Is it possible to control download speed using QNetworkAccessManager

adnan

Can we restrict QNetworkACCessManager from consuming whole bandwidth, by restricting the download speed, as we do see such options available with almost every download manager

lgeyer

What about reducing the "read buffer size":http://doc.qt.nokia.com/4.7-snapshot/qnetworkreply.html#setReadBufferSize and clearing the buffer at the desired download rate?

adnan

Thanks for the reply!
Do you mean that if i set the buffer size to suppose 512kB, it won't cause any data loss.

lgeyer

The download should just stall, which means it stops receiving data after 512 kB and waits for the buffer to be emptied. It then continous receiving the next 512 kB of data once done so.

I haven't verified this, but if you set your read buffer to for example 512 kB an you empty it once a second (using read()), you should theoretically end up with an download rate of (approx.) 512 kB a second.

adnan

How can we empty the buffer exactly after one second, as it is done through signal/slot mechanism?

lgeyer

Use a QTimer and read() once a second.

adnan

When readyRead() signal is emitted then the following slot is invoked
@void Download::saveToDisk()
{

qint64 bytes;
bytes=replySegment->bytesAvailable();
fileSegment.write(replySegment->read(bytes));

}@

How, can i control it using QTimer, it is completely determined by the readyRead() signal. Can you plz explain it more

lgeyer

No, it isn't. ;-)
Just ignore the readyRead() signal and use a QTimer, which periodically empties the read buffer.
@
class Downloader : public QObject
{
Q_OBJECT

public:
Downloader(QObject *object) : QObject(parent)
{
timer.setInterval(1000);
connect(&timer, SIGNAL(timeout()), this, SLOT(processData());
}

bool download(const QUrl &url, const QString &fileName, downloadRate = -1)
{
    file.setFileName(fileName);
    if (file.open(QIODevice::WriteOnly | QIODevice::Truncate) == false)
        return false;

    reply = manager.get(QNetworkRequest(url));
    if (downloadRate > 0)
        reply->setReadBufferSize(downloadRate);

    timer.start();

    return true;
}

signals:
void finished();

private:
QNetworkAccessManager manager;
QNetworkReply *reply;
QTimer timer;

private slots:
void processData()
{
file.write(reply->read(reply->bytesAvailable()));

    if (reply->isFinished() == true)
    {
        timer.stop();

        file.close();

        reply->deleteLater();
        reply = nullptr;

        emit finished();
    }
}

@
Brain to terminal. Not tested. Exemplary.

adnan

Thanks a lot for such a detailed answer!