class ThemeRegistry

class ThemeRegistry extends CacheCollector implements DestructableInterface {
    
    /**
     * Whether the partial registry can be persisted to the cache.
     *
     * This is only allowed if all modules and the request method is GET. _theme()
     * should be very rarely called on POST requests and this avoids polluting
     * the runtime cache.
     *
     * @var bool
     */
    protected $persistable;
    
    /**
     * The complete theme registry array.
     */
    protected $completeRegistry;
    
    /**
     * Constructs a ThemeRegistry object.
     *
     * @param string $cid
     *   The cid for the array being cached.
     * @param \Drupal\Core\Cache\CacheBackendInterface $cache
     *   The cache backend.
     * @param \Drupal\Core\Lock\LockBackendInterface $lock
     *   The lock backend.
     * @param array $tags
     *   (optional) The tags to specify for the cache item.
     * @param bool $modules_loaded
     *   Whether all modules have already been loaded.
     */
    public function __construct($cid, CacheBackendInterface $cache, LockBackendInterface $lock, $tags = [], $modules_loaded = FALSE) {
        $this->cid = $cid;
        $this->cache = $cache;
        $this->lock = $lock;
        $this->tags = $tags;
        $this->persistable = $modules_loaded && \Drupal::hasRequest() && \Drupal::request()->isMethod('GET');
        // @todo: Implement lazyload.
        $this->cacheLoaded = TRUE;
        if ($this->persistable && ($cached = $this->cache
            ->get($this->cid))) {
            $this->storage = $cached->data;
        }
        else {
            // If there is no runtime cache stored, fetch the full theme registry,
            // but then initialize each value to NULL. This allows offsetExists()
            // to function correctly on non-registered theme hooks without triggering
            // a call to resolveCacheMiss().
            $this->storage = $this->initializeRegistry();
            foreach (array_keys($this->storage) as $key) {
                $this->persist($key);
            }
            // RegistryTest::testRaceCondition() ensures that the cache entry is
            // written on the initial construction of the theme registry.
            $this->updateCache();
        }
    }
    
    /**
     * Initializes the full theme registry.
     *
     * @return array
     *   An array with the keys of the full theme registry, but the values
     *   initialized to NULL.
     */
    public function initializeRegistry() {
        // @todo DIC this.
        $this->completeRegistry = \Drupal::service('theme.registry')->get();
        return array_fill_keys(array_keys($this->completeRegistry), NULL);
    }
    
    /**
     * {@inheritdoc}
     */
    public function has($key) {
        // Since the theme registry allows for theme hooks to be requested that
        // are not registered, just check the existence of the key in the registry.
        // Use array_key_exists() here since a NULL value indicates that the theme
        // hook exists but has not yet been requested.
        return isset($this->storage[$key]) || array_key_exists($key, $this->storage);
    }
    
    /**
     * {@inheritdoc}
     */
    public function get($key) {
        // If the offset is set but empty, it is a registered theme hook that has
        // not yet been requested. Offsets that do not exist at all were not
        // registered in hook_theme().
        if (isset($this->storage[$key])) {
            return $this->storage[$key];
        }
        elseif (array_key_exists($key, $this->storage)) {
            return $this->resolveCacheMiss($key);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function resolveCacheMiss($key) {
        if (!isset($this->completeRegistry)) {
            $this->completeRegistry = \Drupal::service('theme.registry')->get();
        }
        $this->storage[$key] = $this->completeRegistry[$key];
        if ($this->persistable) {
            $this->persist($key);
        }
        return $this->storage[$key];
    }
    
    /**
     * {@inheritdoc}
     */
    protected function updateCache($lock = TRUE) {
        if (!$this->persistable) {
            return;
        }
        // @todo: Is the custom implementation necessary?
        $data = [];
        foreach ($this->keysToPersist as $offset => $persist) {
            if ($persist) {
                $data[$offset] = $this->storage[$offset];
            }
        }
        if (empty($data)) {
            return;
        }
        $lock_name = $this->cid . ':' . __CLASS__;
        if (!$lock || $this->lock
            ->acquire($lock_name)) {
            if ($cached = $this->cache
                ->get($this->cid)) {
                // Use array merge instead of union so that filled in values in $data
                // overwrite empty values in the current cache.
                $data = array_merge($cached->data, $data);
            }
            else {
                $registry = $this->initializeRegistry();
                $data = array_merge($registry, $data);
            }
            $this->cache
                ->set($this->cid, $data, Cache::PERMANENT, $this->tags);
            if ($lock) {
                $this->lock
                    ->release($lock_name);
            }
        }
    }

}