class BookNavigationCacheContext

class BookNavigationCacheContext implements CacheContextInterface {
  
  /**
   * The current route match.
   *
   * @var \Drupal\Core\Routing\RouteMatchInterface
   */
  protected $routeMatch;
  
  /**
   * Constructs a new BookNavigationCacheContext service.
   *
   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
   *   The current route match.
   * @param \Drupal\book\BookManagerInterface|null $bookManagerService
   *   The book manager service.
   */
  public function __construct(RouteMatchInterface $route_match, public ?BookManagerInterface $bookManagerService = NULL) {
    $this->routeMatch = $route_match;
    if ($this->bookManagerService === NULL) {
      @trigger_error('Calling ' . __METHOD__ . ' without the $bookManagerService argument is deprecated in drupal:10.2.0 and it will be required in drupal:11.0.0. See https://www.drupal.org/node/3397515', E_USER_DEPRECATED);
      $this->bookManagerService = \Drupal::service('book.manager');
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public static function getLabel() {
    return t("Book navigation");
  }
  
  /**
   * {@inheritdoc}
   */
  public function getContext() {
    // Find the current book's ID.
    $current_bid = 0;
    $node = $this->routeMatch
      ->getParameter('node');
    if ($node instanceof NodeInterface && !empty($node->book['bid'])) {
      $current_bid = $node->book['bid'];
    }
    // If we're not looking at a book node, then we're not navigating a book.
    if ($current_bid === 0) {
      return 'book.none';
    }
    // If we're looking at a book node, get the trail for that node.
    $active_trail = $this->bookManagerService
      ->getActiveTrailIds($node->book['bid'], $node->book);
    return implode('|', $active_trail);
  }
  
  /**
   * {@inheritdoc}
   */
  public function getCacheableMetadata() {
    // The book active trail depends on the node and data attached to it.
    // That information is however not stored as part of the node.
    $cacheable_metadata = new CacheableMetadata();
    $node = $this->routeMatch
      ->getParameter('node');
    if ($node instanceof NodeInterface) {
      // If the node is part of a book then we can use the cache tag for that
      // book. If not, then it can't be optimized away.
      if (!empty($node->book['bid'])) {
        $cacheable_metadata->addCacheTags([
          'bid:' . $node->book['bid'],
        ]);
      }
      else {
        $cacheable_metadata->setCacheMaxAge(0);
      }
    }
    return $cacheable_metadata;
  }

}