Qt Application freezes all threads until mouse is moved or button pressed

jobusch

Hello,
I have a very strange behaviour I guess I could need some experienced help on.

So I made a QtWidgets App which communicates with a USB device with libusb and displays some results. In the beginning I spawn a thread which repeatedly uses some function to check if the sensor is still connected. When the user presses a button, the USB device does some work for like 6 seconds and reports the result, while continously updating a progress bar.
While this work, the spawned thread is not doing anything since I always check in the thread on a variable if the work is ongoing.
After the work, the spawned thread is frozen. It does not seem to do anything anymore. And that is until I move my mouse or press a key. Instantly the thread is working again as nothing happened.

I tried std::thread and QThread, it happens with both.

I'll show you the code of the scanBtnClicked function so maybe you can see if there is something suspicious. I know its bad behaviour that while the scan is running the function does not return. It should be in a separate thread. I am working on this right now to see if it fixes the problem.
MainWidget inherits from QWidget.

void MainWidget::on_scan_button_clicked()
{
    if(!noDoubleClick){
        Ref::isCheckRunning = true;
        ui->products_groupbox->setDisabled(true);
        int errCode;
        ui->scan_button->setEnabled(false);
        ui->continous_scan_button->setEnabled(false);

        clearResponse(false);

        QApplication::processEvents();
        QByteArray b;
        bool result;
        result = check(&b, !ambilight_check, this, &errCode, active_tdf);
        CheckResponse(result);
        QApplication::processEvents();
        noDoubleClick = true;
        Ref::isCheckRunning = false;
        ui->scan_button->setEnabled(true);
        ui->continous_scan_button->setEnabled(true);
        ui->products_groupbox->setDisabled(false);
        QApplication::processEvents();

        QTimer::singleShot(100, this, &MainWidget::resetNoDoubleClick);
    }
}

void MainWidget::resetNoDoubleClick(){
    noDoubleClick = false;
}

Any would be really appreciated!
Best regards,
Jobusch

jobusch

I fixed it by removing the 2 lines with:

QApplication::processEvents();

It is still strange behaviour or am I missing something on how QApplication::processEvents() works?

Best regards,
Jobusch