Qt Concurrent with signals & slots

Question

I'm a novice in threads and someone advises me to use Qt Concurrent (Qt C++).

I'm trying to run a function in a thread by using Qt Concurrent, my functions runs well but my signal/slot is never emitted.

However for your information if I launch my function without using thread everything works fine.

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QFutureWatcher<void> *watcher1 = new QFutureWatcher<void>();
    connect(watcher1, SIGNAL(finished()), this, SLOT(getSizeFinished()));

    QString string = "http://ipv4.download.thinkbroadband.com/50MB.zip";
    QFuture<void> future1 = QtConcurrent::run(this, &MainWindow::getRemoteFileSize, string);
    watcher1->setFuture(future1);
}

void MainWindow::getSizeFinished()
{
       qDebug() << "--- FINISHED ---";
}

void MainWindow::getRemoteFileSize(const QString &url)
{
    qDebug() << "Function - getRemoteFileSize";
    QNetworkRequest req;
    m_netmanager = new QNetworkAccessManager();
    req.setUrl(QUrl(url));
    m_reply = m_netmanager->get(req);
    connect(m_reply, SIGNAL(metaDataChanged()), this, SLOT(remoteHTTPHeader()));
}

void MainWindow::remoteHTTPHeader()
{
    qDebug() << "Function - remoteHTTPHeader";
    remoteSize = m_reply->header(QNetworkRequest::ContentLengthHeader).toInt();
    qDebug() << "Remote File Size: " << remoteSize;
    m_reply->deleteLater();
    m_netmanager->deleteLater();
    qDebug() << "SIZE " << remoteSize;
}

This one: connect(m_reply, SIGNAL(metaDataChanged()), this, SLOT(remoteHTTPHeader())); - remoteHTTPHeader is never called — Sébastien B., Jul 10 '19 at 08:53
Verify the connection is created correctly using `qDebug() << bool(connect(here your code))`, if it returns true it might be that the signal is not emitted for other reasons. — Silvano Cerza, Jul 10 '19 at 08:58

Silvano Cerza · Answer 1 · 2019-07-10T09:26:28.293

0

You probably don't need to create a connection in this case, you could call MainWindow::remoteHTTPHeader() right after m_reply = m_netmanager->get(req);.

You might want to check if the reply is effectively finished like so:

if (m_reply->isFinished()) {
    remoteHTTPHeader();
} else {
    connect(m_reply, &QNetworkReply::finished, this, &MainWindow::remoteHTTPHeader);
}

This way you handle both fast and slow connections. Also notice how I created the connection using function pointers instead of SIGNAL and SLOT macro, this syntax is better since it checks at compile time if the functions exist so you avoid making typos and the like.

edited Jul 10 '19 at 09:26

answered Jul 10 '19 at 09:09

Silvano Cerza

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5
16

Thanks for your help. If I copy/paste your code I always go in the 'else' case. I also get a semantic error with the 'connect' : mainwindow.cpp:110:9: error: no matching member function for call to 'connect' – Sébastien B. Jul 10 '19 at 09:23
Whoops, I left a parens that should have not been there, I fixed it. Let me know if it works now. – Silvano Cerza Jul 10 '19 at 09:27
Indeed, sorry I didn't notice it... thanks. However my function remoteHTTPHeader() is still never executed. – Sébastien B. Jul 10 '19 at 09:31
It's probably going to be executed only on really fast connection, if it's not of use you could remove it. – Silvano Cerza Jul 10 '19 at 09:33
In my case users could have fast and very slow connection – Sébastien B. Jul 10 '19 at 09:34
I'd leave it there just in case then. – Silvano Cerza Jul 10 '19 at 09:36

Qt Concurrent with signals & slots

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