Firebase Login/Logout Example - by Simon Lightfoot

main.dart

/*
This example uses the following packages:

  firebase_auth: 0.14.0+5
  google_sign_in: 4.0.7
  provider: 3.1.0+1

Make sure you have setup your project with Firebase by following these instructions:

  1. Follow Option 1 instructions here up to Step 3
     https://firebase.google.com/docs/android/setup#console

     Firebase Console: https://console.firebase.google.com

     Be sure to configure your SHA-1 or SHA-256 hash in the
     Firebase Project Settings for your app.

     If you are wondering what your package-name is you can get it from:
     <flutter-project>/android/app/build.gradle labelled applicationId.

  2. Place the downloaded 'google-services.json' file from Step 1 above in your
     projects <flutter-project>/android/app/ directory.

  3. Follow the instructions here to enable the Google Services Gradle Plugin.
     https://pub.dev/packages/firebase_auth#android-integration

  4. Go to the Firebase Console and then to the Authentication section and then
     on to the "Sign-in method" tab an enable Email/Password and Google Sign in methods.

  5. Run and enjoy! ... luckily you only have to do this once.

*/
// MIT License
//
// Copyright (c) 2019 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
import 'dart:async';

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart' show PlatformException;
import 'package:google_sign_in/google_sign_in.dart';
import 'package:provider/provider.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  // Async main methods show the platform splash screen
  // before runApp is called to show the flutter UI.
  // So this line waits for Auth.create to finish before
  // showing the rest of the app.
  runApp(App(auth: await Auth.create()));
}

class App extends StatefulWidget {
  const App({
    Key key,
    @required this.auth,
  }) : super(key: key);

  final Auth auth;

  @override
  _AppState createState() => _AppState();
}

class _AppState extends State<App> {
  final _navigatorKey = GlobalKey<NavigatorState>();
  FirebaseUser currentUser;

  @override
  void initState() {
    super.initState();
    currentUser = widget.auth.init(_onUserChanged);
  }

  void _onUserChanged() {
    final user = widget.auth.currentUser.value;
    // User logged in
    if (currentUser == null && user != null) {
      _navigatorKey.currentState.pushAndRemoveUntil(Main.route(), (route) => false);
    }
    // User logged out
    else if (currentUser != null && user == null) {
      _navigatorKey.currentState.pushAndRemoveUntil(Login.route(), (route) => false);
    }
    currentUser = user;
  }

  @override
  void dispose() {
    widget.auth.dispose(_onUserChanged);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MultiProvider(
      providers: <SingleChildCloneableWidget>[
        Provider<Auth>.value(value: widget.auth),
        ValueListenableProvider<FirebaseUser>.value(value: widget.auth.currentUser),
      ],
      child: MaterialApp(
        title: 'Your App',
        theme: ThemeData(
          brightness: Brightness.light,
          primaryColor: Colors.indigo,
          accentColor: Colors.pink,
        ),
        navigatorKey: _navigatorKey,
        home: currentUser == null ? const Login() : const Main(),
      ),
    );
  }
}

/// Logged out view of the app.
class Login extends StatefulWidget {
  static Route<dynamic> route() {
    return MaterialPageRoute(
      builder: (BuildContext context) => const Login(),
    );
  }

  const Login({
    Key key,
  }) : super(key: key);

  @override
  _LoginState createState() => _LoginState();
}

class _LoginState extends State<Login> {
  final _formKey = GlobalKey<FormState>();
  final _email = TextEditingController();
  final _password = TextEditingController();

  Future _loginFuture;

  void _onLoginWithPasswordPressed() {
    if (_formKey.currentState.validate()) {
      _formKey.currentState.save();
      setState(() {
        _loginFuture = Auth.of(context).loginWithEmailAndPassword(_email.text, _password.text);
      });
    }
  }

  void _onLoginWithGooglePressed() {
    setState(() {
      _loginFuture = Auth.of(context).loginWithGoogle();
    });
  }

  String _validateEmail(String value) {
    if (value == null || value.trim().isEmpty) {
      return 'Please enter an email address.';
    } else if (value.contains('@') == false) {
      return 'Please check your email address is correct.';
    }
    return null;
  }

  String _validatePassword(String value) {
    if (value == null || value.trim().isEmpty) {
      return 'Please enter your password.';
    } else if (value.length < 6) {
      return 'Your password needs to be at least 6 characters.';
    }
    return null;
  }

  @override
  void dispose() {
    _email.dispose();
    _password.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Theme.of(context).primaryColorLight,
      body: SafeArea(
        child: SizedBox.expand(
          child: Column(
            children: <Widget>[
              const SizedBox(height: 16.0),
              Text(
                'Login Screen',
                style: Theme.of(context).textTheme.title,
                textAlign: TextAlign.center,
              ),
              const SizedBox(height: 36.0),
              FutureBuilder(
                future: _loginFuture,
                builder: (BuildContext context, AsyncSnapshot snapshot) {
                  if (snapshot.connectionState == ConnectionState.waiting) {
                    return const Center(
                      child: CircularProgressIndicator(),
                    );
                  }
                  return Form(
                    key: _formKey,
                    child: Padding(
                      padding: const EdgeInsets.symmetric(horizontal: 32.0),
                      child: Column(
                        children: <Widget>[
                          TextFormField(
                            controller: _email,
                            decoration: const InputDecoration(
                              labelText: 'Email',
                            ),
                            validator: _validateEmail,
                          ),
                          TextFormField(
                            controller: _password,
                            obscureText: true,
                            decoration: const InputDecoration(
                              labelText: 'Password',
                            ),
                            validator: _validatePassword,
                          ),
                          const SizedBox(height: 16.0),
                          RaisedButton(
                            onPressed: _onLoginWithPasswordPressed,
                            child: const Text('LOGIN'),
                          ),
                          const SizedBox(height: 16.0),
                          RaisedButton(
                            onPressed: _onLoginWithGooglePressed,
                            child: const Text('LOGIN WITH GOOGLE'),
                          ),
                          if (snapshot.hasError)
                            Container(
                              width: double.infinity,
                              margin: const EdgeInsets.symmetric(vertical: 36.0),
                              padding: const EdgeInsets.all(12.0),
                              decoration: BoxDecoration(
                                border: Border.all(color: Theme.of(context).errorColor),
                                borderRadius: const BorderRadius.all(Radius.circular(2.0)),
                                color: Theme.of(context).errorColor.withOpacity(0.6),
                              ),
                              child: Text(
                                snapshot.error.toString(),
                                style: TextStyle(
                                  color: Colors.white,
                                ),
                                textAlign: TextAlign.center,
                              ),
                            ),
                        ],
                      ),
                    ),
                  );
                },
              ),
            ],
          ),
        ),
      ),
    );
  }
}

