youre right:

i had change it to:

customItem.cpp
here ist the member variable for the center: mCenter for the ellipse
Qt Code: 
  1. void customItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
  2. {
  3.     QRectF ellipse = boundingRect();
  4.  
  5.     mCenter = ellipse.center();
  6.  
  7.     qDebug()  << "mCenter: " << mCenter;
  8.  
  9.     if(Pressed)
  10.     {
  11.         QPen pen(Qt::red, 3);
  12.         painter->setPen(pen);
  13.         painter->drawEllipse(ellipse);
  14.     }
  15.     else
  16.     {
  17.         QPen pen(Qt::black, 3);
  18.         painter->setPen(pen);
  19.         painter->drawEllipse(ellipse);
  20.     }
  21. }
  22.  
  23.  
  24. void customItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
  25. {
  26.     Pressed = false;
  27.  
  28.  
  29.  
  30.     if(startOrEnd == false)
  31.     {
  32.        line->line().setP1(mCenter);
  33.     }
  34.  
  35.  
  36.     if(startOrEnd == true)
  37.     {
  38.        line->line().setP2(mCenter);
  39.     }
  40.  
  41.  
  42.     update();
  43.     QGraphicsItem::mouseReleaseEvent(event);
  44. }
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but the line do not move