template_preprocess_book_navigation

template_preprocess_book_navigation(&$variables)

function template_preprocess_book_navigation(&$variables) {
  $book_link = $variables['book_link'];

  // Provide extra variables for themers. Not needed by default.
  $variables['book_id'] = $book_link['bid'];
  $variables['book_title'] = $book_link['link_title'];
  $variables['book_url'] = \Drupal::url('entity.node.canonical', array('node' => $book_link['bid']));
  $variables['current_depth'] = $book_link['depth'];
  $variables['tree'] = '';

  /** @var \Drupal\book\BookOutline $book_outline */
  $book_outline = \Drupal::service('book.outline');

  if ($book_link['nid']) {
    $variables['tree'] = $book_outline->childrenLinks($book_link);

    $build = array();

    if ($prev = $book_outline->prevLink($book_link)) {
      $prev_href = \Drupal::url('entity.node.canonical', array('node' => $prev['nid']));
      $build['#attached']['html_head_link'][][] = array(
        'rel' => 'prev',
        'href' => $prev_href,
      );
      $variables['prev_url'] = $prev_href;
      $variables['prev_title'] = $prev['title'];
    }

    /** @var \Drupal\book\BookManagerInterface $book_manager */
    $book_manager = \Drupal::service('book.manager');
    if ($book_link['pid'] && $parent = $book_manager->loadBookLink($book_link['pid'])) {
      $parent_href = \Drupal::url('entity.node.canonical', array('node' => $book_link['pid']));
      $build['#attached']['html_head_link'][][] = array(
        'rel' => 'up',
        'href' => $parent_href,
      );
      $variables['parent_url'] = $parent_href;
      $variables['parent_title'] = $parent['title'];
    }

    if ($next = $book_outline->nextLink($book_link)) {
      $next_href = \Drupal::url('entity.node.canonical', array('node' => $next['nid']));
      $build['#attached']['html_head_link'][][] = array(
        'rel' => 'next',
        'href' => $next_href,
      );
      $variables['next_url'] = $next_href;
      $variables['next_title'] = $next['title'];
    }
  }

  if (!empty($build)) {
    drupal_render($build);
  }

  $variables['has_links'] = FALSE;
  // Link variables to filter for values and set state of the flag variable.
  $links = array('prev_url', 'prev_title', 'parent_url', 'parent_title', 'next_url', 'next_title');
  foreach ($links as $link) {
    if (isset($variables[$link])) {
      // Flag when there is a value.
      $variables['has_links'] = TRUE;
    }
    else {
      // Set empty to prevent notices.
      $variables[$link] = '';
    }
  }
}