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QGraphicsItem scenePos() broken after aborting previous item drag
http://www.qtcentre.org/forum/showthread.php?t=19050&referrerid=6743
Posted 2-24-2009
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We're trying to abort an item drag when the user hits the ESC key before 
releasing the mouse button. We do that within the item's keyPressEvent() 
method by momentarily disabling the item. (See code, below).

That basically works, but it causes a problem. During the subsequent 
move-drag operation of a different QGraphicsItem, in the processing of 
mouseMoveEvents, the item is "warped" to near the origin of the scene, 
and QGraphicsItem::scenePos() is, in fact, returning those sorts of 
coordinates: it's returning the difference of the drag mouse position 
from the mouse press position which started the item drag.

This is with Qt version Qt 4.3.3, both Solaris and Windows XP. Here's a 
excerpt (and minor simplification) of our code: (1) keyPressEvent, and 
(2) mouseMoveEvent:

//----------------

// virtual from QGraphicsItem
void SimObjGfxItem::keyPressEvent (QKeyEvent* evt)
{
   const QGraphicsItem* mGrabItem = scene()->mouseGrabberItem();
   const bool mouseGrabActive = (this == mGrabItem);
   const bool isEscKey (evt->key() == Qt::Key_Escape);

   if (mouseGrabActive && isEscKey)
   {
      // Schedule restoration of Pre-Move Positions
      _restorePreMovePosOnNextFocusOut = true;

      // DROP THE MOUSE GRAB (by momentarily becomming disabled). 
      // This works, but it causes scenePos() to return strange results
      // during mouseMoveEvents on the subsequent drag-move of a different
      // QGraphicsItem. (See below).
      //
      setEnabled (false);
      qApp->processEvents();   // (same behavior with or without this).
      setEnabled (true);
      setSelected (true);

      return;
      //------->>
   }

   // call base class method
   QGraphicsItem::keyPressEvent (evt);
}

//----------------

// virtual from QGraphicsItem
void SimObjGfxItem::mouseMoveEvent (QGraphicsSceneMouseEvent* evt)
{
   // Call base class method
   QGraphicsItem::mouseMoveEvent (evt);

   ... ... ...

   // NOTE: On the QGraphicsItem drag (only) immediately after aborting a 
   // drag of another item with the ESC key -- see keyPressEvent, above -- 
   // QGraphicsItem::scenePos() returns instead coordinates relative to the
   // starting position of the QGraphicsItem drag -- i.e. on the first 
   // several moves, near (0,0).

   const QPointF objScenePos (scenePos());
   const double sceneX (objScenePos.x());
   const double sceneY (objScenePos.y());

   ... ... ...
   std::cout << ... 
             << " scene (" << sceneX << "," << sceneY << ")"
             << ... << std::endl;
   ... ... ...
}

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