flutter firebase google sign out not working

Question

I'm trying to implement google sign in/ sign out, but my sign out's not working. I get an error signOut() was called on null. When I print out user after the user signs in, I do get all the proper information, but when I print out in my signout function, it says it's null. Custom firebase user does work. Here's my auth.dart file:

import 'dart:async';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:google_sign_in/google_sign_in.dart';

abstract class BaseAuth {
  Future<String> signIn(String email, String password);

  Future<String> signUp(String email, String password);

  Future<FirebaseUser> getCurrentUser();

  Future<void> sendEmailVerification();

  Future<void> signOut();

  Future<bool> isEmailVerified();

  Future<String> signInWithGoogle();
}

class Auth implements BaseAuth {
  final FirebaseAuth _firebaseAuth = FirebaseAuth.instance;
  final GoogleSignIn googleSignIn = GoogleSignIn();
  FirebaseUser user;

  Future<String> signIn(String email, String password) async {

    user = (await _firebaseAuth.signInWithEmailAndPassword(email: email, password: password)).user;
    return user.email;
  }

  Future<String> signUp(String email, String password) async {
    FirebaseUser user = (await _firebaseAuth.createUserWithEmailAndPassword(
        email: email, password: password)).user;
    return user.uid;
  }

  Future<FirebaseUser> getCurrentUser() async {
    user = await _firebaseAuth.currentUser();
    return user;
  }
  signOut() async {
    //print("signed in user: ${authService.user}");
    await _firebaseAuth.signOut();
  }

  Future<void> sendEmailVerification() async {
    FirebaseUser user = await _firebaseAuth.currentUser();
    user.sendEmailVerification();
  }

  Future<bool> isEmailVerified() async {
    FirebaseUser user = await _firebaseAuth.currentUser();
    return user.isEmailVerified;
  }

  Future<String> signInWithGoogle() async {
    final GoogleSignInAccount googleSignInAccount = await googleSignIn.signIn();
    final GoogleSignInAuthentication googleSignInAuthentication =
    await googleSignInAccount.authentication;

    final AuthCredential credential = GoogleAuthProvider.getCredential(
      accessToken: googleSignInAuthentication.accessToken,
      idToken: googleSignInAuthentication.idToken,
    );

   user = (await _firebaseAuth.signInWithCredential(credential)).user;

    assert(!user.isAnonymous);
    assert(await user.getIdToken() != null);

    final FirebaseUser currentUser = await _firebaseAuth.currentUser();
    assert(user.uid == currentUser.uid);
    return 'signInWithGoogle succeeded: $user';
  }

}

Something else strange is if I boot up the app and am already logged in (as google), i hit signout and it appears to work. Nothing in my console, but it goes back to login screen. Then if I log back in as google and signout, error starts happening. Another weird thing is if I happened to already be logged in and I get it to log out after one click, without changing anything if I restart my app in android studio it takes me back to the screen where I'm supposedly already logged in. This is only happening on Google Sign Out (not firebase log out). Any idea what I might be doing wrong? Thanks

pubspec.yaml

dependencies:
  firebase_auth: ^0.14.0+5

  firebase_database: ^3.0.7
  google_sign_in: ^4.0.7
  firebase_storage:
  image_picker:
  cloud_firestore:
  shared_preferences:
  fluttertoast:
  cached_network_image:
  intl:

one page where I use it: (I have many, but implemented similar to this and it doesn't work in any of my pages)

import 'package:flutter/material.dart';
import 'package:pet_helper/chat.dart';
import 'package:pet_helper/lost_and_found.dart';
import 'package:pet_helper/pet_adoption.dart';
import 'authentication.dart';

class HomePage extends StatefulWidget {
  HomePage({Key key, this.auth, this.userId, this.onSignedOut})
      : super(key: key);

  final BaseAuth auth;
  final VoidCallback onSignedOut;
  final String userId;

