField(
                  decoration: InputDecoration(labelText: 'Old Password'),
                  controller: _oldPassController,
                ),
                SizedBox(height: 20.0),
                TextFormField(
                  decoration: InputDecoration(labelText: 'New Password'),
                  controller: _newPassController,
                  obscureText: true,
                ),
                SizedBox(height: 20.0),
                TextFormField(
                  decoration: InputDecoration(labelText: 'Confirm Password'),
                  controller: _confirmPassController,
                  obscureText: true,
                  validator: (value) {
                    final String _newPassword = _newPassController.text;

                    if (_newPassword != value) {
                      return "Password Mismatch";
                    }

                    return null;
                  },
                ),
                SizedBox(height: 20.0),
                RaisedButton(
                  onPressed: () {
                    if (state is! ChangePasswordLoading) {
                      final form = _formKey.currentState;

                      if (form.validate()) {
                        return _onSubmitButtonPressed();
                      }

                      return null;
                    }
                  },
                  child: Text('Submit'),
                ),
              ],
            ),
          );
        },
      ),
    );
  }
}