In this app, I have implemented Google Sign In using Firebase for both Android and iOS, fixing all the issues with the latest Flutter updates. To understand how to fix all the Firebase issues in Flutter make sure you check out my Medium article.

NOTE: The project is tested on Flutter 1.7 (stable) and using all the latest versions of the plugins.

There are three versions of this project available:

• Simple Google Sign In (master)
• Combined with Auto Login (auto_login)
• Combined with Local Authentication using Biometric (local_auth)

If you want to clone and use this app, then you have to complete the following steps:

Use the following command to generate SHA-1:

keytool -list -v \
-alias androiddebugkey -keystore ~/.android/debug.keystore

First of all, complete the whole Firebase setup for both Android and iOS. You will get two files while doing the setup, one for each platform. You have to place the google-services.json & GoogleService-Info.plist files in the respective directory of each platform. For more info, check out my Medium article.

NOTE: USE THE SHA-1 GENERATED FROM YOUR SYSTEM

For the iOS part, you have to do one more step. You will find a TODO in Info.plist file, just complete that.

Now, you can run the app on your device using the command:

flutter run

The plugins used in this project are:

1. firebase_core
2. firebase_auth.
3. google_sign_in.

Add this to your package's pubspec.yaml file to use Firebase & Google Sign In:

dependencies:
  firebase_core: ^0.5.0
  firebase_auth: ^0.18.0+1
  google_sign_in: ^4.5.3

Import using:

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

Following are two useful methods for authentication using Firebase and Google Sign In. You can use these as the basic template for your starting project.

NOTE: These does not check for all edge cases and you should add other security restrictions as per your requirement in your production app

For signing in using a Google account:

Future<String> signInWithGoogle() async {
  await Firebase.initializeApp();

  final GoogleSignInAccount googleSignInAccount = await googleSignIn.signIn();
  final GoogleSignInAuthentication googleSignInAuthentication = await googleSignInAccount.authentication;

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

  final UserCredential authResult = await _auth.signInWithCredential(credential);
  final User user = authResult.user;

  if (user != null) {
    // Checking if email and name is null
    assert(user.email != null);
    assert(user.displayName != null);
    assert(user.photoURL != null);

    name = user.displayName;
    email = user.email;
    imageUrl = user.photoURL;

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

    final User currentUser = _auth.currentUser;
    assert(user.uid == currentUser.uid);

    print('signInWithGoogle succeeded: $user');

    return '$user';
  }

  return null;
}

For signing out of a Google account:

Future<void> signOutGoogle() async {
  await googleSignIn.signOut();

  print("User Signed Out");
}

Copyright (c) 2019 Souvik Biswas

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.