Google Sign In With Firebase in Flutter

dependencies:
     flutter:
       sdk: flutter
     firebase_core: ^3.12.1
     firebase_auth: ^5.5.1
     google_sign_in: ^6.3.0

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

/// A service class to handle Google Sign-In
// and authentication using Firebase.
class GoogleAuthService {
    
    // FirebaseAuth instance to handle authentication.
    final FirebaseAuth _auth = FirebaseAuth.instance;
    
    // GoogleSignIn instance to handle Google Sign-In.
    final GoogleSignIn _googleSignIn = GoogleSignIn();
    
    /// Signs in the user with Google and returns the authenticated Firebase [User].
    /// 
    /// Returns `null` if the sign-in process is canceled or fails.
    Future<User?> signInWithGoogle() async {
        try {
            
            // Trigger the Google Sign-In flow.
            final googleUser = await _googleSignIn.signIn();
            
            // User canceled the sign-in.
            if (googleUser == null) return null; 
            
            // Retrieve the authentication details from the Google account.
            final googleAuth = await googleUser.authentication;
            
            // Create a new credential using the Google authentication details.
            final credential = GoogleAuthProvider.credential(
                    accessToken: googleAuth.accessToken,
                    idToken: googleAuth.idToken,
            );
            
            // Sign in to Firebase with the Google credential.
            final userCredential = await _auth.signInWithCredential(credential);
            
            // Return the authenticated user.
            return userCredential.user; 
        } catch (e) {
            
            // Print the error and return null if an exception occurs.
            print("Sign-in error: $e");
            return null;
        }
    }
    
    /// Signs out the user from both Google and Firebase.
    Future<void> signOut() async {
        
        // Sign out from Google.
        await _googleSignIn.signOut();
        
        // Sign out from Firebase.
        await _auth.signOut(); 
    }
}

import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:geeks_for_geeks/signInScreen.dart';

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

// Main application widget
class MyApp extends StatefulWidget {
    @override
    _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
    @override
    Widget build(BuildContext context) {
        return MaterialApp(debugShowCheckedModeBanner: false, home: SignInScreen());
    }
}

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:geeks_for_geeks/HomeScreen.dart';
import 'package:geeks_for_geeks/signInWithGoogle.dart';

// A stateless widget for the Sign-In screen
class SignInScreen extends StatelessWidget {
  // Instance of GoogleAuthService to handle Google Sign-In
  final GoogleAuthService _authService = GoogleAuthService();

  SignInScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      // AppBar with title and styling
      appBar: AppBar(
        title: Text("GFG Sign In"), 
        backgroundColor: Colors.green, 
        foregroundColor: Colors.white, 
      ),
      // Main body of the screen
      body: Center(
        // ElevatedButton for Google Sign-In
        child: ElevatedButton(
          style: ElevatedButton.styleFrom(
            foregroundColor: Colors.white,
            backgroundColor: Colors.green, 
          ),
          // Asynchronous function triggered on button press
          onPressed: () async {
            // Attempt to sign in with Google
            User? user = await _authService.signInWithGoogle();
            // If sign-in is successful, navigate to the HomeScreen
            if (user != null) {
              Navigator.push(
                context,
                MaterialPageRoute(builder: (_) => HomeScreen(user: user)),
              );
            }
          },
          // Text displayed on the button
          child: Text("Sign in with Google"),
        ),
      ),
    );
  }
}

import 'package:firebase_auth/firebase_auth.dart'; 
import 'package:flutter/material.dart'; 
import 'package:geeks_for_geeks/signInWithGoogle.dart'; 

// Define a stateless widget for the HomeScreen
class HomeScreen extends StatelessWidget {
    
  // Firebase User object to hold user details
  final User user; 
  HomeScreen(
      {super.key,
      
      // Constructor to initialize the user object
      required this.user}); 

  // Instance of GoogleAuthService for authentication
  final GoogleAuthService _authService = GoogleAuthService(); 

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(
            
            // Display user's name in the app bar
            "Welcome ${user.displayName}"), 
      ),
      body: Center(
        child: Column(
          mainAxisAlignment:MainAxisAlignment.center, 
          crossAxisAlignment:CrossAxisAlignment.center,
          children: [
            CircleAvatar(
                
              // Display user's profile picture
              backgroundImage: NetworkImage(user.photoURL ?? ""), 
              
              // Set the radius of the avatar
              radius: 40, 
            ),
            
            // Display user's email
            Text("Email: ${user.email}"),
            
            // Add spacing between elements
            SizedBox(height: 20), 
            ElevatedButton(
              onPressed: () async {
                
                // Sign out from Google and Firebase
                _authService.signOut();
                
                // Navigate back to the sign-in screen
                Navigator.pop(context);
              },
              child: Text("Sign Out"),
              style: ElevatedButton.styleFrom(
                foregroundColor: Colors.white,
                backgroundColor: Colors.red.shade900,
              ),
            ),
          ],
        ),
      ),
    );
  }
}