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() : array {
        $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);
        }
    }

}