Discussion :

[JockdiL]

Hello,
i am new in flutter/dart and i'm trying to build an app with an observer to redirect user for sign in if it wasn't done

Here is my code

Code :

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 class ControlView extends GetWidget<AuthViewModel>{

 

    @override
      Widget build(BuildContext context) {

    ControlViewModel ctrlvwmdl = ControlViewModel();

    return Obx(() {
      return (Get.find<AuthViewModel>().user == null)
          ? LoginView()
          : GetBuilder<ControlViewModel>(
              builder: (controller) => Scaffold(
                body: controller.currentScreen,
                bottomNavigationBar: bottomNavigationBar(),
        ),
      );
    });
    }

    Widget bottomNavigationBar() {
        return GetBuilder<ControlViewModel>(
          init: ControlViewModel(),
          builder: (controller) =>  BottomNavigationBar(
                items: [
                  BottomNavigationBarItem(
                      icon: Icon(Icons.search), label: 'search'.tr),
                  BottomNavigationBarItem(
                      icon: Icon(Icons.home_outlined), label: 'reserve'.tr),
                  BottomNavigationBarItem(
                      icon: Icon(Icons.add), label: 'Déposer'),
                  BottomNavigationBarItem(
                      icon: Icon(Icons.email_outlined), label: 'Messages'),
                  BottomNavigationBarItem(
                      icon: Icon(Icons.person_outline), label: 'Profil'),
                ],
                currentIndex: controller.navigatorValue,
                onTap: (index) {
                  controller.changeSelectedValue(index);
                },
                //elevation: 0,
                //selectedItemColor: Colors.black,
                type: BottomNavigationBarType.fixed,
                backgroundColor: Colors.yellow,
                iconSize: 20,
              ),
        );
      }    
    }

My `AuthViewModel` class is below

Code :

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class AuthViewModel extends GetxController {
      FirebaseAuth _auth = FirebaseAuth.instance;
      String? email, password, name;
      Rxn<User> _firebaseUser = Rxn<User>();
    
      String? get user => _firebaseUser.value?.email;
    
      @override
      void onInit() {
        // TODO: implement onInit
        super.onInit();
        _firebaseUser.bindStream(_auth.authStateChanges());
      }
    
      @override
      void onReady() {
        // TODO: implement onReady
        super.onReady();
      }
    
      @override
      void onClose() {
        // TODO: implement onClose
        super.onClose();
      }
    
      void googleSignInMethod() async {
        print(">> Google Sign In");
        GoogleSignIn googleSignIn = GoogleSignIn(scopes: ['email']);
    
        final GoogleSignInAccount? googleUser = await googleSignIn.signIn();
        print(">> google user : $googleUser");
    
        GoogleSignInAuthentication googleSignInAuthentication =
            await googleUser!.authentication;
    
        final AuthCredential googleCredential = GoogleAuthProvider.credential(
          idToken: googleSignInAuthentication.idToken,
          accessToken: googleSignInAuthentication.accessToken,
        );
    
        await _auth.signInWithCredential(googleCredential).then((user) {
          print(">> app user : $user");
        });
      }
    
      void facebookSignInMethod() async {
        print(">> Facebook Sign In");
        FacebookLogin facebookSignIn = new FacebookLogin();
    
        final FacebookLoginResult result = await facebookSignIn.logIn(['email']);
        print(">> facebook result : $result");
    
        switch (result.status) {
          case FacebookLoginStatus.loggedIn:
            final FacebookAccessToken accessToken = result.accessToken;
            final facebookAuthCredential =
                FacebookAuthProvider.credential(accessToken.token);
    
            await _auth.signInWithCredential(facebookAuthCredential).then((user){
              print(">> app user : $user");
            });
            break;
          case FacebookLoginStatus.cancelledByUser:
            print(">> User cancelled login with FaceBook account");
            break;
          case FacebookLoginStatus.error:
            print(">> Error when login with FaceBook account");
            break;
        }
      }
    
      void signInWithEmailAndPassword() async {
        try {
          print(">> email $email");
          print(">> password $password");
          await _auth
              .signInWithEmailAndPassword(email: email!, password: password!)
              .then((value) {
            print(">> value $value");
          });
        } catch (err) {
          print(err.toString());
          Get.snackbar(
            'Error login account',
            err.toString(),
            colorText: Colors.black,
            snackPosition: SnackPosition.BOTTOM,
          );
        }
      }
    }

And here is my `main.dart`

Code :

