Comments on: Reparenting QGraphicsItem during Mouse Drag

By: Anthony

Anthony — Mon, 27 Feb 2012 01:10:52 +0000

I see this post is old, but I’m experiencing the same problem. As I move around some QGraphicsItems, I want them to “snap” to some other items, and it works visually, but when I try to click on them after moving, it doesn’t work.

Did you ever figure out a nicer solution?

By: Gopala Krishna A

Gopala Krishna A — Wed, 23 Jun 2010 08:47:03 +0000

Probably you can simulate a mouse press on reparenting. (and use timer with 0 timeout just to ensure you are doing things in a fresh event loop iteration)

By: Jason Donenfeld

Jason Donenfeld — Tue, 22 Jun 2010 21:19:41 +0000

@giucam
This works for updating the position as far as reparenting goes, but the mouse event is still tied to the original position, so the item is moved to the correct place initially, but as soon as you move the mouse again after it’s been reparented, it jumps all over.

@aseigo
Good thinking. But I don’t think I really want to do this. The way my application is setup, I have some bounds checking to ensure that child rectangles are never moved outside their parent until they’re reparented when the drag destination is fully enclosed in the new parent. This makes dragging rectangles from one place to another “stick” at the border between them until the user has deliberately dragged it far enough in.

Put more simply, doing something like that is incompatible with this:

QRectF transformed = transform().mapRect(boundingRect());
if (newPoint.x() < -transformed.x())
newPoint.setX(-transformed.x());
if (newPoint.y() < transformed.y())
newPoint.setY(-transformed.y());
if (newPoint.x() > parentItem()->boundingRect().width() – transformed.width() – transformed.x())
newPoint.setX(parentItem()->boundingRect().width() – transformed.width() – transformed.x());
if (newPoint.y() > parentItem()->boundingRect().height() – transformed.height() – transformed.y())
newPoint.setY(parentItem()->boundingRect().height() – transformed.height() – transformed.y());
return newPoint;

By: Aaron Seigo

Aaron Seigo — Tue, 22 Jun 2010 21:03:50 +0000

the first thing i wondered is why you aren’t reparenting on mouse release? you can certainly check what plate is under the food item while it is moving, and even have the plate respond visually in some way (or even “reject” the item, e.g. if it was “full”) .. but maybe just reparent when the movement actually stops?

By: giucam

giucam — Tue, 22 Jun 2010 21:00:18 +0000

I’m doing lot of reparenting when moving QGraphicsItems in my project (playgrund/base/plasma/containments/groupingdesktop) and all i need to do is:

QPointF pos = item->scenePos();
item->setParentItem(newParent);
item->setPos(newParent->mapFromScene(pos));