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Integrating Google Authentication in your Flutter app enhances the user experience by providing a quick and secure sign-in option. This blog walks you through setting up Google Authentication in a Flutter application using Firebase. We'll cover everything from Firebase project setup to Flutter code implementation.

Step 1: Setting Up Firebase

1. Go to the Firebase Console and create a new project.
2. Click on 'Add app' and select the Flutter icon to add your Flutter app to the project.
3. Follow the instructions to download the google-services.json file for Android and GoogleService-Info.plist for iOS, then place them in your app’s respective directories.

Step 2: Adding Firebase to Your Flutter App

Include Firebase in your Flutter project by adding the necessary dependencies in your pubspec.yaml file:

dependencies:
  flutter:
    sdk: flutter
  firebase_core: latest_version
  firebase_auth: latest_version
  google_sign_in: latest_version

Run flutter pub get to install these packages.

Step 3: Implementing Google Sign-In

Start by initializing Firebase in your main.dart file:

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

Create a new Dart file for handling the authentication process, e.g., auth.dart, and add the following code:

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

class AuthService {
  final FirebaseAuth _auth = FirebaseAuth.instance;
  final GoogleSignIn googleSignIn = GoogleSignIn();

  Future signInWithGoogle() async {
    final GoogleSignInAccount? googleSignInAccount = await googleSignIn.signIn();
    if (googleSignInAccount != null) {
      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;
      return user;
    }
    return null;
  }

  void signOutGoogle() async {
    await googleSignIn.signOut();
    print("User Signed Out");
  }
}

Finally, implement the UI and logic in your app to trigger Google sign-in and sign-out. For instance:

ElevatedButton(
  onPressed: () async {
    final user = await AuthService().signInWithGoogle();
    if (user != null) {
      print('Signed in as ${user.displayName}');
    }
  },
  child: Text('Sign in with Google'),
);

Conclusion

Google Authentication enhances the security and ease of use of your app. By following these steps, you can successfully integrate Google sign-in into your Flutter app using Firebase. For any additional configurations, especially for iOS, refer to the Firebase documentation for detailed guidance.