getParentDepthLimit example

if (!isset($this->entity->book)) {
      // The node is not part of any book yet - set default options.       $this->entity->book = $this->bookManager->getLinkDefaults($this->entity->id());
    }
    else {
      $this->entity->book['original_bid'] = $this->entity->book['bid'];
    }

    // Find the depth limit for the parent select.     if (!isset($this->entity->book['parent_depth_limit'])) {
      $this->entity->book['parent_depth_limit'] = $this->bookManager->getParentDepthLimit($this->entity->book);
    }
    $form = $this->bookManager->addFormElements($form, $form_state, $this->entity, $this->currentUser(), FALSE);

    return $form;
  }

  /** * {@inheritdoc} */
  protected function actions(array $form, FormStateInterface $form_state) {
    $actions = parent::actions($form, $form_state);
    

  /** * {@inheritdoc} */
  public function getParentSelectOptions($id = '', array $menus = NULL, CacheableMetadata &$cacheability = NULL) {
    if (!isset($menus)) {
      $menus = $this->getMenuOptions();
    }

    $options = [];
    $depth_limit = $this->getParentDepthLimit($id);
    foreach ($menus as $menu_name => $menu_title) {
      $options[$menu_name . ':'] = '<' . $menu_title . '>';

      $parameters = new MenuTreeParameters();
      $parameters->setMaxDepth($depth_limit);
      $tree = $this->menuLinkTree->load($menu_name, $parameters);
      $manipulators = [
        ['callable' => 'menu.default_tree_manipulators:checkNodeAccess'],
        ['callable' => 'menu.default_tree_manipulators:checkAccess'],
        ['callable' => 'menu.default_tree_manipulators:generateIndexAndSort'],
      ];