Why does my worker thread not receive this signal? - c++

Question

I have my main thread which is in charge with the GUI and my worker thread which is in charge with doing stuff with my webcam.
I am able to emit a signal from my main thread to my webcam thread, but it never receives it! Could somebody explain me why?
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
setThreadsAndObj();
setConnects();
startCamera();
}
void MainWindow::setThreadsAndObj()
{
cameraClassPtr = new cameraClass();
cameraThread = new QThread();
}
void MainWindow::setConnects()
{
....
connect(this, &MainWindow::savePicture, cameraClassPtr, &cameraClass::takePicture);
}
void MainWindow::startCamera()
{
//camera start
connect( cameraThread, &QThread::started, cameraClassPtr, &cameraClass::getVideoFrame );
connect( cameraThread, &QThread::finished, cameraClassPtr, &QThread::deleteLater );
connect( cameraThread, &QThread::finished, cameraThread, &QThread::deleteLater );
//receive camera frames
connect(cameraClassPtr, &cameraClass::videoFrameToGui, this, &MainWindow::updateCameraStream);
//start threads
cameraClassPtr->moveToThread(cameraThread);
cameraThread->start();
}
void MainWindow::foo()
{
emit savePicture();
}
void cameraClass::takePicture()
{
qDebug()<<"camera class received signal"; //THIS NEVER GETS PRINTED
}
Could someone explain me why my signal is never received and "camera class received signal" is never printed and how to solve this?

Answer 1

I had this loop in my worker thread:
...
static cv::VideoCapture cap(-1);
if( !cap.isOpened() )
{
qDebug()<< "Could not initialize capturing...\n";
}
qDebug()<<"starting live camera";
cap.set(CV_CAP_PROP_FRAME_WIDTH,240);
cap.set(CV_CAP_PROP_FRAME_HEIGHT,320);
while(1)
{
cap >> imgFrame;
int fd = open("/home/John/Desktop/mainImglock.txt", O_RDWR |O_CREAT, 0666);
int rc = flock(fd, LOCK_EX | LOCK_NB);
if(rc==0)
{
cv::imwrite("/home/John/Desktop/camera.jpg", imgFrame);
}
flock(fd, LOCK_UN);
close(fd);
}
I solved the issue by adding: QApplication::processEvents(); in the while(1) loop, so that the event loop isn't hanging.

Answer 2

You need to subclass QThread and call QThread::exec(); in order to process events in this thread.