code: protected function transform(DocumentInterface $document, $entity, $skip = null)
      {
          $entityMethods = get_class_methods($entity);
          $documentMethods = get_class_methods($document);
          if ($skip === null) {
              $skip = $this->getCopySkipFields();
          }
  
          foreach ($entityMethods as $method) {
              if (strpos($method, 'get') !== 0) {
                  continue;
              }
              $property = substr($method, 3);
              if (in_array(lcfirst($property), $skip)) {
                  continue;
              }
              $setter = 'set' . $property;
              if (in_array($setter, $documentMethods)) {
                  $document->{$setter}($entity->{$method}());
              }
          }
      }
docs: Copies properties that have matching getter and setter.
  
  @param DocumentInterface $document
  @param object            $entity
  @param string[]          $skip