Prevent QGraphicsItem from moving outside of QGraphicsScene - qt

Question

I have a scene which has fixed dimensions from (0;0) to (481;270):
scene->setSceneRect(0, 0, 481, 270);
Inside of it, I have a custom GraphicsItem and I can move it thanks to the flag ItemisMovable, but I would like it to stay within the Scene; I actually mean I don't want it to have coordinates neither under (0;0) nor over (481;270).
I tried several solutions like overriding QGraphicsItem::itemChange() or even QGraphicsItem::mouseMoveEvent() but I still cannot manage to reach what I want to do.
What is the suitable solution for my needs? Do I use QGraphicsItem::itemChange() badly?
Thanks in advance.

Answer 1

You can override QGraphicsItem::mouseMoveEvent() like this:
YourItem::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
QGraphicsItem::mouseMoveEvent(event); // move the item...
// ...then check the bounds
if (x() < 0)
setPos(0, y());
else if (x() > 481)
setPos(481, y());
if (y() < 0)
setPos(x(), 0);
else if (y() > 270)
setPos(x(), 270);
}

Answer 2

This code keeps your complete item in scene. Not only the upper left pixel of your item.
void YourItem::mouseMoveEvent( QGraphicsSceneMouseEvent *event )
{
QGraphicsItem::mouseMoveEvent(event);
if (x() < 0)
{
setPos(0, y());
}
else if (x() + boundingRect().right() > scene()->width())
{
setPos(scene()->width() - boundingRect().width(), y());
}
if (y() < 0)
{
setPos(x(), 0);
}
else if ( y()+ boundingRect().bottom() > scene()->height())
{
setPos(x(), scene()->height() - boundingRect().height());
}
}

Answer 3

Warning: The suggested solutions won't work for multiply selected items. The problem is, only one of the items receives a mouse move event in that case.
Actually, the Qt Documentation on QGraphicsItem provides an example which exactly solves the problem of limiting the movement of items to the scene rect:
QVariant Component::itemChange(GraphicsItemChange change, const QVariant &value)
{
if (change == ItemPositionChange && scene()) {
// value is the new position.
QPointF newPos = value.toPointF();
QRectF rect = scene()->sceneRect();
if (!rect.contains(newPos)) {
// Keep the item inside the scene rect.
newPos.setX(qMin(rect.right(), qMax(newPos.x(), rect.left())));
newPos.setY(qMin(rect.bottom(), qMax(newPos.y(), rect.top())));
return newPos;
}
}
return QGraphicsItem::itemChange(change, value);
}
Note I: You'd have to enable the QGraphicsItem::ItemSendsScenePositionChanges flag:
item->setFlags(QGraphicsItem::ItemIsMovable
| QGraphicsItem::ItemIsSelectable
| QGraphicsItem::ItemSendsScenePositionChanges);
Note II: If you only want to react on finished movement, consider using the GraphicsItemChange flag ItemPositionHasChanged