validate method

@override ValidationException? validate(Object? value)

Return null if valid, otherwise a ValidationException.

Implementation

@override
ValidationException? validate(Object? value) {
  if (value is String) {
    for (var char in value.split('')) {
      if (!allowedChars.contains(char)) {
        return ValidationException(this, value);
      }
    }

    return null;
  } else {
    return ValidationException(
      this,
      value,
      'Value must be a String type (is ${value.runtimeType})',
    );
  }
}