How to redraw QGraphicsScene background in running

In my QGraphicsScene object declare, I have draw a picture in drawBackground(), see below:

Code:

---

class MyScene : public QGraphicsScene
{    
public: 
    MyScene(qreal x, qreal y, qreal w, qreal h)
          : QGraphicsScene(x, y, w, h)      { } 
 
protected:
    void drawBackground(QPainter *painter, const QRectF &rect)
{    //draw an old  background
};

---

In my program running, I need redraw the background, how I will do?
Very Thanks

Call QGraphicsScene::invalidate() with QGraphicsScene::BackgroundLayer.

Thank you. But I not understand. Do you give me a particular explain or a source code

Follow the link in my post.