lambda slots and QNetworkReply

Mark81

I'm doing something wrong in this code:

void Packages::download(Package_t package)
{
    _currentFilename = package.filename;
    _currentFile.setFileName(QString("/mnt/resources/packages/%1").arg(_currentFilename));
    if (!_currentFile.open(QIODevice::WriteOnly))
    {
        qWarning() << "Cannot open" << _currentFile.fileName();
        return;
    }

    QUrlQuery query;
    query.addQueryItem("filename", package.filename);

    QUrl url(API_HOST + API_DOWNLOAD);
    url.setQuery(query);

    QNetworkRequest request(url);
    _managerDownload.setTransferTimeout(10000);
    QNetworkReply *reply = _managerDownload.get(request);

    connect(reply, &QNetworkReply::downloadProgress, [reply, this](qint64 bytesReceived, qint64 bytesTotal)
    {
        _currentFile.write(reply->readAll());
        qInfo() << "progress" << bytesReceived << "/" << bytesTotal;
    });

    connect(&_managerDownload, &QNetworkAccessManager::finished, [this](QNetworkReply *reply)
    {
        reply->deleteLater();
        int status = reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
        qInfo() << "HTTP status:" << status << reply->url();
        switch (status)
        {
        case 200: qInfo() << "Download completed successfully"; break;
        case 404: qInfo() << "File not found"; break;
        default: break;
        }

        _currentFile.close();
        emit downloadFinished(status, _currentFilename);
    });

    qInfo() << "GET request" << request.url();
}

Basically it should:

open a local file where to store the downloaded binary data
create the HTTP GET request
bind the downloadProgress and finished signals to the two lambda functions

It works, but every time I call this function I see more and more messages, like it does not destroy the previous objects.
I don't get the mistake because:

_managerDownload is a class member, not a pointer
reply is deleted inside the finished slot

Can you help me to understand my fault please?

Christian Ehrlicher

@Mark81 said in lambda slots and QNetworkReply:

connect(reply, &QNetworkReply::downloadProgress, [reply, this](qint64 bytesReceived, qint64 bytesTotal)
{
    _currentFile.write(reply->readAll());
    qInfo() << "progress" << bytesReceived << "/" << bytesTotal;
});

You should do this once since there's no disconnect because you forgot to specify the receiver's object (third parameter): https://doc.qt.io/qt-5/qobject.html#connect-4 and https://doc.qt.io/qt-5/qobject.html#connect-5 --> set reply as context to make sure the slot gets disconnected or connect only once (which I would prefer)

J.Hilk

@Mark81 said in lambda slots and QNetworkReply:

change this

connect(&_managerDownload, &QNetworkAccessManager::finished, [this](QNetworkReply *reply)

to

connect(&_managerDownload, &QNetworkAccessManager::finished, reply, [this](QNetworkReply *reply)

the reply object in 3rd place should guarantee, that this connect is dissolved once reply is deleted