code: protected function getValue($event)
      {
          if (!$this->isNewSlugNeeded()) {
              return $this->owner->{$this->slugAttribute};
          }
  
          if ($this->attribute !== null) {
              $slugParts = [];
              foreach ((array) $this->attribute as $attribute) {
                  $part = ArrayHelper::getValue($this->owner, $attribute);
                  if ($this->skipOnEmpty && $this->isEmpty($part)) {
                      return $this->owner->{$this->slugAttribute};
                  }
                  $slugParts[] = $part;
              }
              $slug = $this->generateSlug($slugParts);
          } else {
              $slug = parent::getValue($event);
          }
  
          return $this->ensureUnique ? $this->makeUnique($slug) : $slug;
      }
docs: {@inheritdoc}