  @override
  State<StatefulWidget> createState() => new _HomePageState();
}

class _HomePageState extends State<HomePage> {

  final GlobalKey<FormState> formKey = GlobalKey<FormState>();
  int _currentIndex = 0;

  @override
  void initState() {
    super.initState();
  }
  final List<Widget> _children = [
    new LostAndFoundPage(),
    new PetAdoptionPage(),
    new ChatPage(),
  ];
  _signOut() async {
    try {
      await widget.auth.signOut();
      widget.onSignedOut();
    } catch (e) {

      print(e);
    }
  }

  onTabTapped(int index) {
    setState(() {
      _currentIndex = index;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        appBar: new AppBar(
          automaticallyImplyLeading:false,
          title: new Text('Pet Helper'),
          actions: <Widget>[
            new FlatButton(
                child: new Text('Logout',
                    style: new TextStyle(fontSize: 17.0, color: Colors.white)),
                onPressed: _signOut)
          ],
        ),
        body: _children[_currentIndex], // new
        bottomNavigationBar: BottomNavigationBar(
          onTap: onTabTapped, // new
          currentIndex: _currentIndex, // new
          items: [
            new BottomNavigationBarItem(
              icon: Icon(Icons.home),
              title: Text('Lost & Found'),
            ),
            new BottomNavigationBarItem(
              icon: Icon(Icons.pets),
              title: Text('Pet Adoption'),
            ),
            new BottomNavigationBarItem(
                icon: Icon(Icons.person), title: Text('Chat'))
          ],
        ));
  }
}

and for completion, here's my login page:


import 'package:flutter/material.dart';
import 'package:pet_helper/home_page.dart';
import 'authentication.dart';

class LoginSignUpPage extends StatefulWidget {
  LoginSignUpPage({this.auth, this.onSignedIn});

  final BaseAuth auth;
  final VoidCallback onSignedIn;

  @override
  State<StatefulWidget> createState() => new _LoginSignUpPageState();
}

enum FormMode { LOGIN, SIGNUP }
class _LoginSignUpPageState extends State<LoginSignUpPage> {
  final _formKey = new GlobalKey<FormState>();
  String _email;
  String _password;
  String _errorMessage;

  // Initial form is login form
  FormMode _formMode = FormMode.LOGIN;
  bool _isIos;
  bool _isLoading;

  // Check if form is valid before perform login or signup
  bool _validateAndSave() {
    final form = _formKey.currentState;
    if (form.validate()) {
      form.save();
      return true;
    }
    return false;
  }

  // Perform login or signup
  void _validateAndSubmit() async {
    setState(() {
      _errorMessage = "";
      _isLoading = true;
    });
    if (_validateAndSave()) {
      String userId = "";
      try {
        if (_formMode == FormMode.LOGIN) {
          userId = await widget.auth.signIn(_email, _password);
          print('Signed in: $userId');
        } else {
          userId = await widget.auth.signUp(_email, _password);
          widget.auth.sendEmailVerification();
          _showVerifyEmailSentDialog();
          print('Signed up user: $userId');
        }
        setState(() {
          _isLoading = false;
        });

        if (userId.length > 0 && userId != null && _formMode == FormMode.LOGIN) {
          widget.onSignedIn();
        }

      } catch (e) {
        print('Error: $e');
        setState(() {
          _isLoading = false;
          if (_isIos) {
            _errorMessage = e.details;
          } else
            _errorMessage = 'Incorrect user or password';
        });
      }
    }
  }


  @override
  void initState() {
    _errorMessage = "";
    _isLoading = false;
    super.initState();
  }

  void _changeFormToSignUp() {
    _formKey.currentState.reset();
    _errorMessage = "";
    setState(() {
      _formMode = FormMode.SIGNUP;
    });
  }

  void _changeFormToLogin() {
    _formKey.currentState.reset();
    _errorMessage = "";
    setState(() {
      _formMode = FormMode.LOGIN;
    });
  }

