Create Thread in QDialog and emit signals to QDialog in Qt - c++

Question

I created an application that has a mainwindow and from this window creates a QDialog. This QDialog should create a RenderThread that emits received images from the camera or in the example emits text. When i debug this it seems that the connection is never made as adding a breakpoint in the slot CameraWindow::debugSomething does not get caught. Whats wrong ?
I followed this example: http://qt-project.org/doc/qt-4.8/threads-mandelbrot.html but it seems that i've done something wrong.
qtDEVC::qtDEVC(QWidget *parent, Qt::WFlags flags)
: QMainWindow(parent, flags)
{
ui.setupUi(this);
connect (ui.pushButton_startCam, SIGNAL( clicked() ),this,SLOT( startCam() ) );
/**Threading*/
CameraWindow camWindow = new CameraWindow(this);
}
int qtDEVC::startCam()
{
camWindow.show();
camWindow.startCaptureThread();
}
CameraWindow Class:
CameraWindow::CameraWindow(QWidget *parent)
: QDialog(parent)
{
ui.setupUi(this);
connect(&thread, SIGNAL(sendText(std::string)),
this, SLOT(debugSomething(std::string)));
}
void CameraWindow::debugSomething(std::string something){
QString somethings(something.c_str());
qDebug()<<somethings;
}
int CameraWindow::startCaptureThread(){
RenderThread *thread = new RenderThread(this, guid, CLEYE_COLOR_RAW, CLEYE_VGA, 50);
thread->StartCapture(); //starts thread in low priority and sets _running to true
}
CameraWindow header
class CameraWindow : public QDialog
{
Q_OBJECT
public:
CameraWindow(QWidget *parent = 0);
~CameraWindow();
Ui::CameraWindow ui;
public slots:
int startCaptureThread();
void debugSomething(QString);
private:
RenderThread thread;
};
RenderThread Class
void RenderThread::run()
{
// image capturing loop
while(_running)
{
qDebug()<<("render while()"); //is printed with qdebug correctly
if (restart)
break;
if (abort)
return;
qDebug("it"); //is also printed with qdebug correctly
emit sendText(text);
}
RenderThread header
class RenderThread : public QThread
{
Q_OBJECT
public:
RenderThread(QObject *parent, GUID cameraGUID, CLEyeCameraColorMode mode, CLEyeCameraResolution resolution, float fps);
RenderThread();
~RenderThread();
bool StartCapture();
signals:
void sendText(QString &text);
protected:
void run();
private:
QMutex mutex;
QWaitCondition condition;
//some more private variables for construction
};
I think that this creation seems somehow wrong: RenderThread *thread = new RenderThread(this);

Answer 1

The first thing that's worrying about the question is the word "RenderThread". Note that Qt only allows rendering on the main thread. You can create separate threads for calculations of image data, but whenever you use a painter and draw objects, that must happen on the main thread. However, If you're just going to capture the image and pass that to the main thread, via signals and slots, then that should be ok.
Secondly, whilst you've not shown all your code, I'm assuming from the function called RenderThread::run() and from the Qt example that you may have inherited from QThread here. If this is the case, please note that this is not how to use QThread. Instead, you should have your class inherit from QObject and move that to a QThread. You can read about how to use QThread properly here.
I know it's a Qt example that you've followed, but even the guys at Qt think it's a bad idea. Here's an article of how to really use QThreads.
With that in mind, here's an outline of how I would use QThread for this: -
class CameraWindow : public QDialog
{
private:
CameraObject* m_pCamObject;
};
class CameraObject : public QObject
{
Q_OBJECT
public:
private slots:
startCaptureThread();
private:
};
int CameraWindow::startCaptureThread()
{
m_pCamObject = new CameraObject;
QThread* pThread = new QThread;
this->moveToThread(pThread); // CameraObject will run on the new thread
connect(pThread, SIGNAL(started()), m_pCamObject, SLOT(startCaptureThread()));
connect(pThread, SIGNAL(finished()), pThread, SLOT(deleteLater()); // clear up when
finished
pThread->start();
}
Note that here is a CameraObject, separated from the CameraWindow. The QThread is just a controller of the thread and keeps a clean separation from the CameraObject.
Another reason for handling threads this way is that you can move multiple QObject instances to the new thread, rather than always creating a new thread per object. If you've more threads than CPU cores, you're unlikely to gain by creating yet another thread.

Answer 2

In the constructor of CameraWindow class, you are connecting a RenderThread's signal but it is not the same object which is started in startCaptureThread. Change your startCaptureThread like this:
int CameraWindow::startCaptureThread()
{
thread.StartCapture(); //starts thread in low priority and sets _running to true
}
In this method, the thread member of CameraWindow is started.
P.S.: post the headers too, we can't see the members from this code.

Answer 3

In the mentioned example, CameraWindow class holds a RenderThread thread variable, not a RenderThread *thread.
You are connecting a pointer address in your connect call:
connect(&thread, SIGNAL(sendText(std::string)),
this, SLOT(debugSomething(std::string)));
Try to use a good address:
connect(thread, SIGNAL(sendText(std::string)),
this, SLOT(debugSomething(std::string)));
This is not a tested answer.