private function getResourceLookup($format)
    {
        if(in_array($format, self::$formats)){
            if(!isset($this->lookups)){
                $this->lookups = [
                    'list' => $this->getServiceManager()->get(ListResourceLookup::SERVICE_ID),
                    'tree' => $this->getServiceManager()->get(TreeResourceLookup::SERVICE_ID)
                ];
            }
            return $this->lookups[$format];
        }
        return null;
    }