/**
* Add one or more fields to the list of fields that are required.
*
* @param string|string[] $fields
*
* @throws InvalidArgumentException
* @return $this
*/
public function required($fields)
{
Arguments::define(Boa::either(Boa::string(), Boa::arrOf(Boa::string())))->check($fields);
if (!is_array($fields)) {
$fields = [$fields];
}
$this->required = $this->required->append(ArrayList::of($fields));
return $this;
}
