Example of applying Google and Email Auth in Flutter

LoginScreen.dart

import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_learn/presentation/auth/register_screen.dart';
import 'package:firebase_learn/presentation/main_screen.dart';
import 'package:flutter/material.dart';
import 'package:google_sign_in/google_sign_in.dart';

class LoginScreen extends StatefulWidget {
  const LoginScreen({Key? key});

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

class LoginScreenState extends State<LoginScreen> {
  final _emailController = TextEditingController();
  final _passwordController = TextEditingController();

  String _errorMessage = '';

  Future<UserCredential> loginWithGoogle() async {
    final GoogleSignInAccount? googleUser = await GoogleSignIn().signIn();
    final GoogleSignInAuthentication? googleAuth =
        await googleUser?.authentication;

    final credential = GoogleAuthProvider.credential(
        accessToken: googleAuth?.accessToken, idToken: googleAuth?.idToken);

    return await FirebaseAuth.instance.signInWithCredential(credential);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sign In'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(16.0),
        child: SingleChildScrollView(
          child: Column(
            children: [
              const SizedBox(height: 32.0),
              TextField(
                controller: _emailController,
                decoration: const InputDecoration(
                  labelText: 'Email',
                  border: OutlineInputBorder(),
                ),
              ),
              const SizedBox(height: 16.0),
              TextField(
                controller: _passwordController,
                decoration: const InputDecoration(
                  labelText: 'Password',
                  border: OutlineInputBorder(),
                ),
                obscureText: true,
              ),
              const SizedBox(height: 16.0),
              ElevatedButton(
                onPressed: () async {
                  try {
                    await FirebaseAuth.instance.signInWithEmailAndPassword(
                      email: _emailController.text,
                      password: _passwordController.text,
                    );
                    Navigator.of(context).pushReplacement(
                      MaterialPageRoute(
                          builder: (context) => const MainScreen()),
                    );
                  } catch (error) {
                    setState(() {
                      _errorMessage = error.toString();
                    });
                    ScaffoldMessenger.of(context).showSnackBar(
                      SnackBar(
                        content: Text(_errorMessage),
                      ),
                    );
                  }
                },
                child: const Text('Sign In'),
              ),
              const SizedBox(height: 16.0),
              TextButton(
                onPressed: () async {
                  try {
                    UserCredential userCredential = await loginWithGoogle();
                    if (userCredential != null) {
                      Navigator.of(context).pushReplacement(
                        MaterialPageRoute(
                            builder: (context) => const MainScreen()),
                      );
                    }
                  } catch (error) {
                    setState(() {
                      _errorMessage = error.toString();
                    });
                    ScaffoldMessenger.of(context).showSnackBar(
                      SnackBar(
                        content: Text(_errorMessage),
                      ),
                    );
                  }
                },
                child: const Text('Login with Google'),
              ),
              const SizedBox(height: 32.0),
              TextButton(
                onPressed: () {
                  Navigator.push(
                    context,
                    MaterialPageRoute(
                        builder: (context) => const RegisterScreen()),
                  );
                },
                child: const Text('Don\'t have an account? Sign up'),
              ),
            ],
          ),
        ),
      ),
    );
  }
}