bookTreeOutput example

if ($node instanceof NodeInterface && !empty($node->book['bid'])) {
      $current_bid = $node->book['bid'];
    }
    if ($this->configuration['block_mode'] == 'all pages') {
      $book_menus = [];
      $pseudo_tree = [0 => ['below' => FALSE]];
      foreach ($this->bookManager->getAllBooks() as $book_id => $book) {
        if ($book['bid'] == $current_bid) {
          // If the current page is a node associated with a book, the menu           // needs to be retrieved.           $data = $this->bookManager->bookTreeAllData($node->book['bid'], $node->book);
          $book_menus[$book_id] = $this->bookManager->bookTreeOutput($data);
        }
        else {
          // Since we know we will only display a link to the top node, there           // is no reason to run an additional menu tree query for each book.           $book['in_active_trail'] = FALSE;
          // Check whether user can access the book link.           $book_node = $this->nodeStorage->load($book['nid']);
          $book['access'] = $book_node->access('view');
          $pseudo_tree[0]['link'] = $book;
          $book_menus[$book_id] = $this->bookManager->bookTreeOutput($pseudo_tree);
        }
        

$link = array_shift($flat);
      } while ($link && ($link['nid'] != $book_link['nid']));
      // Continue though the array and collect the links whose parent is this page.       while (($link = array_shift($flat)) && $link['pid'] == $book_link['nid']) {
        $data['link'] = $link;
        $data['below'] = '';
        $children[] = $data;
      }
    }

    if ($children) {
      return $this->bookManager->bookTreeOutput($children);
    }
    return '';
  }

}

  public function testMultilingualBookManager(string $langcode) {
    $this->setCurrentLanguage($langcode);
    /** @var \Drupal\book\BookManagerInterface $bm */
    $bm = $this->container->get('book.manager');
    $books = $bm->getAllBooks();
    $this->assertNotEmpty($books);
    foreach ($books as $book) {
      $bid = $book['bid'];
      $build = $bm->bookTreeOutput($bm->bookTreeAllData($bid));
      $items = $build['#items'];
      $this->assertBookItemIsCorrectlyTranslated($items[$bid], $langcode);
      $this->assertBookItemIsCorrectlyTranslated($items[$bid]['below'][$bid + 1], $langcode);
      $this->assertBookItemIsCorrectlyTranslated($items[$bid]['below'][$bid + 1]['below'][$bid + 3], $langcode);
      $this->assertBookItemIsCorrectlyTranslated($items[$bid]['below'][$bid + 1]['below'][$bid + 4], $langcode);
      $this->assertBookItemIsCorrectlyTranslated($items[$bid]['below'][$bid + 2], $langcode);
      $this->assertBookItemIsCorrectlyTranslated($items[$bid]['below'][$bid + 2]['below'][$bid + 5], $langcode);
      $this->assertBookItemIsCorrectlyTranslated($items[$bid]['below'][$bid + 2]['below'][$bid + 6], $langcode);
      $toc = $bm->getTableOfContents($bid, 4);
      // Root entry does not have an indent.       $this->assertToCEntryIsCorrectlyTranslated($toc, $langcode, $bid, '');