Firebase Setup & Auth

Real-world analogy

What is it?

Firebase is Google's suite of cloud services for mobile apps. Firebase Auth handles user authentication (email, Google, Apple sign-in). Crashlytics captures and reports crashes. In team_mvp_kit, Firebase Auth is used for social logins -- the Firebase ID token is exchanged with the backend for app-specific access and refresh tokens.

Real-world relevance

team_mvp_kit uses Firebase as a bridge for authentication. Users sign in with Google or Apple through Firebase Auth, then the app sends the Firebase ID token to the backend server. The backend verifies it and returns its own JWT tokens. Crashlytics catches every crash in production so the team can fix issues before users even report them.

Key points

• What is Firebase? — Firebase is a suite of cloud services by Google. For Flutter apps, the most useful services are: Authentication (login/signup), Crashlytics (crash reporting), Remote Config (change app behavior without updates), Cloud Firestore (real-time database), and Cloud Messaging (push notifications). team_mvp_kit uses Auth and Crashlytics.
• Adding Firebase to Flutter — Use the FlutterFire CLI to configure Firebase automatically. Run 'flutterfire configure' and it generates the firebase_options.dart file with your project's API keys and configuration for each platform (Android, iOS, web). This replaces the old manual google-services.json setup.
• Firebase Initialization — Initialize Firebase in main() before runApp(). This must happen before any Firebase service is used. In team_mvp_kit, Firebase init is part of the dependency injection setup to ensure correct initialization order.
• Firebase Authentication Basics — FirebaseAuth provides methods for creating accounts, signing in, signing out, and listening to auth state changes. It supports email/password, Google, Apple, phone number, and anonymous sign-in. team_mvp_kit uses Firebase Auth primarily for social logins and backend token exchange.
• Auth State Listener — FirebaseAuth provides an authStateChanges() stream that emits whenever the user signs in or out. Use this to reactively update your UI or routing. When the stream emits null, the user is signed out. When it emits a User, they are signed in.
• Getting Firebase ID Token — After signing in with Firebase, get the ID token to send to your backend server. The backend verifies this token with Firebase Admin SDK to confirm the user's identity. This is how team_mvp_kit bridges Firebase Auth with its own backend API.
• Crashlytics Setup — Firebase Crashlytics automatically captures and reports app crashes. Initialize it after Firebase and enable it in main(). Uncaught Flutter errors and Dart errors are automatically sent. You can also log custom errors and add user context for better debugging.
• Custom Crashlytics Logging — Add custom keys, log messages, and record non-fatal errors to Crashlytics for better debugging. Set the user ID so you can filter crashes by user. team_mvp_kit sets the user ID after login and logs important state changes.

Code example

import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_crashlytics/firebase_crashlytics.dart';
import 'firebase_options.dart';

// 1. App initialization with Firebase
void main() async {
  WidgetsFlutterBinding.ensureInitialized();

  // Initialize Firebase
  await Firebase.initializeApp(
    options: DefaultFirebaseOptions.currentPlatform,
  );

  // Setup Crashlytics error handlers
  FlutterError.onError =
      FirebaseCrashlytics.instance.recordFlutterError;

  PlatformDispatcher.instance.onError = (error, stack) {
    FirebaseCrashlytics.instance.recordError(error, stack);
    return true;
  };

  await configureDependencies();
  runApp(const MyApp());
}

// 2. FirebaseAuthService wraps Firebase Auth (team_mvp_kit)
@lazySingleton
class FirebaseAuthService {
  final FirebaseAuth _auth;
  FirebaseAuthService(this._auth);

  // Current user
  User? get currentUser => _auth.currentUser;
  bool get isSignedIn => currentUser != null;

  // Auth state stream
  Stream<User?> get authStateChanges =>
      _auth.authStateChanges();

  // Email/password sign up
  Future<UserCredential> signUpWithEmail({
    required String email,
    required String password,
  }) async {
    return _auth.createUserWithEmailAndPassword(
      email: email,
      password: password,
    );
  }

  // Email/password sign in
  Future<UserCredential> signInWithEmail({
    required String email,
    required String password,
  }) async {
    return _auth.signInWithEmailAndPassword(
      email: email,
      password: password,
    );
  }

