I'm attempting to enable auto-login by obtaining a token saved in the local mobile instance. I'm encountering this error. I'm using Provider for state management. In my main file , I want to retrieve my token to check whether user has been logged in or not. What i'm, doing wrong ?

My Error

E/flutter ( 6547): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: No ScaffoldMessenger widget found.
E/flutter ( 6547): MyApp widgets require a ScaffoldMessenger widget ancestor.
E/flutter ( 6547): The specific widget that could not find a ScaffoldMessenger ancestor was:
E/flutter ( 6547):   MyApp
E/flutter ( 6547): The ancestors of this widget were:
E/flutter ( 6547):   _InheritedProviderScope<UserProvider?>
E/flutter ( 6547):   ChangeNotifierProvider<UserProvider>
E/flutter ( 6547):   _NestedHook
E/flutter ( 6547):   MultiProvider
E/flutter ( 6547):   [root]
E/flutter ( 6547): Typically, the ScaffoldMessenger widget is introduced by the MaterialApp at the top of your application widget tree.

Main.dart file

void main() {
  runApp(MultiProvider(
      providers: [ChangeNotifierProvider(create: (context) => UserProvider())],
      child: const MyApp()));
}

class MyApp extends StatefulWidget {
  const MyApp({Key? key}):super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  final AuthService authService = AuthService();
  @override
  void initState() {
    // TODO: implement initState
    authService.getUserData(context);
    super.initState();
  }

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'ShopOne',
      theme: ThemeData(
        scaffoldBackgroundColor: GlobalVariables.backgroundColor,
        textTheme: Theme.of(context)
            .textTheme
            .apply(bodyColor: Colors.white, displayColor: Colors.white),

        colorScheme:
            const ColorScheme.light(primary: GlobalVariables.secondaryColor),
        appBarTheme: const AppBarTheme(
            elevation: 0, iconTheme: IconThemeData(color: Colors.white)),
        //
        primarySwatch: Colors.blue,
      ),
      debugShowCheckedModeBanner: false,
      onGenerateRoute: ((settings) => generateRoute(settings)),
      home: Provider.of<UserProvider>(context).user.token.isNotEmpty
                  ? const HomeScreen()
                  : const AuthScreen()
    );
    if (Provider.of<UserProvider>(context).user.token.isNotEmpty) {
      print('true');
    } else {
      print('false');
      ;
    }
  }
}

get user function

  void getUserData(
    BuildContext context,
  ) async {
    try {
      
      SharedPreferences prefs = await SharedPreferences.getInstance();
      String? token = prefs.getString("auth-token");
      if (token == null) {
        prefs.setString('auth-token', '');
      }

      var tokenRes = await http.post(Uri.parse('$uri//tokenisvalid'),
          headers: <String, String>{
            'Content-Type': 'application/json;charset=UTF-8',
            'auth-token': token!
          });
      print('tokenres working 1');

      var response = jsonDecode(tokenRes.body);
      if (response == true) {
        http.Response userRes = await http.get(Uri.parse('$uri/'),
            headers: <String, String>{
              'Content-type': 'application/json;charset=UTF-8',
              'auth-token': token
            });
        print('tokenres working 2');

        var userProvider = Provider.of<UserProvider>(context, listen: false);
        userProvider.setUser(userRes.body);
      }
    } catch (e) {
      showSnackBar(context, e.toString());
    }
  }

CodePudding user response：

This happened because the context that you used in showSnackBar, doesn't belongs to any Scaffold, you need to separate the MaterialApp's home to new StatefulWidget class like this:

class ScreenManager extends StatefulWidget {
  const ScreenManager({Key? key}) : super(key: key);

  @override
  State<ScreenManager> createState() => _ScreenManagerState();
}

class _ScreenManagerState extends State<ScreenManager> {
  @override
  void initState() {
    // TODO: implement initState
    authService.getUserData(context);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Provider.of<UserProvider>(context).user.token.isNotEmpty
          ? const HomeScreen()
          : const AuthScreen(),
    );
  }

  void getUserData(BuildContext context) async {
   

    try {

      ...

    } catch (e) {
      WidgetsBinding.instance.addPostFrameCallback((_) {
         showSnackBar(context, e.toString());
      });
    }
  }
}

then use it like this:

home: ScreenManager();

Note: don't forget to remove authService.getUserData(context) from _MyAppState's initState.