Freeze using QTimer with interval 0

[Alundra]

Hi all,
I have a class who inherit from QWidget to render a 3D content inside.
I use a QTimer to refresh the window like that :

m_Timer.setInterval( 0 );
connect( &m_Timer, SIGNAL( timeout() ), this, SLOT( repaint() ) );
m_Timer.start();

m_Timer.setInterval( 0 );
connect( &m_Timer, SIGNAL( timeout() ), this, SLOT( repaint() ) );
m_Timer.start();

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When I launch the application and I move the window, the application freeze.
When I undock a dock window and I try to move it, the application freeze.
The only one solution I have is to set the interval to 10ms.
Thanks for the help

[wysota]

Use update() instead of repaint(). Besides, what is the point of constantly updating the widget if nothing in it changes? Redraw only when the scene is updated.

[zeeshanaayan07]

great coding Freeze using QTime with interval

[Alundra]

using update() instead of repaint() doesn't change anything, still freeze.

[wysota]

Please provide a minimal compilable example reproducing the problem.

[Alundra]

#include <QMainWindow>
#include <QDockWidget>
#include <QApplication>
#include <QTimer>
 
class TestWidget : public QWidget
{
public:
 
    TestWidget(QWidget* parent = 0) :
    QWidget(parent),
    m_Initialized(false)
    {
        m_Timer.setInterval(0);
    }
 
    QPaintEngine* paintEngine() const
    {
        return NULL;
    }
 
    void showEvent(QShowEvent* event)
    {
        if(m_Initialized == false)
        {
            connect(&m_Timer, SIGNAL(timeout()), this, SLOT(repaint()));
            m_Timer.start();
            m_Initialized = true;
        }
    }
 
private:
 
    QTimer m_Timer;
    bool m_Initialized;
};
 
class MainWindow : public QMainWindow
{
public:
 
    MainWindow(QWidget* parent = 0) :
    QMainWindow(parent)
    {
        QDockWidget* NewDock = new QDockWidget("Test", this);
        NewDock->setWidget(new TestWidget(this));
        addDockWidget(Qt::BottomDockWidgetArea, NewDock);
    }
};
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

#include <QMainWindow>
#include <QDockWidget>
#include <QApplication>
#include <QTimer>

class TestWidget : public QWidget
{
public:

    TestWidget(QWidget* parent = 0) :
    QWidget(parent),
    m_Initialized(false)
    {
        m_Timer.setInterval(0);
    }

    QPaintEngine* paintEngine() const
    {
        return NULL;
    }

    void showEvent(QShowEvent* event)
    {
        if(m_Initialized == false)
        {
            connect(&m_Timer, SIGNAL(timeout()), this, SLOT(repaint()));
            m_Timer.start();
            m_Initialized = true;
        }
    }

private:

    QTimer m_Timer;
    bool m_Initialized;
};

class MainWindow : public QMainWindow
{
public:

    MainWindow(QWidget* parent = 0) :
    QMainWindow(parent)
    {
        QDockWidget* NewDock = new QDockWidget("Test", this);
        NewDock->setWidget(new TestWidget(this));
        addDockWidget(Qt::BottomDockWidgetArea, NewDock);
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

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[wysota]

What is returning an empty paint engine meant to do in this example? What exactly would you expect to receive in the widget?

[ChrisW67]

Doesn't freeze here, not that it actually does anything useful. It does, however, tie up a CPU core at near 100% utilisation... but then that is what you are asking it to do with a zero timer.

[Alundra]

The real widget in the application initialize a render window from a 3D Engine.
The return NULL is to remove the flickering.
To reproduce the freeze you have to undock and move the dock or the application window.

[wysota]

No freeze here... sorry...

Did you check that the small code snippet that you posted actually freezes?

[Alundra]

I have retested again and the freeze is real and not on this computer only.
Only try to move the window and the freeze will be there.
I have to ctrl+alt+enter to end the application.

[wysota]

Which platform did you test it on? Which Qt version?