  @override
  Widget build(BuildContext context) {
    _isIos = Theme.of(context).platform == TargetPlatform.iOS;
    return new Scaffold(
        appBar: new AppBar(
          title: new Text('Pet Helper'),
        ),
        body: Stack(
          children: <Widget>[
            _showBody(),
            _showCircularProgress(),
          ],
        ));
  }

  Widget _showCircularProgress(){
    if (_isLoading) {
      return Center(child: CircularProgressIndicator());
    } return Container(height: 0.0, width: 0.0,);

  }

  void _showVerifyEmailSentDialog() {
    showDialog(
      context: context,
      builder: (BuildContext context) {
        // return object of type Dialog
        return AlertDialog(
          title: new Text("Verify your account"),
          content: new Text("Link to verify account has been sent to your email"),
          actions: <Widget>[
            new FlatButton(
              child: new Text("Dismiss"),
              onPressed: () {
                _changeFormToLogin();
                Navigator.of(context).pop();
              },
            ),
          ],
        );
      },
    );
  }

  Widget _showBody(){
    return new Container(
        padding: EdgeInsets.all(16.0),
        child: new Form(
          key: _formKey,
          child: new ListView(
            shrinkWrap: true,
            children: <Widget>[
              _showLogo(),
              _showEmailInput(),
              _showPasswordInput(),
              _showErrorMessage(),
              _showPrimaryButton(),
              _showSecondaryButton(),
              _googleSignInButton(),

            ],
          ),
        ));
  }

  Widget _showErrorMessage() {
    if (_errorMessage.length > 0 && _errorMessage != null) {
      return new Text(
        _errorMessage,
        style: TextStyle(
            fontSize: 13.0,
            color: Colors.red,
            height: 1.0,
            fontWeight: FontWeight.w300),
      );
    } else {
      return new Container(
        height: 0.0,
      );
    }
  }

  Widget _showLogo() {
    return new Hero(
      tag: 'hero',
      child: Padding(
        padding: EdgeInsets.fromLTRB(0.0, 30.0, 0.0, 0.0),
        child: CircleAvatar(
          backgroundColor: Colors.transparent,
          radius: 120.0,
          child: Image.asset('assets/babies.png'),
        ),
      ),
    );
  }

  Widget _showEmailInput() {
    return Padding(
      padding: const EdgeInsets.fromLTRB(0.0, 25.0, 0.0, 0.0),
      child: new TextFormField(
        maxLines: 1,
        keyboardType: TextInputType.emailAddress,
        autofocus: false,
        decoration: new InputDecoration(
            hintText: 'Email',
            icon: new Icon(
              Icons.mail,
              color: Colors.grey,
            )),
        validator: (String value) {
        if (value.isEmpty) {
          _isLoading = false;
          return 'Email can\'t be empty';
        }
        else{
          return null;
        }
      },
        onSaved: (value) => _email = value.trim(),
      ),
    );
  }

  Widget _showPasswordInput() {
    return Padding(
      padding: const EdgeInsets.fromLTRB(0.0, 15.0, 0.0, 0.0),
      child: new TextFormField(
        maxLines: 1,
        obscureText: true,
        autofocus: false,
        decoration: new InputDecoration(
            hintText: 'Password',
            icon: new Icon(
              Icons.lock,
              color: Colors.grey,
            )),
        validator: (String value) {
          if (value.isEmpty) {
            _isLoading = false;
            return 'Password can\'t be empty';
          }
          else{
            return null;
          }
        },
        onSaved: (value) => _password = value.trim(),
      ),
    );
  }

  Widget _showSecondaryButton() {
    return new FlatButton(
      child: _formMode == FormMode.LOGIN
          ? new Text('Create an account',
          style: new TextStyle(fontSize: 18.0, fontWeight: FontWeight.w300))
          : new Text('Have an account? Sign in',
          style:
          new TextStyle(fontSize: 18.0, fontWeight: FontWeight.w300)),
      onPressed: _formMode == FormMode.LOGIN
          ? _changeFormToSignUp
          : _changeFormToLogin,
    );
  }

