validate method

@override String? validate(String input)

Validates the provided input using the custom validation rule.

The input parameter is the value to be validated. Returns the error message if the validation fails, otherwise returns null.

Implementation

@override
String? validate(String input) {
  // Use the predicate function to determine if the input passes the custom validation rule.
  // If the predicate returns true, the validation is considered successful, and null is returned.
  // Otherwise, the error message is returned.
  return predicate(input) ? null : message;
}