validate method

@override String? validate(String? value)

Returns String containing the error message if value is invalid. or null when value is valid.

Implementation

@override
String? validate(String? value) {
  if (value == null) return null;
  if (min != null && value.length < min!) return message;
  if (max != null && value.length > max!) return message;
  return null;
}