How to get multitouch to work in QGraphicsView, Qt 5.0.2 in Windows 8

Question

I am struggling with getting multi-touch to work on a couple of QWidgets that I have added to a QGraphicsView. I have created a subclass of QWidget in which I set up a QGraphicsScene and QGraphicsView. This is my (test) subclass of QWidget:

#include "qttest1.h"

#include <QtWidgets>
#include <QTouchEvent>

qttest1::qttest1(QWidget *parent)
    : QWidget(parent)
{
    setEnabled(true);

    if(!QCoreApplication::testAttribute(Qt::AA_DontCreateNativeWidgetSiblings))
        setAttribute(Qt::WA_NativeWindow);

    setAttribute(Qt::WA_AcceptTouchEvents);

    scene = new QGraphicsScene(this);
    scene->setSceneRect(0, 0, 1920, 1080);
    graphicsView = new QGraphicsView(scene, this);
    graphicsView->setRenderHints(QPainter::Antialiasing);
    graphicsView->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    graphicsView->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    graphicsView->setAttribute(Qt::WA_AcceptTouchEvents);
    graphicsView->viewport()->setAttribute(Qt::WA_AcceptTouchEvents);

    QBoxLayout *layout = new QVBoxLayout;
    layout->addWidget(graphicsView);

    setLayout(layout);
}

qttest1::~qttest1() {}

void qttest1::showGraphics() 
{
    for(int i = 0; i < 10; i++)
    {
        Dial *dial = new QDial();

        dial->move(i * 120 + 50, 200);
        dial->resize(120, 120);
        dial->setAttribute(Qt::WA_AcceptTouchEvents);

        QGraphicsProxyWidget *proxy = scene->addWidget(dial);
        proxy->setAcceptTouchEvents(true);
    }
}

This is my main:

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    app.setAttribute(Qt::AA_DontCreateNativeWidgetSiblings);

    QRect rect = app.desktop()->screenGeometry();

    qttest1 test;
    test.resize(rect.width(), rect.height());
    test.showFullScreen();
    test.showGraphics();

    return app.exec(); 
}

I know the code isn't pretty and probably leaking a bit, but the point is to try to get multi-touch to work.

I can see and use every kind of widget I add to the scene, but as soon as I touch a dial it swallows every touch that comes after the first. Which makes the dial jump between several positions. What I want is that every dial (or any type of widget) can be used individually and at the same time. I am using QT 5.0.2, Windows 8 with a monitor that supports up to 10 touches.

Answer 1

The Qt docs state : -

Reimplement QWidget::event() or QAbstractScrollArea::viewportEvent() for widgets and QGraphicsItem::sceneEvent() for items in a graphics view to receive touch events.

With that, I believe that you need to handle the QEvent::TouchBegin, QEvent::TouchUpdate and QEvent::TouchEnd events, which I don't see in the code you've posted.

Qt may handle the first touch for you, but it's not going to know what you want to do with the second, third, fourth etc. simultaneous touches. For example, you may want your app to do any of the following with the second touch moving: -

1) Rotate the object that the first item is over

2) Scale the object that the first item is over

3) Select the second item

4) Translate the view

5) etc.

So, you need to handle the consecutive touches to do what you want it to do. Also, you may want to look at Gestures in Qt.