dependencies:
  flutter:
    sdk: flutter
  firebase_core: ^2.0.0
  firebase_auth

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

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Firebase Auth',
      home: const LoginPage(),
    );
  }
}

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

class LoginPage extends StatefulWidget {
  const LoginPage({super.key});
  @override
  State<LoginPage> createState() => _LoginPageState();
}

class _LoginPageState extends State<LoginPage> {
  final _emailCtl = TextEditingController();
  final _passCtl = TextEditingController();
  final _auth = FirebaseAuth.instance;

  Future<void> _signIn() async {
    try {
      await _auth.signInWithEmailAndPassword(
        email: _emailCtl.text.trim(),
        password: _passCtl.text.trim(),
      );
    } catch (e) {
      ScaffoldMessenger.of(context).showSnackBar(
        SnackBar(content: Text('Error: ${e.toString()}')),
      );
    }
  }

  Future<void> _register() async {
    try {
      await _auth.createUserWithEmailAndPassword(
        email: _emailCtl.text.trim(),
        password: _passCtl.text.trim(),
      );
    } catch (e) {
      ScaffoldMessenger.of(context).showSnackBar(
        SnackBar(content: Text('Error: ${e.toString()}')),
      );
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Login')),
      body: Padding(
        padding: const EdgeInsets.all(16),
        child: Column(
          children: [
            TextField(controller: _emailCtl, decoration: const InputDecoration(labelText: 'Email')),
            TextField(controller: _passCtl, decoration: const InputDecoration(labelText: 'Password'), obscureText: true),
            const SizedBox(height: 20),
            ElevatedButton(onPressed: _signIn, child: const Text('Sign In')),
            TextButton(onPressed: _register, child: const Text('Register')),
          ],
        ),
      ),
    );
  }
}

StreamBuilder<User?>(
  stream: FirebaseAuth.instance.authStateChanges(),
  builder: (context, snapshot) {
    if (snapshot.connectionState == ConnectionState.waiting) {
      return const CircularProgressIndicator();
    } else if (snapshot.hasData) {
      return const HomePage();  // User is signed in
    } else {
      return const LoginPage(); // User is signed out
    }
  },
)