Qt: Signal Slot not working

Question

I am trying to implement Signal and Slot system between the main gui and another object moved to another thread...the following is how the class design looks like...unfortunately cannot implement it...

MainWindow.h

signals:
    void StopDisplayWidget();
    void StartDisplayWidget();
    void signalFromGUI();

private slots:
    void on_pushButton_start_display_clicked();

    void on_pushButton_stop_display_clicked();

    void on_pushButton_check_clicked();

private:
    Ui::MainWindow *ui;
    displaythread *threadforDisplay;
    display *displayWidget;
    QThread *WorkerDisplay;

MainWindow.cpp

{
    threadforDisplay = new displaythread;
    threadforDisplay->setptr2display(displayWidget);

    WorkerDisplay = new QThread;
    QObject::connect(WorkerDisplay,SIGNAL(started()),threadforDisplay,SLOT(Process()));

    QObject::connect(this,SIGNAL(StartDisplayWidget()),threadforDisplay,SLOT(StartDisplay()));
    QObject::connect(this,SIGNAL(StopDisplayWidget()),threadforDisplay,SLOT(StopDisplay()));
    QObject::connect(this,SIGNAL(signalFromGUI()),threadforDisplay,SLOT(Check()));

    threadforDisplay->moveToThread(WorkerDisplay);
}

void MainWindow::on_pushButton_start_display_clicked()
{
    if(!threadforDisplay->IsDisplayActive())
        emit this->StartDisplayWidget();

    if(!WorkerDisplay->isRunning())
        WorkerDisplay->start();
}

void MainWindow::on_pushButton_stop_display_clicked()
{
    if(threadforDisplay->IsDisplayActive())
    {
        emit this->StopDisplayWidget();
    }
}

void MainWindow::on_pushButton_check_clicked()
{
    std::cout<<"CHECKING SIGNAL SLOT"<<std::endl;
    emit this->signalFromGUI();
}

threadforDisplay is a pointer to displaythread class which looks like

displaythread.h

#include <QObject>
#include <QWaitCondition>
#include <QMutex>
#include "display.h"

class displaythread : public QObject
{
    Q_OBJECT
public:
    explicit displaythread(QObject *parent = 0);
    bool IsDisplayActive() const;
    void setptr2display(display *);

signals:

public slots:
    void Process();
    void StartDisplay();
    void StopDisplay();
    void Check();


private:
    void SleepThread();

    volatile bool stopped,running;
    QMutex mutex;
    QWaitCondition waitcondition;
    display *displayinGUI;

displaythread.cpp

void displaythread::setptr2display(display *ptr)
{
    displayinGUI = ptr;
}

void displaythread::Process()
{
    std::cout<<"RECEIVED START PROCESS SIGNAL"<<std::endl;
    running = true;
    while(true)
    {
        if(!stopped)
        {
            displayinGUI->update();
            this->SleepThread();
        }
    }

}

void displaythread::SleepThread()
{
    mutex.lock();
    waitcondition.wait(&mutex,20);
    mutex.unlock();
}

void displaythread::StartDisplay()
{
    std::cout<<"RECEIVED START SIGNAL"<<std::endl;
    stopped = false;
    running = true;
}

void displaythread::StopDisplay()
{
    std::cout<<"RECEIVED STOP SIGNAL"<<std::endl;
    stopped = true;
    running = false;
}

bool displaythread::IsDisplayActive() const
{
    return running;
}

void displaythread::Check()
{
    std::cout<<"SIGNAL FROM GUI RECEIVED"<<std::endl;
}

display.h

class display : public QWidget
{
    Q_OBJECT
public:
    explicit display(QWidget *parent = 0);
    ~display();

signals:

public slots:

private:
    void paintEvent(QPaintEvent *);

    IplImage *image_opencvBGR,*image_opencvRGB;
    QImage image;
    CvCapture *webcam;

display.cpp

display::display(QWidget *parent) :
    QWidget(parent)
{
    image_opencvRGB = cvCreateImage(cvSize(640,480),8,3);
    webcam = cvCaptureFromCAM(-1);
}

display::~display()
{
    cvReleaseCapture(&webcam);
}

void display::paintEvent(QPaintEvent *)
{
    //std::cout<<"IN PAINT LOOP"<<std::endl;
    image_opencvBGR = cvQueryFrame(webcam);

    cvCvtColor(image_opencvBGR,image_opencvRGB,CV_BGR2RGB);
    image = QImage((const unsigned char*)image_opencvRGB->imageData,image_opencvRGB->width,image_opencvRGB->height,QImage::Format_RGB888);
    QRectF target(0.0,0.0,image.width(),image.height());
    QRectF source(0.0,0.0,image.width(),image.height());
    QPainter painter(this);
    painter.drawImage(target,image,source);
}

OUTPUT :

RECEIVED START PROCESS SIGNAL

However except the Process slot no other slot is working when signals are emitted from the main gui i.e. MainWindow..is it due to movetoThread command? Donno where i am going wrong..

Answer 1

The answer is simple : Qwidgets don't work outside the main thread. So you cannot execute GUI code using displaythread.

Furthermore your while loop may cause issues (I know the variable is volatile but I dont have the time to analyze properly if it is correct)

See the documentation for more information.

ps: It seems you are overdoing things. Rework your whole design. GUI operations are in the main thread. Use threads for computations only. If the communication between thread and access to their variables use signal and slots only, you dont need locking mechanisms.