  Widget _showPrimaryButton() {
    return new Padding(
        padding: EdgeInsets.fromLTRB(0.0, 35.0, 0.0, 0.0),
        child: SizedBox(
          height: 40.0,
          child: new RaisedButton(
            elevation: 5.0,
            shape: new RoundedRectangleBorder(borderRadius: new BorderRadius.circular(30.0)),
            color: Colors.blue,
            child: _formMode == FormMode.LOGIN
                ? new Text('Login',
                style: new TextStyle(fontSize: 20.0, color: Colors.white))
                : new Text('Create account',
                style: new TextStyle(fontSize: 20.0, color: Colors.white)),
            onPressed: _validateAndSubmit,
          ),
        ));
  }

void submitGoogleLogin() async{
  setState(() {
    _errorMessage = "";
    _isLoading = true;
  });
  String userId = "";
  userId = await widget.auth.signInWithGoogle().whenComplete(() {
    widget.onSignedIn();
    Navigator.of(context).push(
      MaterialPageRoute(
        builder: (context) {
          return new HomePage();
        },
      ),
    );
  });
  print('Signed in: $userId');
}

  Widget _googleSignInButton() {
    return OutlineButton(
      splashColor: Colors.grey,
      onPressed: submitGoogleLogin,
      shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(40)),
      highlightElevation: 0,
      borderSide: BorderSide(color: Colors.grey),
      child: Padding(
        padding: const EdgeInsets.fromLTRB(0, 10, 0, 10),
        child: Row(
          mainAxisSize: MainAxisSize.min,
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Image(image: AssetImage("assets/google_logo.png"), height: 30.0),
            Padding(
              padding: const EdgeInsets.only(left: 10),
              child: Text(
                'Sign in with Google',
                style: TextStyle(
                  fontSize: 15,
                  color: Colors.grey,
                ),
              ),
            )
          ],
        ),
      ),
    );
  }
}

Oh and root page changes the state of the user:

import 'package:flutter/material.dart';
import 'login_signup_page.dart';
import 'authentication.dart';
import 'home_page.dart';

class RootPage extends StatefulWidget {
  RootPage({this.auth});

  final BaseAuth auth;

  @override
  State<StatefulWidget> createState() => new _RootPageState();
}

enum AuthStatus {
  NOT_DETERMINED,
  NOT_LOGGED_IN,
  LOGGED_IN,
}

class _RootPageState extends State<RootPage> {
  AuthStatus authStatus = AuthStatus.NOT_DETERMINED;
  String _userId = "";

  @override
  void initState() {
    super.initState();
    widget.auth.getCurrentUser().then((user) {
      setState(() {
        if (user != null) {
          _userId = user?.uid;
        }
        authStatus =
        user?.uid == null ? AuthStatus.NOT_LOGGED_IN : AuthStatus.LOGGED_IN;
      });
    });
  }

  void _onLoggedIn() {
    widget.auth.getCurrentUser().then((user){
      setState(() {
        _userId = user.uid.toString();
      });
    });
    setState(() {
      authStatus = AuthStatus.LOGGED_IN;

    });
  }

  void _onSignedOut() {
    setState(() {
      authStatus = AuthStatus.NOT_LOGGED_IN;
      _userId = "";
    });
  }

  Widget _buildWaitingScreen() {
    return Scaffold(
      body: Container(
        alignment: Alignment.center,
        child: CircularProgressIndicator(),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    switch (authStatus) {
      case AuthStatus.NOT_DETERMINED:
        return _buildWaitingScreen();
        break;
      case AuthStatus.NOT_LOGGED_IN:
        return new LoginSignUpPage(
          auth: widget.auth,
          onSignedIn: _onLoggedIn,
        );
        break;
      case AuthStatus.LOGGED_IN:
        if (_userId.length > 0 && _userId != null) {
          return new HomePage(
            userId: _userId,
            auth: widget.auth,
            onSignedOut: _onSignedOut,
          );
        } else return _buildWaitingScreen();
        break;
      default:
        return _buildWaitingScreen();
    }
  }
}

initializing root page Main.dart

void main() async{
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
        title: 'Pet Helper',
        debugShowCheckedModeBanner: false,
        theme: new ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: new RootPage(auth: new Auth()));
  }
}

