isValid method

@override String? isValid(BuildContext context, String? value)

Checks if the value contains the substring.

Returns null if the value contains the substring, otherwise returns the error message.

Implementation

@override
String? isValid(BuildContext context, String? value) {
  return value != null && value.contains(substring)
      ? null
      : errorMessage ??
          defaultMessage[Localizations.localeOf(context).languageCode] ??
          defaultMessage['en'];
}