Firebase-Einrichtung und Authentifizierung

Firebase for Flutter Apps

Firebase Project Setup

Adding Firebase to Flutter

Initialize Firebase Core

import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';
// import 'firebase_options.dart'; // Generated by FlutterFire CLI

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp(
    // options: DefaultFirebaseOptions.currentPlatform, // Use if generated
  );
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Firebase App',
      home: Scaffold(
        appBar: AppBar(title: Text('Firebase Init')),
        body: Center(child: Text('Firebase initialized!')),
      ),
    );
  }
}

Config File Placement

Email/Password Sign Up

import 'package:firebase_auth/firebase_auth.dart';

class AuthService {
  final FirebaseAuth _auth = FirebaseAuth.instance;

  Future<User?> signUp(String email, String password) async {
    try {
      UserCredential result = await _auth.createUserWithEmailAndPassword(
        email: email,
        password: password,
      );
      return result.user;
    } catch (e) {
      print(e.toString());
      return null;
    }
  }

  // This main is for demonstration of the function logic.
  // In a real app, Firebase.initializeApp() would be called once at app start.
  static void main() async {
    // Simulate Firebase being initialized
    // await Firebase.initializeApp(); 
    
    AuthService auth = AuthService();
    User? user = await auth.signUp("test@example.com", "password123");
    if (user != null) {
      print("Signed up user: ${user.email}");
    } else {
      print("Sign up failed.");
    }
  }
}

Email/Password Sign In

import 'package:firebase_auth/firebase_auth.dart';

class AuthService {
  final FirebaseAuth _auth = FirebaseAuth.instance;

  Future<User?> signIn(String email, String password) async {
    try {
      UserCredential result = await _auth.signInWithEmailAndPassword(
        email: email,
        password: password,
      );
      return result.user;
    } catch (e) {
      print(e.toString());
      return null;
    }
  }

  // This main is for demonstration of the function logic.
  // In a real app, Firebase.initializeApp() would be called once at app start.
  static void main() async {
    // Simulate Firebase being initialized
    // await Firebase.initializeApp(); 
    
    AuthService auth = AuthService();
    User? user = await auth.signIn("test@example.com", "password123");
    if (user != null) {
      print("Signed in user: ${user.email}");
    } else {
      print("Sign in failed.");
    }
  }
}

Monitoring Auth State

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Auth State App',
      home: StreamBuilder<User?>(
        stream: FirebaseAuth.instance.authStateChanges(),
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.waiting) {
            return CircularProgressIndicator();
          }
          if (snapshot.hasData) {
            return HomeScreen(user: snapshot.data!); // User is signed in
          } else {
            return LoginScreen(); // User is signed out
          }
        },
      ),
    );
  }
}

class HomeScreen extends StatelessWidget {
  final User user;
  HomeScreen({required this.user});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Welcome')),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('Logged in as: ${user.email}'),
            ElevatedButton(
              onPressed: () async {
                await FirebaseAuth.instance.signOut();
              },
              child: Text('Sign Out'),
            ),
          ],
        ),
      ),
    );
  }
}

Google Sign-In Setup

Implementing Google Sign-In

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

class GoogleAuthService {
  final FirebaseAuth _auth = FirebaseAuth.instance;
  final GoogleSignIn _googleSignIn = GoogleSignIn();

  Future<User?> signInWithGoogle() async {
    try {
      final GoogleSignInAccount? googleUser = await _googleSignIn.signIn();
      if (googleUser == null) return null; // User cancelled the sign-in

      final GoogleSignInAuthentication googleAuth = await googleUser.authentication;

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

      UserCredential result = await _auth.signInWithCredential(credential);
      return result.user;
    } catch (e) {
      print(e.toString());
      return null;
    }
  }

  // This main is for demonstration of the function logic.
  // In a real app, Firebase.initializeApp() would be called once at app start.
  static void main() async {
    // Simulate Firebase being initialized
    // await Firebase.initializeApp(); 
    
    GoogleAuthService auth = GoogleAuthService();
    User? user = await auth.signInWithGoogle();
    if (user != null) {
      print("Signed in with Google: ${user.email}");
    } else {
      print("Google Sign-In failed or cancelled.");
    }
  }
}

Firebase Auth Quick Check

Recap & Next Steps