Comment ajouter une chargeuse à écran complet dans le flutter

Ici, j'essaie d'ajouter une chargeuse en plein écran, mais cela ne fonctionne pas, j'ai déjà terminé le code mais quand j'essaie d'ajouter une chargeuse en plein écran, elle ne fonctionnera pas. Alors, ici, je veux simplement ajouter le chargeur de plein écran lorsque je clique sur le bouton de connexion. Dans ce code, j'ai déjà défini la variable _isloader lorsque son chargeur vrai sera affiché.

Voici le code que j'ai essayé. P>

class LoginScreen extends StatefulWidget {
  LoginScreen({Key key}) : super(key: key);

  @override
  _LoginScreenState createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  @override
  Widget build(BuildContext context) {
    final formKey = new GlobalKey<FormState>();
    final _emailFocusNode = new FocusNode();
    final _passwordFocusNode = new FocusNode();

    String _username;
    String _password;
    bool rememberMe = false;
    bool _isLoading = false;

    @override
    void initState() {
      super.initState();
    }

    void _showErrorDialog(String message) {
      showDialog(
          barrierDismissible: false,
          context: context,
          builder: (context) => ShowErrorDialog(
                title: Text('An Error Occurred!'),
                content: Text(message),
              ));
    }

    Widget _buildUserNameField() {
      return EnsureVisibleWhenFocused(
        focusNode: _emailFocusNode,
        child: TudoEmailWidget(
          focusNode: _emailFocusNode,
          prefixIcon: Icon(Icons.email),
          labelText: AppConstantsValue.appConst['login']['email']
              ['translation'],
          validator: (val) => Validators.validateEmail(val.trim()),
          onSaved: (val) => _username = val.trim(),
          // onChanged:(val) => _username = val.trim(),
        ),
      );
    }

    Widget _buildPasswordField() {
      return EnsureVisibleWhenFocused(
        focusNode: _passwordFocusNode,
        child: TudoPasswordWidget(
          focusNode: _passwordFocusNode,
          prefixIcon: Icon(Icons.vpn_key),
          hintText: AppConstantsValue.appConst['login']['password']
              ['translation'],
          labelText: AppConstantsValue.appConst['login']['password']
              ['translation'],
          validator: Validators().validatePassword,
          onSaved: (val) => _password = val.trim(),
        ),
      );
    }

    Widget _buildLoginButton(BuildContext context, LoginViewModel loginVm) {
      return GestureDetector(
        child: TudoLoginButtonWidget.buildRoundedRectButton(
            "Log In", signInGradients, false),
        onTap: () async {
          if (!formKey.currentState.validate()) {
            // Invalid!
            return;
          }
          formKey.currentState.save();
          print("User");

          setState(() {
            _isLoading = true;
          });
          try {
            LoginRepository _loginRepository = LoginRepository();
            Map<String, dynamic> loginResponse =
                await _loginRepository.loginUser(_username, _password);
            if (loginResponse['error'] != null) {
              var errorMessage = 'Invalid email or password';
              _showErrorDialog(errorMessage);
            } else {
              LoginUser userModel = LoginUser(
                token: loginResponse['data']['loginUser']['token'],
                user: User.fromJson(
                  loginResponse['data']['loginUser']['user'],
                ),
              );
              SharedPreferences preferences =
                  await SharedPreferences.getInstance();
              preferences.setString('user', loginUserToJson(userModel));
              loginVm.loginMe(context, userModel);
            }
            setState(() {
              _isLoading = false;
            });
          } catch (error) {
            print('error');
            print(error);
            setState(() {
              _isLoading = false;
            });
            var errorMessage = 'Authentication failed';
            _showErrorDialog(errorMessage);
          }
        },
      );
    }


    Widget content(context, loginVm) {
      ProgressDialog pr =
          new ProgressDialog(context, type: ProgressDialogType.Normal);
      pr.style(message: 'Showing some progress...');
      return new SafeArea(
        top: false,
        bottom: false,
        child: Form(
          key: formKey,
          child: Scrollbar(
            child: SingleChildScrollView(
              dragStartBehavior: DragStartBehavior.down,
              padding: const EdgeInsets.symmetric(horizontal: 16.0),
              child: new Container(
                margin: EdgeInsets.fromLTRB(30, 100, 30, 0),
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  crossAxisAlignment: CrossAxisAlignment.center,
                  children: <Widget>[
                    TudoLogoWidget(),
                    _buildUserNameField(),
                    SizedBox(
                      height: 20,
                    ),
                    _buildPasswordField(),

                    SizedBox(
                      height: 20.0,
                    ),
                    _buildLoginButton(context, loginVm),
                    SizedBox(
                      height: 20,
                    ),

                  ],
                ),
              ),
            ),
          ),
        ),
      );
    }

    return new WillPopScope(
      onWillPop: () =>
          SystemChannels.platform.invokeMethod('SystemNavigator.pop'),
      child: Scaffold(
        body: Container(
          height: double.infinity,
          width: double.infinity,
          child: Stack(
            children: <Widget>[
              Background(),
              SingleChildScrollView(
                  child: StoreConnector<AppState, LoginViewModel>(
                converter: (Store<AppState> store) =>
                    LoginViewModel.fromStore(store),
                builder: (BuildContext context, LoginViewModel loginVm) =>
                    content(context, loginVm),
              )),
            ],
          ),
        ),
      ),
    );
  }
}

class LoginViewModel {
  final Function(BuildContext context, LoginUser loginUser) loginMe;
  LoginViewModel({
    this.loginMe,
  });

  static LoginViewModel fromStore(Store<AppState> store) {
    return LoginViewModel(
      loginMe: (context, loginUser) {
        store.dispatch(
          login(context, loginUser),
        );
      },
    );
  }
}