public function refresh(array $providers = array(), array &$results = array()) {
        if (empty($providers)) {
            foreach ($this->caches as $cacheKey => $cache) {
                $providers[$cacheKey] = array();
            }
        }
        foreach ($providers as $key => $options) {
            if (array_key_exists($key, $this->caches) && !array_key_exists($key, $results)) {
                $results[$key] = $this->clean(array_merge($options, array(xPDO::OPT_CACHE_KEY => $key)));
            }
        }
        return (array_search(false, $results, true) === false);
    }