How to use the QGraphicsItem::setPos() function - c++

Question

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();
}

Answer 1

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)