Flutter example for async form validation

async_validation_form.dart

class UsernameForm extends StatefulWidget {
  final Function onSuccess;

  UsernameForm(this.onSuccess);

  @override
  _UsernameFormState createState() => _UsernameFormState(onSuccess);
}

class _UsernameFormState extends State<UsernameForm> {
  final _formKey = GlobalKey<FormState>();
  String _username;
  bool _usernameTaken = false;
  bool _success = false;
  final Function onSuccess;

  _UsernameFormState(this.onSuccess);

  @override
  Widget build(BuildContext context) {
    // auto validate the form when the state changes
    _formKey.currentState?.validate();

    return Container(
      child: Form(
        key: _formKey,
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            TextFormField(
              validator: (value) {
                print("validating...");
                if (value.isEmpty) {
                  return 'Please enter a name';
                }
                if (_usernameTaken) {
                  return 'Username is taken';
                } else if (!_success) {
                  _saveUsername();
                }
              },
              onSaved: (value) {
                _username = value;
                _checkUsername(value);
              },
              cursorColor: Palette.colorTextPrimary,
              decoration: new InputDecoration(
                hintText: "Username",
                labelText: "Pick a username",
              ),
            ),
            Padding(
              padding: const EdgeInsets.symmetric(vertical: 16.0),
              child: RaisedButton(
                onPressed: () {
                  // Validate will return true if the form is valid, or false if
                  // the form is invalid.
                  _formKey.currentState.save();
                },
                child: Text('Submit'),
              ),
            ),
          ],
        ),
      ),
    );
  }

  _saveUsername() {
    FirebaseAuth.instance.currentUser().then((value) {
      Firestore.instance
          .collection('usernames')
          .document(_username)
          .setData({'a': true}).then((val) {
        Firestore.instance
            .collection('users')
            .document(value.uid)
            .setData({'name': _username}).then((val) {
          print("success");
          setState(() {
            _success = true;
          });
          this.onSuccess();
        });
      });
    });
  }

  _checkUsername(String username) {
    if (username != null && !username.isEmpty) {
      Firestore.instance
          .collection('usernames')
          .document(username)
          .get()
          .then((value) {
        setState(() {
          _usernameTaken = value.data != null;
        });
      });
    } else {
      setState(() {
        _usernameTaken = true;
      });
    }
  }
}