fltkeep-login_screen.dart

class LoginScreen extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  final _auth = FirebaseAuth.instance;
  final _googleSignIn = GoogleSignIn();
  String _errorMessage;
  
  @override
  Widget build(BuildContext context) => Scaffold(
    body: Center(
      child: Column(
        children: <Widget>[
          RaisedButton(
            onPressed: _signInWithGoogle,
            child: const Text('Continue with Google'),
          ),
          if (_errorMessage != null) Text(
            _errorMessage,
            style: const TextStyle(color: Colors.red),
          ),
        ],
      ),
    ),
  );
  
  void _signInWithGoogle() async {
    _setLoggingIn(); // show progress
    String errMsg;

    try {
      final googleUser = await _googleSignIn.signIn();
      final googleAuth = await googleUser.authentication;
      final credential = GoogleAuthProvider.getCredential(
        idToken: googleAuth.idToken,
        accessToken: googleAuth.accessToken,
      );
      await _auth.signInWithCredential(credential);
    } catch (e) {
      errMsg = 'Login failed, please try again later.';
    } finally {
      _setLoggingIn(false, errMsg); // always stop the progress indicator
    }
  }
  
  /// update the logging-in indicator, & show error message if any
  void _setLoggingIn([bool loggingIn = true, String errMsg]) {
    if (mounted) {
      setState(() {
        _loggingIn = loggingIn;
        _errorMessage = errMsg;
      });
    }
  }
}