code: public function scheduleUpdate($id)
  	{
  		// The onsubmit_callback passes a DataContainer object
  		if (\is_object($id))
  		{
  			$id = $id->id;
  		}
  
  		// Return if there is no ID
  		if (!$id || Contao\Input::get('act') == 'copy')
  		{
  			return;
  		}
  
  		/** @var Symfony\Component\HttpFoundation\Session\SessionInterface $objSession */
  		$objSession = Contao\System::getContainer()->get('session');
  
  		// Store the ID in the session
  		$session = $objSession->get('style_sheet_updater');
  		$session[] = $id;
  		$objSession->set('style_sheet_updater', array_unique($session));
  	}
docs: Schedule a style sheet update
  
  This method is triggered when a single style sheet or multiple style
  sheets are modified (edit/editAll) or duplicated (copy/copyAll).
  
  @param mixed $id