Find screen position of a QGraphicsItem

Question

Use case: This should be a fairly common problem. In a normal QMainWindow with QMdiArea lives an mdiChild with a QGraphicsView. This view displays a QGraphicsScene with QGraphicsItems inside. A right-click at one of these items selects (focusses) the item and opens a context menu, which is conveniently placed at the screen coordinates QGraphicsSceneMouseEvent::screenPos(). This is working as expected.

Now I'd like to show the same context menu when the user presses a key (e.g. Qt::Key_Menu). I know the selected (focussed) item, I know the view which displays the scene.

So my question is:
What is the correct way to get the position (in global, screen coordinates) of the visible representation of a QGraphicsItem within a scene?

Here is what's not working:

QGraphicsItem *item = ...; // is the currently selected item next to which the context menu shall be opened
QGraphicsScene *scene = ...; // is the scene that hosts the item
QGraphicsView *graphicsView = ...; // is the view displaying the scene, this inherits from QWidget

// get the position relative to the scene
QPointF sp = item->scenePos();
// or use
QPointF sp = item->mapToScene(item->pos());

// find the global (screen) position of the item
QPoint global = graphicsView->mapToGlobal(graphicsView->mapFromScene(sp));

// now
myContextMenu.exec(global);
// should open the context menu at the top left corner of the QGraphicsItem item, but it goes anywhere

The doc says: If you want to know where in the viewport an item is located, you can call QGraphicsItem::mapToScene() on the item, then QGraphicsView::mapFromScene() on the view.
Which is exactly what I'm doing, right?

Just stumbled upon a thread in a german forum that hints to:

QGraphicsView *view = item->scene()->views().last();

or even nicer:

QGraphicsView *view;
foreach (view,  this->scene()->views())
{
    if (view->underMouse() || view->hasFocus()) break;
}
// (use case in the forum thread:) // QMenu *menu = new QMenu(view);

Using that might allow a more generalized answer to my question...

I was about to post a response, but from re-reading the docs I think I agree with your analysis: QGraphicsView::mapFromScene should give viewport coordinates (worth checking). Only question would be if there's a latent bug in mapToGlobal on widgets inside MDI children. — James Turner
@JamesTurner So what would be your first guess (which you would have written in response)? — Martin Hennings

Martin Hennings Martin Hennings · Accepted Answer · 2013-04-04T10:45:16

I found a working solution.
The QGraphicsItem must be visible on the screen. (Probably if it's not visible because the view shows some other point of the scene, one could restrain the point to the view's viewport's rect.)

// get the screen position of a QGraphicsItem
// assumption: the scene is displayed in only one view or the first view is the one to determine the screen position for
QPoint sendMenuEventPos; // in this case: find the screen position to display a context menu at
QGraphicsItem *pFocusItem = scene()->focusItem();

if(scene() != NULL // the focus item belongs to a scene
    && !scene()->views().isEmpty() // that scene is displayed in a view...
    && scene()->views().first() != NULL // ... which is not null...
    && scene()->views().first()->viewport() != NULL // ... and has a viewport
    )
{
    QGraphicsView *v = scene()->views().first();
    QPointF sceneP = pFocusItem->mapToScene(pFocusItem->boundingRect().bottomRight());
    QPoint viewP = v->mapFromScene(sceneP);
    sendMenuEventPos = v->viewport()->mapToGlobal(viewP);
}

if(sendMenuEventPos != QPoint())
{
    // display the menu:
    QMenu m;
    m.exec(sendMenuEventPos);
}

It is important to use the view's viewport for mapping the view coords to global coords.

The detection of the context menu key (Qt::Key_Menu) happens in the keyPressEvent() of the "main" QGraphicsItem (due to the structure of my program).

Find screen position of a QGraphicsItem

3 Answers