How to keep a user signed in using Shared Preferences

Question

I have a scanning app that uses Firebase for Authorisation, I used Shared Preferences to keep the user signed in after closing the app but I get an error that says,

The getter 'uid' was called on null. Receiver: null Tried calling: uid

Below is my Main.dart page

In this main block, there is a ternary condition that is supposed to determine which page to go to based on wether the app was just closed or signed out

Future <void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  SharedPreferences preferences = await SharedPreferences.getInstance();
 var email = preferences.get("email");
  await Firebase.initializeApp();
  runApp(MaterialApp(
      debugShowCheckedModeBanner: false,
      title: "Jinjer Pay",
      theme: ThemeData(
          primaryColor: Colors.amber,
          visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: email != null ? Login() : Scan_QR()));
     // home: Login()));
}

Code for Login Page I put the shared preferences block at the end of the Auth. block. I am not sure if I have initialised the shared preferences block correctly with the Auth block.

 RaisedButton(
                                  child: Text(
                                    "Log In",
                                    style: TextStyle(fontSize: 19),
                                  ),
                                  color: Colors.amberAccent,
                                  elevation: 10,
                                  shape: RoundedRectangleBorder(
                                      borderRadius:
                                      BorderRadius.circular(15)),
                                  onPressed: () async {
                                    if (formKey.currentState.validate()) {
                                      Loading();
                                      formKey.currentState.save();

                                      setState(() => loading = true);

                                      try {
                                        UserCredential res;
                                        String userID;

                                        res = (await FirebaseAuth.instance
                                            .signInWithEmailAndPassword(
                                            email:
                                            _emailController.text,
                                            password:
                                            _passwordController
                                                .text));
                                        userID = res.user.uid;


                                        if (res != null) {
                                          setState(() {
                                            loading = false;
                                          });
                                          Navigator.pop(context);
                                          Navigator.of(context).push(
                                            MaterialPageRoute(
                                                builder: (context) =>
                                                    Scan_QR()),
                                          );
                                        }
                                      } catch (e) {
                                        if (FirebaseAuth
                                            .instance.currentUser.uid !=
                                            e.code) {
                                          setState(() {
                                            loading = false;
                                          });
                                        }

                                        if (e.code == "unknown") {
                                          Fluttertoast.showToast(
                                              msg:
                                              "Email/Password Field cannot be Empty",
                                              toastLength:
                                              Toast.LENGTH_SHORT,
                                              gravity: ToastGravity.CENTER,
                                              timeInSecForIosWeb: 1,
                                              backgroundColor:
                                              Colors.redAccent,
                                              textColor: Colors.white,
                                              fontSize: 16.0);
                                        }
                                        if (e.code == "wrong-password") {
                                          Fluttertoast.showToast(
                                              msg:
                                              "Your password is incorrect",
                                              toastLength:
                                              Toast.LENGTH_SHORT,
                                              gravity: ToastGravity.CENTER,
                                              timeInSecForIosWeb: 1,
                                              backgroundColor:
                                              Colors.redAccent,
                                              textColor: Colors.white,
                                              fontSize: 16.0);
                                        }
                                        if (e.code == "invalid-email") {
                                          Fluttertoast.showToast(
                                              msg:
                                              "Email/Password Field cannot be Empty",
                                              toastLength:
                                              Toast.LENGTH_SHORT,
                                              gravity: ToastGravity.CENTER,
                                              timeInSecForIosWeb: 1,
                                              backgroundColor:
                                              Colors.redAccent,
                                              textColor: Colors.white,
                                              fontSize: 16.0);
                                        }
                                        if (e.code == "user-not-found") {
                                          Fluttertoast.showToast(
                                              msg:
                                              "No User found with this email",
                                              toastLength:
                                              Toast.LENGTH_SHORT,
                                              gravity: ToastGravity.CENTER,
                                              timeInSecForIosWeb: 1,
                                              backgroundColor:
                                              Colors.redAccent,
                                              textColor: Colors.white,
                                              fontSize: 16.0);
                                        }
                                        if (e.code == "user-disabled") {
                                          Fluttertoast.showToast(
                                              msg:
                                              "user with this email has been disabled",
                                              toastLength:
                                              Toast.LENGTH_SHORT,
                                              gravity: ToastGravity.CENTER,
                                              timeInSecForIosWeb: 1,
                                              backgroundColor:
                                              Colors.redAccent,
                                              textColor: Colors.white,
                                              fontSize: 16.0);
                                        }
                                        if (e.code ==
                                            "network-request-failed") {
                                          Fluttertoast.showToast(
                                              msg:
                                              "A network error has occurred",
                                              toastLength:
                                              Toast.LENGTH_SHORT,
                                              gravity: ToastGravity.CENTER,
                                              timeInSecForIosWeb: 1,
                                              backgroundColor:
                                              Colors.redAccent,
                                              textColor: Colors.white,
                                              fontSize: 16.0);
                                        }
                                        if (e.code == "too-many-requests") {
                                          Fluttertoast.showToast(
                                              msg:
                                              "Too many requests, try again later",
                                              toastLength:
                                              Toast.LENGTH_SHORT,
                                              gravity: ToastGravity.CENTER,
                                              timeInSecForIosWeb: 1,
                                              backgroundColor:
                                              Colors.redAccent,
                                              textColor: Colors.white,
                                              fontSize: 16.0);
                                        }

                                         SharedPreferences preferences = await SharedPreferences.getInstance();
                                         preferences.setString("email", _emailController.text);
                                        Navigator.of(context).push(
                                          MaterialPageRoute(
                                              builder: (context) =>
                                                  Scan_QR()),
                                        );
                                        
                                        print(e);
                                       
                                        _passwordController.text = "";
                                      }

                                      
                                    }
                                  },
                                ),

I have a Sign Out Block that is supposed to sign out the previously signed-in user data from the app and wait for a new user to sign in.

Future<void> SignOut() async {
     SharedPreferences preferences = await SharedPreferences.getInstance();
     preferences.remove("email");

   await FirebaseAuth.instance.signOut().then((value) => Navigator.of(context)
       .push(MaterialPageRoute(builder: (context) => Login())));

  }

Answer 1

You could use StreamBuilder to check the authentication process when open the app instead of using the shared preference.

FirebaseAuth mAuth = FirebaseAuth.instance;
@override
Widget build(BuildContext context) {
    return StreamBuilder<FirebaseUser>(
        stream: mAuth.onAuthStateChanged,
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.active) {
            FirebaseUser user = snapshot.data;
            if (user == null) {
              //add the logic when user is null
            } else {
              // add the logic when user is not null
            }
            
          } else {
            // add the logic when connection state not active
          }
        },
    );
  }

Then you could do your authentication process without using the shared preference.

Hope this answer will solve your problem!