  // Get ID token for backend exchange
  Future<String?> getIdToken() async {
    return currentUser?.getIdToken();
  }

  // Sign out
  Future<void> signOut() async {
    await _auth.signOut();
  }
}

// 3. Module to register FirebaseAuth instance
@module
abstract class FirebaseModule {
  @singleton
  FirebaseAuth get firebaseAuth => FirebaseAuth.instance;

  @singleton
  FirebaseCrashlytics get crashlytics =>
      FirebaseCrashlytics.instance;
}

Line-by-line walkthrough

1. 1. Import firebase_core for initialization
2. 2. Import firebase_auth for authentication
3. 3. Import firebase_crashlytics for crash reporting
4. 4. Import the generated Firebase config for this project
5. 5.
6. 6. Comment: App initialization with all Firebase services
7. 7. Main function marked as async for await calls
8. 8. Ensure Flutter is ready before async operations
9. 9.
10. 10. Comment: Initialize Firebase with platform-specific config
11. 11. Call Firebase.initializeApp with the generated options
12. 12. DefaultFirebaseOptions.currentPlatform picks Android/iOS/web config
13. 13. Closing the Firebase.initializeApp call
14. 14.
15. 15. Comment: Set up Crashlytics to catch all errors
16. 16. Assign Flutter framework errors to Crashlytics recorder
17. 17. Closing the FlutterError.onError assignment
18. 18.
19. 19. Set up platform error handler for async Dart errors
20. 20. Record the error and stack trace to Crashlytics
21. 21. Return true to mark the error as handled
22. 22. Closing the PlatformDispatcher setup
23. 23.
24. 24. Initialize dependency injection (after Firebase is ready)
25. 25. Start the app
26. 26. Closing main
27. 27.
28. 28. Comment: FirebaseAuthService wraps FirebaseAuth
29. 29. @lazySingleton for DI registration
30. 30. Class declaration
31. 31. Private FirebaseAuth instance (injected)
32. 32. Constructor receives FirebaseAuth
33. 33.
34. 34. Comment: Current user access
35. 35. Getter returns the currently signed-in user or null
36. 36. Getter checks if a user is signed in
37. 37.
38. 38. Comment: Auth state stream for reactive listening
39. 39. Returns the authStateChanges stream from FirebaseAuth
40. 40. Closing the getter
41. 41.
42. 42. Comment: Email/password registration
43. 43. signUpWithEmail method with required email and password
44. 44. Calls FirebaseAuth createUserWithEmailAndPassword
45. 45. Passes email and password
46. 46. Closing the method
47. 47.
48. 48. Comment: Email/password sign in
49. 49. signInWithEmail method with required email and password
50. 50. Calls FirebaseAuth signInWithEmailAndPassword
51. 51. Passes email and password
52. 52. Closing the method
53. 53.
54. 54. Comment: Get Firebase ID token for backend exchange
55. 55. getIdToken returns the current user's ID token
56. 56. Calls getIdToken on the current user (null-safe)
57. 57. Closing the method
58. 58.
59. 59. Comment: Sign out
60. 60. signOut method clears the Firebase session
61. 61. Calls signOut on FirebaseAuth instance
62. 62. Closing signOut and FirebaseAuthService
63. 63.
64. 64. Comment: Module to provide Firebase instances to DI
65. 65. @module annotation for third-party dependencies
66. 66. Abstract class for the module
67. 67. @singleton provides one FirebaseAuth instance
68. 68. Returns FirebaseAuth.instance
69. 69.
70. 70. @singleton provides one Crashlytics instance
71. 71. Returns FirebaseCrashlytics.instance
72. 72. Closing the module

Spot the bug

void main() {
  runApp(const MyApp());
  Firebase.initializeApp(
    options: DefaultFirebaseOptions.currentPlatform,
  );
}

Explain like I'm 5

Fun fact

Hands-on challenge

More resources

• FlutterFire Overview (FlutterFire)
• Firebase Auth for Flutter (FlutterFire)
• Firebase Crashlytics for Flutter (FlutterFire)