login.dart

//Function to be called after submitting the form.

final _formKey = GlobalKey<FormState>();
  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
  Map<String, String> _authData22 = {
    'username': '',
  };
Future<void> _submit(BuildContext context) async {
    if (!_formKey.currentState.validate()) {
      // Invalid! test
      return;
    }
    _formKey.currentState.save();
    setState(() {
      isLoading = true;
    });

    final data = await Provider.of<Auth>(context, listen: false)
        .login(_authData22['username']);
    setState(() {
      isLoading = false;
    });
    if (data) {
      Navigator.pushReplacement(context, FadeNavigation(widget: HomeScreen()));
    } else {
      final snackBar = new SnackBar(
        content: new Text('Invalid Username'),
        action: new SnackBarAction(
          label: 'OK',
          onPressed: () async {},
        ),
        duration: new Duration(seconds: 3),
      );
      _scaffoldKey.currentState.showSnackBar(snackBar);
    }
  }