QGraphicsItem parent transformation

[sDmt]

Hi,
I have two QGraphicsItem objects as in the image

What I need is:

• Make the ChildObject moves as I drag the parent which was easily done by setParentItem to ParentObject.

• I want to be able to rotate the ParentObject while keeping the ChildObject fixed.

The best I could come up with was setting the parent of the childobject to nullptr when rotating and setting back to ParenObject when moving the ParentObject

QVariant MyShape::itemChange(GraphicsItemChange change, const QVariant &value)
{
 if(change == QGraphicsItem::ItemPositionChange)
       childobject->setParentItem(this); 
 else if (change == QGraphicsItem::ItemRotationChange)
    {
        childobject->setParentItem(nullptr);
        scene()->update();
    }
}

I have initialized the ChildObject in the constructor of the ParentObject shape to appear under the ParentObject

ChildObject *childobj = new ChildObject;

    QPointF mPos;
    mPos.setX(rect().bottomLeft().x() + rect().width()/2 -
                  childObject->boundingRect().width()/2);

    mPos.setY(rect().bottomLeft().y() + rect().height()/3);
    childobj ->setPos(mapFromItem(this,mPos));
    childobj->setFlag(QGraphicsItem::ItemIgnoresTransformations,true);
    childobj->setParentItem(this);

However, when I run the code, the ChildObject moves when I rotate the ParentObject. I don't know what I am missing, I think its about the coordinate of the Parent/Scene issue but not sure. Please help.

Thanks in advance

[SGaist]

Hi,

Based on the flag description:

The item ignores inherited transformations (i.e., its position is still anchored to its parent, but the parent or view rotation, zoom or shear transformations are ignored).

I would say that this is the expected behaviour as the rotation will make the parent time move.

[AlexMaly]

How about creating a QGraphicsItemGroup , add them both item and move the group, then you can rotate the child you need independently.
Or just do what you do, but listen to the parent item QGraphicsItem::ItemRotationChange and adjust child item rotation there to compensate parent rotation?

[sDmt]

Thank you SGaist for pointing out the flaw in my approach

[sDmt]

@AlexMaly Thanks for the valuable suggestion. I've managed to do it as you suggested