QT slot not getting called on main thread

Question

From the thread context of a slot in my QT GUI application (upon button push), I am trying to launch a worker thread to update other another part of the GUI with the results of a CPU intensive calculation - these results will update a table or a google like map widget - so this needs to occur in the main QT application thread where it is safe to update these widgets.

The problem I have is that the updateGUIWidget slot never gets called unless I change the connection type to Qt::DirectConnection - in which case it gets called in the worker thread (where it is unsafe to update the GUI). I checked the results of each of the connect calls and they are fine, it seems that there is some issue with the event loop somewhere. I'm not sure if I need to allocate the thread and the worker objects as members of the mainwindow or if its OK to do so from stack variables in the slot.

void
mainwindow::on_importSimulatedFlight_clicked()
{
    // experimental worker thread model adapted from YouTube tutorial
    // by Giuseppe di Angelo https://www.youtube.com/watch?v=BgqT6SIeRn4
    // auto thread = new QThread;
    // note worker created in gui thread here - we will move its thread
    // affinity to 'thread' below before starting it.
    auto thread = new QThread;
    auto worker = new Worker;
    connect(thread, &QThread::started, worker, &Worker::doWork);
    // connect(worker, &Worker::progressUpdate, this, &mainwindow::updateGUIWidget, Qt::DirectConnection);
    connect(worker, &Worker::progressUpdate, this, &mainwindow::updateGUIWidget, Qt::QueuedConnection);
    connect(worker, &Worker::workDone, thread, &QThread::quit);
    connect(thread, &QThread::finished, worker, &Worker::deleteLater);
    // move worker to separate thread
    worker->moveToThread(thread);
    thread->start();
}

The mainwindow has a slots declared in mainwindow.h as follows:

class mainwindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit mainwindow(QWidget *parent = Q_NULLPTR);
    ~mainwindow();
    ...
public slots:
    void on_importSimulatedFlight_clicked();
    void updateGUIWidget(const param& rParam);
    ...
}

and implemented in mainwindow.cpp as follows:

void
mainwindow::updateGUIWidget(const param& rParam)
{
... update widget components with rParam partial result here
}

and my worker is as follows:

class Worker : public QObject
{
    Q_OBJECT
public slots:
    void doWork() {
        const QString result;
        for (int i=0; i<5; i++) {
            const MMRTimedRecord foo;
//            QThread::sleep(1);
            emit progressUpdate(foo);
        }
        emit workDone(result);
    }
signals:
    void progressUpdate(const MMRTimedRecord&);
    void workDone(const QString &result);
};

Answer 1

The reason it isn't working is because there's a serious flaw in your code: you are trying to emit a reference to a local variable to be handled in an slot on a different thread. That is a recipe for disaster.

When you are using Qt::QueuedConnection, you MUST emit by value, like this:

void progressUpdate(MMRTimedRecord val);

That means that your MMRTimedRecord must be copyable, and correspondingly, your slot must also accept by value. And why is there a mismatch between the signal progressUpdate(const MMRTimedRecord&) and the slot updateGUIWidget(const param& rParam); ?