public function onNotification($notification_code, $severity, $message, $message_code, $bytes_transferred, $bytes_max)
    {
        switch($notification_code) {
            case STREAM_NOTIFY_PROGRESS:
                $this->emit('progress', [$bytes_transferred]);
                break;
            case STREAM_NOTIFY_FILE_SIZE_IS:
                $this->emit('request.start', [$this->url, $bytes_max]);
                break;
        }
    }