In order to Authenticate to Api, I created AuthProvider class to authenticate, after getting accessToken from the Api,I stored the token using sharedPreferences,I wanted to check if the user loggedIn or not, so I initialized sharedPreferences to return a boolean value if it contains the token... as shown in the code bellow:

class AuthProvider with ChangeNotifier{
  
    bool _isLoading = false;
  bool get isLoading => _isLoading;
  User user = User();
  late SharedPreferences prefs ;
  
   
 String token = '';
  Map<String, String>   _mainHeaders = {
      'Content-Type': 'application/json; charset=UTF-8',
       
    };

   void updateHeader(String token) {
    _mainHeaders = {
      'Content-Type': 'application/json; charset=UTF-8',
      'Authorization': 'Bearer $token',
    };
  }
  

 

Future<ResponseModel> login(String username, String password) async {
    print("Getting token");
    //print(authRepo.getUserToken().toString());
    _isLoading = true;
 
       notifyListeners();
    http.Response response = await dologin(username, password);
   
    // print('${response.body.}');
   var answer = User.fromJson(jsonDecode(response.body));
   print('the result is : ${answer.token}');
    
   print('level 1: ${response.body.toString()}');
    late ResponseModel responseModel;
    print(response.statusCode);
    if (response.statusCode == 200) {
         
      //authRepo.saveUserToken((response.body["token"]).toString());
       print("Backend token: ${response.body.toString()}");

     responseModel = ResponseModel(true, answer.token!);
       
    //  SharedPreferences prefs = await SharedPreferences.getInstance();
    //  prefs.setString(ApiConstants.kEY_ACCESS_TOKEN, answer.token!);
      storeToken(answer.token!);

    } else {
      responseModel = ResponseModel(false, response.reasonPhrase!);
    
    }
    _isLoading = false;

       notifyListeners();
    return responseModel;
  }

  
  bool userLoggedIn() =>
      prefs.containsKey(ApiConstants.kEY_ACCESS_TOKEN) ? true : false;


  // bool clearSharedData() => authRepo.clearSharedData();

 void storeToken(String token) async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    await prefs.setString(ApiConstants.kEY_ACCESS_TOKEN, token);
    print('the token is here : ${token}');

  }

...

in main.dart I created a consumer to check if the user is loggedIn or not :

 child: MaterialApp(
          home: Consumer<AuthProvider>(
            builder: ((context, auth, _){
              return auth.userLoggedIn() ? const  HomePage() : const loginScreen();
            })
            ),
         ...

when I hit run I got the following error in my console:

The following LateError was thrown building Consumer(dirty, dependencies: [_InheritedProviderScope<AuthProvider?>]): LateInitializationError: Field 'prefs' has not been initialized. The relevant error-causing widget was Consumer. PS : I tried to initialize sharedPreferences : SharedPreferences? prefs; the error would be: Null check operator used on a null value. ...

CodePudding user response：

You're getting the error because you've marked your prefs as late but you haven't initialised it (as written in the error message). Just get rid of the line late SharedPreferences prefs; at the top and ensure that you initialise it each time you need it within the method. I.e. call await SharedPreferences.getInstance(); each time.

CodePudding user response：

Future<bool> userLoggedIn() async{
      prefs = await SharedPreferences.getInstance();
      prefs.containsKey(ApiConstants.kEY_ACCESS_TOKEN) ? true : false;
}

Change userLoggedIn getter to this.

CodePudding user response：

You can call a function from init state and check the sharedprefrences in it like

late SharedPreferences prefs;

//initialize shared prefrences then access it 
  void initState() {
    super.initState();
    sharedData();
  }

void sharedData() async {
    prefs = await SharedPreferences.getInstance(); 
  }

 void storeToken(String token) async {
    prefs = await SharedPreferences.getInstance();
    prefs.setString(ApiConstants.kEY_ACCESS_TOKEN, token);
    print('the token is here : ${token}');
  }