Hi,

you have to use option->rect and implement the pure virtual function boundingRect()!

Qt Code: 
  1. class LayoutItem : public QGraphicsWidget {
  2. public:
  3.   LayoutItem(QGraphicsItem *parent = 0) : QGraphicsWidget( parent ) {}
  4.   void paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
  5.              QWidget *widget = 0) {
  6.  
  7.     qDebug() << "LayoutItem::paint, size = " << option->rect;
  8.     painter->drawRect( option->rect );
  9.   }
  10.  QRectF boundingRect() const
  11.  {
  12.      return QRectF(0,0,400.0,400.0);
  13.  }
  14.  
  15. };
  16.  
  17. class Window : public QGraphicsWidget {
  18. public:
  19.   Window(QGraphicsWidget *parent = 0) 
  20.   {
  21.     QGraphicsGridLayout *grid = new QGraphicsGridLayout( this );
  22.     LayoutItem* item = new LayoutItem;
  23.     grid->addItem(item, 0, 0, 1, 1);
  24.   }
  25. };
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