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, ?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;
    }

}