Where is the code that initialize RootPage? If you didn't write this, you might want to download the complete source code from github from the guide that you saw and start from there. It seems like you are jumping too much ahead when there are a lot of things going on in these file set. — user482594
I modified everything to avoid duplicating code and cleaned up extra unnecessary code that was in my project. — Mike1982
By the way, I did follow a tutorial to create this abstract class and everything working with only the login/signup page. The tutorial didn't include google login and I had to do figure out how it works anyways to get it to work with my specific project modifications to get it to work with my app specifically. — Mike1982
What I am saying is that I recommend you to first fully understand how existing Firebase signing in work in Flutter (state management, initialization, Statefulness, Event Listener) and then take a step by step to add Google Auth. — user482594

Benjamin Benjamin · Accepted Answer · 2019-12-08T02:32:27

abstract class BaseAuth {
  Future<String> signIn(String email, String password);

  Future<String> signUp(String email, String password);

  Future<FirebaseUser> getCurrentUser();

  Future<void> sendEmailVerification();

  Future<void> signOut();

  Future<bool> isEmailVerified();

  Future<String> signInWithGoogle();

  void signOutGoogle();
}

class Auth implements BaseAuth {
  final FirebaseAuth _firebaseAuth = FirebaseAuth.instance;
  final GoogleSignIn googleSignIn = GoogleSignIn();
  FirebaseUser user;

  Future<String> signIn(String email, String password) async {

    user = (await _firebaseAuth.signInWithEmailAndPassword(email: email, password: password)).user;
    return user.email;
  }

  Future<String> signUp(String email, String password) async {
    FirebaseUser user = (await _firebaseAuth.createUserWithEmailAndPassword(
        email: email, password: password)).user;
    return user.uid;
  }

  Future<FirebaseUser> getCurrentUser() async {
    user = await _firebaseAuth.currentUser();
    return user;
  }
  signOut() async {
    print("signed in user: $user");
    await _firebaseAuth.signOut();
  }

  Future<void> sendEmailVerification() async {
    FirebaseUser user = await _firebaseAuth.currentUser();
    user.sendEmailVerification();
  }

  Future<bool> isEmailVerified() async {
    FirebaseUser user = await _firebaseAuth.currentUser();
    return user.isEmailVerified;
  }

  Future<String> signInWithGoogle() async {
    final GoogleSignInAccount googleSignInAccount = await googleSignIn.signIn();
    final GoogleSignInAuthentication googleSignInAuthentication =
    await googleSignInAccount.authentication;

    final AuthCredential credential = GoogleAuthProvider.getCredential(
      accessToken: googleSignInAuthentication.accessToken,
      idToken: googleSignInAuthentication.idToken,
    );

   user = (await _firebaseAuth.signInWithCredential(credential)).user;

    assert(!user.isAnonymous);
    assert(await user.getIdToken() != null);

    final FirebaseUser currentUser = await _firebaseAuth.currentUser();
    assert(user.uid == currentUser.uid);
    return 'signInWithGoogle succeeded: $user';
  }
}

final Auth authService = Auth(); // add this to the bottom outside the class

Using a global variable

So let me explain. Each time you call Auth(), it creates a new instance of that class. So in one instance, you might sign the user in. In another instance, you might sign the user out. The variables will contain different values for the variables. So if you use a different instance for signing users out than you did for signing them in, the user variable will be null, therefore not allowing you to sign them out and printing null.

Solution

The solution is a global variable to access the Auth class once. Ex. var authService = Auth();

Put the variable outside the Auth class so it can be accessed in any class, anywhere

flutter firebase google sign out not working

2 Answers

Using a global variable

Solution