How to measure download percent using QNetwork?

Question

In the example below sometimes the file is downloaded correctly, and sometimes, I'm getting these values in the qDebug() added into the downloadProgress lambda:

Percent complete:  -1.08905e+09
Downloaded  11623330  of  -1

And then the download fails, I mean it saves a zip file with 0 bytes.

        QNetworkAccessManager *manager = new QNetworkAccessManager(this);

        QNetworkRequest request;
        // Random link just to test:
        request.setUrl(
            QUrl("https://github.com/FFmpeg/FFmpeg/archive/refs/tags/n5.0.2.zip"));

        QNetworkReply *reply = manager->get(request);

        connect(reply, &QNetworkReply::downloadProgress,
            [this, reply](qint64 bytesReceived, qint64 bytesTotal)
        {
            qDebug() << "Downloaded " << bytesReceived << " of " << bytesTotal;
            double percentComplete = (bytesReceived * 100.0) / bytesTotal;
            qDebug() << "Percent complete: " << percentComplete;
        });

        connect(reply, &QNetworkReply::finished, [this, reply]() 
        {
            if (reply->error() != QNetworkReply::NoError)
            {
                qDebug() << "Error: " << reply->errorString();
            } else 
            {
                QString fileName = "C:/Users/Raja/Downloads/file.zip";
                QFile file(fileName);
                if (file.open(QIODevice::WriteOnly)) 
                {
                    file.write(reply->readAll());
                    file.close();
                    qDebug() << "File downloaded successfully";
                } else
                    qDebug() << "Error: Unable to open the file";
            }
            reply->deleteLater();
        });

What i'm missing?

score 1 · Answer 1 · answered Jan 30 '23 at 11:05

Did you read the documentation?

It says that bytesTotal is -1 if the total size is unknown and

The download is finished when bytesReceived is equal to bytesTotal. At that time, bytesTotal will not be -1.

In other words: That behavior is expected and just means the download is still in progress. This probably happens when the server doesn't send a content-length header. See What's the "Content-Length" field in HTTP header?

How to measure download percent using QNetwork?

1 Answers1