class AliasWhitelist

class AliasWhitelist extends CacheCollector implements AliasWhitelistInterface {
    
    /**
     * The Key/Value Store to use for state.
     *
     * @var \Drupal\Core\State\StateInterface
     */
    protected $state;
    
    /**
     * The path alias repository.
     *
     * @var \Drupal\path_alias\AliasRepositoryInterface
     */
    protected $pathAliasRepository;
    
    /**
     * Constructs an AliasWhitelist object.
     *
     * @param string $cid
     *   The cache id to use.
     * @param \Drupal\Core\Cache\CacheBackendInterface $cache
     *   The cache backend.
     * @param \Drupal\Core\Lock\LockBackendInterface $lock
     *   The lock backend.
     * @param \Drupal\Core\State\StateInterface $state
     *   The state keyvalue store.
     * @param \Drupal\path_alias\AliasRepositoryInterface $alias_repository
     *   The path alias repository.
     */
    public function __construct($cid, CacheBackendInterface $cache, LockBackendInterface $lock, StateInterface $state, AliasRepositoryInterface $alias_repository) {
        parent::__construct($cid, $cache, $lock);
        $this->state = $state;
        $this->pathAliasRepository = $alias_repository;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function lazyLoadCache() {
        parent::lazyLoadCache();
        // On a cold start $this->storage will be empty and the whitelist will
        // need to be rebuilt from scratch. The whitelist is initialized from the
        // list of all valid path roots stored in the 'router.path_roots' state,
        // with values initialized to NULL. During the request, each path requested
        // that matches one of these keys will be looked up and the array value set
        // to either TRUE or FALSE. This ensures that paths which do not exist in
        // the router are not looked up, and that paths that do exist in the router
        // are only looked up once.
        if (empty($this->storage)) {
            $this->loadMenuPathRoots();
        }
    }
    
    /**
     * Loads menu path roots to prepopulate cache.
     */
    protected function loadMenuPathRoots() {
        if ($roots = $this->state
            ->get('router.path_roots')) {
            foreach ($roots as $root) {
                $this->storage[$root] = NULL;
                $this->persist($root);
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function get($offset) {
        $this->lazyLoadCache();
        // This may be called with paths that are not represented by menu router
        // items such as paths that will be rewritten by hook_url_outbound_alter().
        // Therefore internally TRUE is used to indicate whitelisted paths. FALSE is
        // used to indicate paths that have already been checked but are not
        // whitelisted, and NULL indicates paths that have not been checked yet.
        if (isset($this->storage[$offset])) {
            if ($this->storage[$offset]) {
                return TRUE;
            }
        }
        elseif (array_key_exists($offset, $this->storage)) {
            return $this->resolveCacheMiss($offset);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function resolveCacheMiss($root) {
        $exists = $this->pathAliasRepository
            ->pathHasMatchingAlias('/' . $root);
        $this->storage[$root] = $exists;
        $this->persist($root);
        if ($exists) {
            return TRUE;
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function clear() {
        parent::clear();
        $this->loadMenuPathRoots();
    }

}