private function prepareApnsHeaders(Notification $notification)
    {
        if (!empty($notification->getId())) {
            $this->headers[self::HEADER_APNS_ID] =  $notification->getId();
        }

        if ($notification->getExpirationAt() instanceof \DateTime) {
            $this->headers[self::HEADER_APNS_EXPIRATION] = $notification->getExpirationAt()->getTimestamp();
        }

        if (is_int($notification->getPriority())) {
            $this->headers[self::HEADER_APNS_PRIORITY] =  $notification->getPriority();
        }

        if (!empty($notification->getCollapseId())) {
            $this->headers[self::HEADER_APNS_COLLAPSE_ID ] = $notification->getCollapseId();
        }
    }