code: public function scheduleUpdate(Contao\DataContainer $dc)
  	{
  		// Return if there is no ID
  		if (!$dc->activeRecord || !$dc->activeRecord->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('sitemap_updater');
  		$session[] = Contao\PageModel::findWithDetails($dc->activeRecord->id)->rootId;
  		$objSession->set('sitemap_updater', array_unique($session));
  	}
docs: Schedule a sitemap update
  
  This method is triggered when a single page or multiple pages are
  modified (edit/editAll), moved (cut/cutAll) or deleted
  (delete/deleteAll). Since duplicated pages are unpublished by default,
  it is not necessary to schedule updates on copyAll as well.
  
  @param Contao\DataContainer $dc