How to measure download percent using QNetwork? - c++

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?

Answer 1

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?