I can't figure out how the setPos() function of the QGraphicsItem class works.

My Rect class has no parent, so its origin is relative to the scene.

I try to put the rectangle back at (0, 0) after it is moved with the mouse but it is placed in a different place depending on where I had moved it. I suppose that means that the origin of the scene moves but what causes this change?

class Rect : public QGraphicsItem {
public:
    Rect(): QGraphicsItem()
    {
        setFlag(ItemIsMovable);
    }

    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override
    {
        painter->drawRect(0, 0, 20, 20);
    }

    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override
    {
            setPos(0, 0);
            update();
            QGraphicsItem::mouseReleaseEvent(event);
    }

    QRectF boundingRect() const
    {
            return QRectF(0, 0, 20, 20);
    }

private:
};

int main(int argc, char *argv[])
{
        QApplication a(argc, argv);
        QGraphicsScene scene;
        QGraphicsView view(&scene);
        Rect obj;
        scene.addItem(&obj);

        view.show();
        return a.exec();
}

CodePudding user response：

When you create a QGraphicsView you initially accept the default settings. A standard setting is, for example, that it is horizontally centered.

Another factor is that the default area size is probably up to the maximum size.

what you can do set a custom size for the scene. You do that with graphicsView->setSceneRect(0,0,300,300); (for example)

    scene       = new QGraphicsScene(this);
    
    ui->graphicsView->setScene(scene);
    ui->graphicsView->setRenderHint(QPainter::Antialiasing);
    ui->graphicsView->setSceneRect(0,0, 300,300);
    
    rectItem    = new QGraphicsRectItem(0,0, 100, 100);
    rectItem->setPen(QPen(Qt::darkMagenta, 2));
    rectItem->setBrush(QGradient(QGradient::SaintPetersburg));
    rectItem->setPos(190,10);

    scene->addItem(rectItem);

So in summary: if you want to work with fixed values. maybe it is better to know the total size. (that was not clear from your code, that's why I gave this example)