qt - Need QGraphicsScene signal or event for _after_ change

Question

I use QGraphicsScene of the Qt framework. Inside the scene I have some QGraphicsItems which the user can select and move. I would like to have an info label where the current x and y coordinate of the currently moved selection (can consist of many items) is displayed.

I have tried with the signal changed of QGraphicsScene. But it is fired before the x() and y() property of the items is set to the new values. So the labels always show the second-to-last coordinates. If one moves the mouse slowly, the display is not very wrong. But with fast moves and sudden stops, the labels are wrong. I need a signal that is fired after the scene hast changed.

I have also tried to override the itemChange method of QGraphicsItem. But it is the same. It is fired before the change. (The new coordinates are inside the parameters of this method, but I need the new coordinates of all selected items at once)

I have also tried to override the mouseMove events of QGraphicsScene and of QGraphicsView but they, too, are before the new coordinates are set.

I did a test: I used a oneshot timer so that the labels are updated 100 ms after the signals. Then everything works fine. But a timer is no solution for me.

What can I do? Make all items un-moveable and handle everything by my own?

Answer 1

QGraphicsItem::itemChange() is the correct approach, you were probably just checking the wrong flag. Something like this should work fine:

QVariant::myGraphicsItem( GraphicsItemChange change, const QVariant &value )
{
  if( change == QGraphicsItem::ItemPositionHasChanged )
  {
     // ...
  }
}

Note the use of QGraphicsItem::ItemPositionHasChanged rather than QGraphicsItem::ItemPositionChange, the former is called after the position changes rather than before.