class PrepareLayout

class PrepareLayout implements EventSubscriberInterface {
  use StringTranslationTrait;
  
  /**
   * The layout tempstore repository.
   *
   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
   */
  protected $layoutTempstoreRepository;
  
  /**
   * The messenger service.
   *
   * @var \Drupal\Core\Messenger\MessengerInterface
   */
  protected $messenger;
  
  /**
   * Constructs a new PrepareLayout.
   *
   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
   *   The tempstore repository.
   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
   *   The messenger service.
   */
  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, MessengerInterface $messenger) {
    $this->layoutTempstoreRepository = $layout_tempstore_repository;
    $this->messenger = $messenger;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events[LayoutBuilderEvents::PREPARE_LAYOUT][] = [
      'onPrepareLayout',
      10,
    ];
    return $events;
  }
  
  /**
   * Prepares a layout for use in the UI.
   *
   * @param \Drupal\layout_builder\Event\PrepareLayoutEvent $event
   *   The prepare layout event.
   */
  public function onPrepareLayout(PrepareLayoutEvent $event) {
    $section_storage = $event->getSectionStorage();
    // If the layout has pending changes, add a warning.
    if ($this->layoutTempstoreRepository
      ->has($section_storage)) {
      $this->messenger
        ->addWarning($this->t('You have unsaved changes.'));
    }
    else {
      // If the layout is an override that has not yet been overridden, copy the
      // sections from the corresponding default.
      if ($section_storage instanceof OverridesSectionStorageInterface && !$section_storage->isOverridden()) {
        $sections = $section_storage->getDefaultSectionStorage()
          ->getSections();
        foreach ($sections as $section) {
          $section_storage->appendSection($section);
        }
      }
      // Add storage to tempstore regardless of what the storage is.
      $this->layoutTempstoreRepository
        ->set($section_storage);
    }
  }

}