code: protected function isNewSlugNeeded()
      {
          if (empty($this->owner->{$this->slugAttribute})) {
              return true;
          }
  
          if ($this->immutable) {
              return false;
          }
  
          if ($this->attribute === null) {
              return true;
          }
  
          foreach ((array) $this->attribute as $attribute) {
              if ($this->owner->isAttributeChanged($attribute)) {
                  return true;
              }
          }
  
          return false;
      }
docs: Checks whether the new slug generation is needed
  This method is called by [[getValue]] to check whether the new slug generation is needed.
  You may override it to customize checking.
  @return bool
  @since 2.0.7