C++ Qt get HTTP headers from QNetworkRequest

Question

I am developing a Qt C++ application. I need to download some files (which can be large) and show downloading progress to user. To perform this task, I use this code:

QNetworkAccessManager* networkManager = new QNetworkAccessManager();

QNetworkRequest request(fileUrl); //fileUrl is a QUrl variable
QVariant responseLength = request.header(QNetworkRequest::ContentLengthHeader);
int fileSize = responseLength.toInt();
ui->progressBar->setMaximum(fileSize);
QNetworkReply reply = networkManager->get(request);
QObject::connect(reply, SIGNAL(downloadProgress(qint64,qint64)),
                 this, SLOT(downloadProgressChanged(qint64,qint64)));

Where downloadProgressChanged is a slot with this code:

void downloadProgressChanged(qint64 downloaded, qint64 total)
{
    ui->progressBar->setValue(ui->progressBar->value() + 1);
    ui->labelProgress->setText(QString::number((downloaded / 1024)));
}

(I use QProgressBar named progressBar to show progress and QLabel named labelProgress to show downloaded kilobytes).

My problem is that I can't access Content-Length header (int fileSize value is 0) and so I am not able to show the progress of the operation. I checked HTTP headers on my web-server - Content-Length works fine.

In this SO question I read that I can use QNetworkReply::metaDataChanged() signal, but how can I use it to show progress? Documentation says that the signal can be emitted when downloading has been already started, but I need to get header content before downloading will start - to set up my progressBar.

Answer 1

Have you tried using readyRead signal? And in slot you could prepare the GUI. Something like this shoud do the work:

connect(reply, SIGNAL(readyRead()), this, SLOT(updateProgressBar()))

Answer 2

This isn't how you would get header information from a request:

QNetworkRequest request(fileUrl); //fileUrl is a QUrl variable
QVariant responseLength = request.header(QNetworkRequest::ContentLengthHeader);
int fileSize = responseLength.toInt();
ui->progressBar->setMaximum(fileSize);

Try making the request with the QNetworkAccessManager and then getting the header you want from the reply that it returns. There is a special method for retrieving only header information from a request:

QNetworkAccessManager::head(const QNetworkRequest & request)

Since Qt's network API is asynchronous, you have to connect the QNetworkAccessManager's finished(QNetworkReply*) signal to a slot, and get the header information in the slot.

Here's how I'd do it:

void MainWindow::on_download_button_clicked(){
    QUrl url("http://someurl");
    QNetworkAccessManager * manager = new QNetworkAccessManager(this);
    connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(getHeaders(QNetworkReply*)));
    manager->head(QNetworkRequest(url));
}

void MainWindow::getHeaders(QNetworkReply * reply){
    if (reply->operation() == QNetworkAccessManager::HeadOperation){
        int content_length = reply->header(QNetworkRequest::ContentLengthHeader).toInt();
    }
}