validate method

@override FutureOr<String?> validate(Object? value, { String? fieldName, })

This validator type supports asynchronous validation It might come useful e.g. if you need to make some backend validation of the data entered into your form. With this validator you don't have to invent a custom approach to validate anything. Just do all of your asynchronous work inside the validator's body and then return the result. returns Returns an error text if the validation fails. If it returns null then the validation is successful

Implementation

@override
FutureOr<String?> validate(
  Object? value, {
  String? fieldName,
}) {
  if (value is String) {
    if (value.isNotEmpty != true) {
      return '$fieldName is required';
    }
    final error = _validateName(value.toString());
    if (error?.isNotEmpty == true) {
      return '$fieldName is required';
    }
  }

  return null;
}