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 void main() async {
      WidgetsFlutterBinding.ensureInitialized();
      await Firebase.initializeApp();
      runApp(MyApp());
    }
    
    class MyApp extends StatelessWidget {
    
      @override
      Widget build(BuildContext context) {
    
        return GetMaterialApp(
          title: 'Flutter App',
          theme: ThemeData(
            primarySwatch: Colors.blue,
            appBarTheme: AppBarTheme(
              color: Colors.orange
            ),
          ),
          initialBinding: Binding(),
          home: Scaffold(
            appBar: AppBar(
              title: Text('Training App'),
              centerTitle: true,
              actions: <Widget>[
                IconButton(
                  icon: Icon(
                    Icons.settings,
                    color: Colors.white,
                  ),
                  onPressed: () {
                    debugPrint('Settings button tapped');
                  },
                ),
                IconButton(
                  icon: Icon(
                    Icons.more_horiz,
                    color: Colors.white,
                  ),
                  onPressed: () {
                    debugPrint('Menu button tapped');
                  },
                )
              ],
            ),
            drawer: SideBar(),
            body: ControlView(),
          ),
          translations: Translate(),
          locale: Locale('fr'), //Default language
          fallbackLocale: Locale('fr'), //Default language in case of error
        );
      }
    }

I'm getting the strange behavior below:
- when i run the application for the first time, all is ok and i can login either with google sign in method, and the facebook sign in method and also with password and email sign in method
- when i ask the app to logout it does and all is ok
- since then, whenever i try to sign in again, it seem (after troubleshooting of course) that the app logs the user in successfully but when building the UI the exception below is thrown

> Another exception was thrown: Null check operator used on a null value

and I get this ugly screen:

[enter image description here][1]


[1]: https://i.stack.imgur.com/BJqmP.png

without signin out, i performed a hot reload and i got the same error with a new hint in addition, it says

> ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY╞═══════════════════════════════════════════
>
> The following _CastError was thrown building Obx(has builder, state:
> _ObxState#37426): Null check operator used on a null value
>
> The relevant error-causing widget was: Obx
> Obx:file:///C:/Users/yoozer/StudioProjects/swim_test/lib/view/control_view.dart:14:12

Here is the stack when the exception was thrown

The relevant error-causing widget was:
Obx Obx:file:///C:/Users/yoozer/StudioProjects/swim_test/lib/view/control_view.dart:14:12

> When the exception was thrown, this was the stack:
> #0 GetBuilderState.initState (package:get/get_state_manager/src/simple/get_state.dart:134:40)
> #1 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4805:57)
> #2 ComponentElement.mount (package:flutter/src/widgets/framework.dart:4638:5)
> #3 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3673:14)
> #4 Element.updateChild (package:flutter/src/widgets/framework.dart:3422:20)
> #5 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4690:16)
> #6 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4840:11)
> #7 Element.rebuild (package:flutter/src/widgets/framework.dart:4355:5)
> #8 StatefulElement.update (package:flutter/src/widgets/framework.dart:4872:5)
> #9 Element.updateChild (package:flutter/src/widgets/framework.dart:3412:15)
> #10 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4690:16)
> #11 GetWidgetCacheElement.performRebuild (package:get/get_state_manager/src/simple/get_widget_cache.dart:35:11)
> #12 Element.rebuild (package:flutter/src/widgets/framework.dart:4355:5)
> #13 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2620:33)
> #14 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:882:21)
> #15 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:319:5)
> #16 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1143:15)
> #17 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1080:9)
> #18 SchedulerBinding.scheduleWarmUpFrame.<anonymous closure> (package:flutter/src/scheduler/binding.dart:863:7) (elided 4 frames
> from class _RawReceivePortImpl, class _Timer, and dart:async-patch)

I spent long days troubleshooting the error and trying to understand what happen, I looked on the internet for a solution, a similar problem or a way to deeply inspect this behavior but i'm still stuck.

Any help will be welcome
I will be very grateful

[JockdiL]

i finally found the solution. Using the flutter dev tools for debugging, i noticed that the GetBuilder returned by the Obx((){}) inline function wasn't instantiated each time the error occurred. So i created a GetBuilder variable in the top of the build() method, and i instantiated it. Then in the Obx((){}) inline function i modified my code to return the object i instantiated above.

here is the only modification i made in the code, other classes and functions didn't change:

Code :

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class ControlView extends GetWidget<AuthViewModel>{

  @override
  Widget build(BuildContext context) {

    LoginView loginView = LoginView();
    //ControlViewModel ctrlvwmdl = ControlViewModel();
    GetBuilder<ControlViewModel> getbuilder = GetBuilder<ControlViewModel>(
      init: ControlViewModel(),
      builder: (controller) => Scaffold(
        body: controller.currentScreen,
        bottomNavigationBar: bottomNavigationBar(),
      ),
    );

    print(">> ControlView.build() : new call of the method.");
    return Obx(() {
      return (Get.find<AuthViewModel>().user == null)
          ? loginView
          : getbuilder;
    });
  }
}