/// Main logged in view of the app.
class Main extends StatefulWidget {
  static Route<dynamic> route() {
    return MaterialPageRoute(
      builder: (BuildContext context) => const Main(),
    );
  }

  const Main({
    Key key,
  }) : super(key: key);

  @override
  _MainState createState() => _MainState();
}

class _MainState extends State<Main> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Theme.of(context).primaryColorDark,
      body: SafeArea(
        child: SizedBox.expand(
          child: Column(
            children: <Widget>[
              const SizedBox(height: 16.0),
              Text(
                'Main Screen',
                style: Theme.of(context).textTheme.title,
                textAlign: TextAlign.center,
              ),
              const SizedBox(height: 36.0),
              const UserAvatar(),
              const SizedBox(height: 16.0),
              RaisedButton(
                onPressed: () {
                  Auth.of(context).logout();
                },
                child: const Text('LOGOUT'),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

/// Displays the user's image
class UserAvatar extends StatelessWidget {
  const UserAvatar({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final user = Provider.of<FirebaseUser>(context, listen: false);
    return Material(
      elevation: 4.0,
      shape: const CircleBorder(
        side: BorderSide(color: Colors.blue, width: 6.0),
      ),
      color: Colors.black,
      clipBehavior: Clip.antiAlias,
      child: SizedBox(
        width: 96.0,
        height: 96.0,
        child: user?.photoUrl != null
            ? Image.network(user.photoUrl)
            : Icon(
                Icons.person,
                color: Colors.white,
                size: 72.0,
              ),
      ),
    );
  }
}

/// Auth backend
class Auth {
  static Future<Auth> create() async {
    final currentUser = await FirebaseAuth.instance.currentUser();
    return Auth._(currentUser);
  }

  static Auth of(BuildContext context) {
    return Provider.of<Auth>(context, listen: false);
  }

  Auth._(
    FirebaseUser currentUser,
  ) : this.currentUser = ValueNotifier<FirebaseUser>(currentUser);

  final ValueNotifier<FirebaseUser> currentUser;

  final _googleSignIn = GoogleSignIn();
  final _firebaseAuth = FirebaseAuth.instance;
  StreamSubscription<FirebaseUser> _authSub;

  FirebaseUser init(VoidCallback onUserChanged) {
    currentUser.addListener(onUserChanged);
    _authSub = _firebaseAuth.onAuthStateChanged.listen((FirebaseUser user) {
      currentUser.value = user;
    });
    return currentUser.value;
  }

  void dispose(VoidCallback onUserChanged) {
    currentUser.removeListener(onUserChanged);
    _authSub.cancel();
  }

  Future<void> loginWithEmailAndPassword(String email, String password) async {
    try {
      await _firebaseAuth.signInWithEmailAndPassword(email: email, password: password);
    } catch (e, st) {
      throw _getAuthException(e, st);
    }
  }

  Future<void> loginWithGoogle() async {
    try {
      final account = await _googleSignIn.signIn();
      if (account == null) {
        throw AuthException.cancelled;
      }
      final auth = await account.authentication;
      await _firebaseAuth.signInWithCredential(
        GoogleAuthProvider.getCredential(idToken: auth.idToken, accessToken: auth.accessToken),
      );
    } catch (e, st) {
      throw _getAuthException(e, st);
    }
  }

  Future<void> logout() async {
    try {
      await _firebaseAuth.signOut();
      await _googleSignIn.signOut();
    } catch (e, st) {
      FlutterError.reportError(FlutterErrorDetails(exception: e, stack: st));
    }
  }

  AuthException _getAuthException(dynamic e, StackTrace st) {
    if (e is AuthException) {
      return e;
    }
    FlutterError.reportError(FlutterErrorDetails(exception: e, stack: st));
    if (e is PlatformException) {
      switch (e.code) {
        case 'ERROR_INVALID_EMAIL':
          throw const AuthException('Please check your email address.');
        case 'ERROR_WRONG_PASSWORD':
          throw const AuthException('Please check your password.');
        case 'ERROR_USER_NOT_FOUND':
          throw const AuthException('User not found. Is that the correct email address?');
        case 'ERROR_USER_DISABLED':
          throw const AuthException('Your account has been disabled. Please contact support');
        case 'ERROR_TOO_MANY_REQUESTS':
          throw const AuthException('You have tried to login too many times. Please try again later.');
      }
    }
    throw const AuthException('Sorry, an error occurred. Please try again.');
  }
}

class AuthException implements Exception {
  static const cancelled = AuthException('cancelled');

  const AuthException(this.message);

  final String message;

  @override
  String toString() => message;
}

Nice example!
Simon, is it the way you code for your production ready apps? Or this is only for example? Can you please write a post for the way to write code for scalable apps?