What should I do if I want to drag QGraphicsVIew with the mouse middleButton？

[John Van]

Hello!
The default is to drag the view with the leftButton, I want to change it to the middleButton.
Another question is why adding the line "setSceneRect(-100, -100, 200, 200);"makes it impossible to drag the view.

#include <QApplication>
#include <QGraphicsSceneMouseEvent>
class MyScene : public QGraphicsScene
{
public:
    QImage m_image;
protected:
    void drawBackground(QPainter* painter, const QRectF& rect)
    {
        if (!m_image.isNull())
        {
            auto r = sceneRect();
            painter->drawImage(r, m_image);
        }
    }
    
    void mousePressEvent(QGraphicsSceneMouseEvent* e)
    {
        switch (e->buttons())
        {
        case Qt::LeftButton:
            printf("do something\n");
            break;
        case Qt::RightButton:
            break;
        case Qt::MiddleButton:
            QGraphicsScene::mousePressEvent(e);
            break;
        }
    }
};
class MyView :public QGraphicsView
{
public:
    MyView()
    {
        auto  m_scene = new MyScene;
        m_scene->m_image = QImage("D:/100.png");
        m_scene->setBackgroundBrush(Qt::gray);
        m_scene->setSceneRect(0, 0, 400, 400);
        //setSceneRect(-100, -100, 200, 200);
        setScene(m_scene);

        setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
        setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
        setOptimizationFlags(QGraphicsView::DontSavePainterState);
        setViewportUpdateMode(QGraphicsView::SmartViewportUpdate);
        setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
        setRenderHint(QPainter::Antialiasing, false);
        setDragMode(QGraphicsView::ScrollHandDrag);
    }
    void wheelEvent(QWheelEvent* event)
    {
        auto r = sceneRect();
        double scaleFactor = pow(2.0, event->angleDelta().y() / 240.0);
        scale(scaleFactor, scaleFactor);
    };
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    auto  v = new MyView();
    v->resize(800, 600);
    v->show();
    return a.exec();
}

[Pl45m4]

@John-Van said in What should I do if I want to drag QGraphicsVIew with the mouse middleButton？:

I want to change it to the middleButton.

Quick google search

• https://gist.github.com/gboeer/a00760b6d7af32c01357fb7ff76ad86a

Another question is why adding the line "setSceneRect(-100, -100, 200, 200);"makes it impossible to drag the view.

Because you set a static scene rect. There is nothing to move. The scene is fixed 200x200 to that point...
If you add items and the scene will grow, you can move again.