authentication.dart

Future<String> signInWithEmailPassword(String email, String password) async {
  // Initialize Firebase
  await Firebase.initializeApp();

  final UserCredential userCredential = await _auth.signInWithEmailAndPassword(
    email: email,
    password: password,
  );

  final User user = userCredential.user;

  if (user != null) {
    // checking if uid or email is null
    assert(user.uid != null);
    assert(user.email != null);

    uid = user.uid;
    userEmail = user.email;

    assert(!user.isAnonymous);
    assert(await user.getIdToken() != null);

    final User currentUser = _auth.currentUser;
    assert(user.uid == currentUser.uid);

    SharedPreferences prefs = await SharedPreferences.getInstance();
    prefs.setBool('auth', true);

    return 'Successfully logged in, User UID: ${user.uid}';
  }

  return null;
}