Unexpected behaviour with QMouseMoveEvent and QKeyEvent modifiers

Question

I am encountering unexpected behaviour in my code. I have a QGraphicsView containing a QGraphicsScene. Now I want to detect the mouse wheel for zooming the view and mouse moving for moving items in the scene, the latter only while controll is pressed. Now I have two Problems:

1. MouseMoveEvent is called even when the mouse is not moved but only the mouse wheel.

2. Moving with and without controll pressed works fine but when I stop moving while controll is pressed and continue using the mouse wheel not only the mousemoveevent is called but also the controllmodifier is still active. What is the problem?

main.cpp

#include "ppi.h"
#include <QtGui/QApplication>

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

ppi.h

#ifndef PPI_H
#define PPI_H

#include <QtGui/QMainWindow>
#include <QGraphicsView>
#include <QDebug>
#include <QWheelEvent>

#include "ui_ppi.h"
#include "ppiView.h"
#include "ppiscene.h"

class PPI : public QMainWindow
{
    Q_OBJECT

public:
    PPI(QWidget *parent = 0, Qt::WFlags flags = 0);
    ~PPI();
    int i;

private:
    Ui::ppiClass ui;
    PPIScene* ppiScene;

protected slots:
    void onZoom(QWheelEvent *event);
    void onMouseMoved(QGraphicsSceneMouseEvent *event);
};

#endif // PPI_H

ppi.cpp

#include "ppi.h"


PPI::PPI(QWidget *parent, Qt::WFlags flags)
    : QMainWindow(parent, flags)
{
    ui.setupUi(this);

    ppiScene = new PPIScene(this);

    connect(ppiScene, SIGNAL(mouseMoved(QGraphicsSceneMouseEvent*)), this, SLOT(onMouseMoved(QGraphicsSceneMouseEvent*)));
    connect(ui.gVPPI, SIGNAL(zoom(QWheelEvent*)), this, SLOT(onZoom(QWheelEvent*)));

    ppiScene->setSceneRect(0,0,1024,1024);
    ui.gVPPI->setScene(ppiScene);
    ui.gVPPI->setMouseTracking(true);

    i = 0;
}

PPI::~PPI()
{

}



void PPI::onZoom(QWheelEvent *event)
{
        if(event->delta() > 0)
            ui.gVPPI->scale(1.01, 1.01);
        else
            ui.gVPPI->scale(1/1.01, 1/1.01);
}


void PPI::onMouseMoved(QGraphicsSceneMouseEvent *event)
{
    i++;
    qDebug() << "slot" << i << event->modifiers();
    if(event->modifiers() & Qt::ControlModifier)
    {
        qDebug() << "ctrl pressed";
    }
}

ppiview.h

#ifndef PPIVIEW_H
#define PPIVIEW_H

#include <QGraphicsView>
#include <QMouseEvent>

class PPIView : public QGraphicsView
{
    Q_OBJECT

public:
    PPIView(QWidget * parent = 0);
    ~PPIView();

private:
    void wheelEvent(QWheelEvent *event);

signals:
    void zoom(QWheelEvent *event);

};

#endif // PPIVIEW_H

ppiview.cpp

#include "ppiview.h"

PPIView::PPIView(QWidget * parent)
    : QGraphicsView(parent)
{

}

PPIView::~PPIView()
{

}

void PPIView::wheelEvent(QWheelEvent *event)
{
    emit zoom(event);
}

ppiscene.h

#ifndef PPISCENE_H
#define PPISCENE_H

#include <QGraphicsScene>
#include <QGraphicsSceneMouseEvent>
#include <QDebug>

class PPIScene : public QGraphicsScene
{
    Q_OBJECT

public:
    PPIScene(QObject *parent);
    ~PPIScene();
    int i;

private:
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event);

signals:
    void mouseMoved(QGraphicsSceneMouseEvent *event);

};

#endif // PPISCENE_H

ppiscene.cpp

#include "ppiscene.h"

PPIScene::PPIScene(QObject *parent)
    : QGraphicsScene(parent)
{
    i = 0;
}

PPIScene::~PPIScene()
{

}

void PPIScene::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    i++;
    qDebug() << "signal" << i << event->modifiers();
    emit mouseMoved(event);
}

Answer 1

It is really strange. It looks like the Qt::KeyboardModifiers status, which QGraphicsSceneMouseEvent::modifiers returns, is only updated, when the mouse is physically moved. What is even more strange, is that in your code QGraphicsSceneMouseEvents of type QGraphicsSceneMouseMove are sent even when the mouse is not moved at all, but only the wheel is turned. Maybe the relative movement due to your scaling counts as movement, but not as movement, which updates the modifiers.

I was able to reproduce your problem: The status of the modifiers don't change unless the mouse is physically moved.

Fortunately there is an easy workaround. In

void PPI::onMouseMoved(QGraphicsSceneMouseEvent *event)
{
    i++;
    qDebug() << "slot" << i << event->modifiers();
    if(event->modifiers() & Qt::ControlModifier)
    {
        qDebug() << "ctrl pressed";
    }
}

replace:

if(event->modifiers() & Qt::ControlModifier)

with:

if(QApplication::queryKeyboardModifiers() & Qt::ControlModifier)

QApplication::queryKeyboardModifiers() is updated immediately when you press or release the control key.