validation.dart

  String validator(String text) {
    if (widget.validator == null) return null;
    String error = widget.validator(text);
    InputStatus inst;
    if (error == null)
      inst = InputStatus.valid;
    else
      inst = InputStatus.invalid;

    if (inst != inputStatus) {
      inputStatus = inst;
      setInputDetails(error);
      setState(() {});
    }

    return error;
  }