[Qt-interest] translate vs setPos

[Oliver.Knoll at comit.ch]

On 2010-11-30 Brian Brian McKinnon wrote:

> ...
> I create it from the overloaded drop event in my graphics view.  I 
> wasn't able to get the drag event inside the scene to activate so I drop it in from the scene.

Okay, so I understand you want to insert some data upon a drag and drop operation upon the scene. As a matter of fact I just did the same a couple of days ago, and I also struggled with getting the drag and drop support work in QGraphicsSCENE: you overwrote the corresponding methods in the QGraphicsVIEW, as I understand. 

(But I was too lazy to extend from the view, because then I would have had to insert custom widgets ("promote", define #include paths etc.) in Qt Designer - but this just as a side note).

The CRUCIAL thing - and a bit counterintuitive - is to know that you ALSO have to overwrite the

void MyGraphicsScene::dragMoveEvent(QGraphicsSceneDragDropEvent *event) {
    if (event->mimeData()->hasUrls()) {
        event->accept(); // THIS IS CRUCIAL: ACCEPT AGAIN!
    }
}

See the Qt-Interest archive "QGraphicsScene: drag'n'drop onto empty scene? (dropEvent() never called) [SOLVED]"  on Friday last week for the whole story

The explanation is that the DEFAULT implementation of QGraphicsScene::dragMoveEvent REJECTS the drop event IF there is no item under the mouse! Which is very likely the case, because you just want to insert a new item at this empty place ;) Sounds reasonable, but is nowhere documented in the Qt docs and you and I are apparently not the first persons to stumble across this issue! 


Add a comment (as I did) here, if you also feel this should be mentioned in a special "Drag and drop on QGraphicsScene" chapter: http://doc.trolltech.com/4.7/qgraphicsscene.html

(There is a TINY LITTLE "[+] Documentation Feedback" link in LIGHT-BRIGHT-GREY ON WHITE colour, with a TINY MICRO-FONT-SIZE of about 4 px, at the VERY BOTTOM RIGHT of that page ;)


You read the explanation here again: http://www.qtcentre.org/threads/8022-QGraphicsScene-doesn-t-accept-Drops


Anyway, this works for me: in the QGraphicsScene, in the dropEvent() method, I get the position of the mouse like this:

void ScreenieGraphicsScene::dropEvent(QGraphicsSceneDragDropEvent *event)
{
  ...
  QPointF position = event->scenePos();
  ...
}

I then do (basically) the following (in some slot which is connected to my own scene model):

  MyGraphicsPixmapItem *item = new MyGraphicsPixmapItem (...);
  item ->setPos(position);
  m_graphicsScene.addItem(item);

This places the item (a QGraphicsPixmapItem based item) with its top-left corner at where the mouse was when dropping the data.

> ...
>         QGraphicsRectItem * rect = new
>         DragableRectangleItem(mapToScene(event- pos()),

QGraphicsRectItem *rect = new DragableRectangleItem(event->scenePos(), ...

(instead of mapToScene(event->pos()).

Should be equivalent here (to my understanding of QGraphicsView/Scene) - but maybe it makes the difference already? It works for me...


Cheers, Oliver
--
Oliver Knoll
Dipl. Informatik-Ing. ETH
COMIT AG - ++41 79 520 95 22