protected function retrieveCollection()
    {
        $cache = new FilesystemAdapter($this->namespace);
        $item = $cache->getItem('collection');

        $collection = null;
        if ($item->isHit()) {
            $collection = $item->get('collection');
        }

        if (empty($collection) || !$collection instanceof ValuesCollection) {
            $collection = new ValuesCollection();
        }

        return $collection;
    }