validate method

Implementation

@override
String? validate(dynamic value) {
  final validationResult = validateFunction(
    value,
  );
  if (validationResult.isError) {
    return errorMessageBuilder == null
        ? validationResult.firstErrorToString
        : errorMessageBuilder!(
            validationResult.firstError.fieldName,
            validationResult.firstError.errorMessage,
          );
  }
  return null;
}