FYI, I've just created https://bugreports.qt.io/browse/QTBUG-58988.
Regards,
Chris
On 16 February 2017 at 17:52, Ch'Gans <chg...@gna.org> wrote:
> Hi there,
>
> Here is my use-case:
> An item has "handles" child items, when the user moves around an handles,
> the parent item update it's geometry and position accordingly. Simple
> concept to let the user change an item geometry by dragging handles.
> Imagine, you're using a drawing application, you select a circle and 5
> handles appear on the screen, one in each corner to resize the circle, and
> one in the center to move the circle around.
>
> The way i implement it is to let the circle item filter an handle item's
> 'itemChange()' notification. This work nicely for resizing handles, but
> fails for the 'move handle'. Here is the simplified pseudo code:
> QVariant HandleItem::itemChange(change, value)
> {
> // Move the parent to where the handle wants to go...
> parentItem()->setPos(parentItem()->mapToParent(value);
> // ... and leave the handle at parent's origin
> return QPointF(0, 0);
> }
>
> After banging my head for a while, i finally pin-pointed what I think is
> the problem:
>
> This is related to how items are moved in qt5/qtbase/src/widgets/
> graphicsview/qgraphicsitem.cpp. When a move begins, the 'initial
> position' of the item is stored in the scene. This initial position is
> simply the value of item->pos(), which is in parent's item CS:
>
> initialPositions = d_ptr->scene->d_func()->movingItemsInitialPositions;
> if (initialPositions.isEmpty()) {
> foreach (QGraphicsItem *item, selectedItems)
> initialPositions[item] = item->pos();
> initialPositions[this] = pos();
> }
> d_ptr->scene->d_func()->movingItemsInitialPositions = initialPositions;
>
> As the move goes on, the item's pos is updated based on this 'initial
> position' (expressed in an outdated parent's CS) and the parent position
> relative to the item's position:
>
> currentParentPos = item->mapToParent(item->mapFromScene(event->scenePos()
> ));
> buttonDownParentPos = item->mapToParent(item->mapFromScene(event->
> buttonDownScenePos(Qt::LeftButton)));
> [...]
> item->setPos(initialPositions.value(item) + currentParentPos -
> buttonDownParentPos);
>
> Basically the above 3 lines would be correct only and only if the
> initialPositions were expressed in a dynamic way, that is, if they were
> independent of the parent's CS since it can change during the move.
>
> The following pseudo-code modifications fixes that:
>
> // 'Capture' time
> initialPosition = item->mapToScene(item->mapFromParent(item->pos())); //
> Independent of parent CS
> [...]
> // 'Update' time
> initialPosition = item->mapToParent(item->mapFromScene(initialPosition));
> // up-to-date w/ changed parent's CS
> item->setPos(initialPosition + currentParentPos - buttonDownParentPos);
>
>
> This modification fixes my use-cases, and doesn't seem to break the "40000
> chips" and other demo/example.
> The scene's private 'movingItemsInitialPositions' is only set/used by
> QGI::mouseMoveEvent() and cleared by QGI::mouseReleaseEvent(), so this is
> not an invasive change
>
>
> So my chain of question is:
> - Is changing the item's parent position in an item's itemChange() a
> supported feature?
> - if it is, then isn't there a bug in qgraphicsitem.ccp as i described
> above?
> - if positive, is the above fix correct and would you accept this sort of
> things if i push that to gerrit?
>
>
> Chris
>
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