/**
  * Sets a view mode or get current view mode.
  *
  * @param string|null $slug Slug name of the view mode
  * @return void
  * @see \CMS\View\ViewModeRegistry::uses()
  */
 public function viewMode($slug = null)
 {
     if ($slug === null) {
         return ViewModeRegistry::current();
     }
     return ViewModeRegistry::uses($slug);
 }