The code is similar:

// Scene
void CGraphicsScene::drawBackground(QPainter *painter, const QRectF &rect)
{
    painter->drawImage(rect, QImage(":/background/background.jpg"));
}

bool CGraphicsScene::event(QEvent *event)
{
    switch (event->type()){
    case QEvent::GraphicsSceneMousePress : {
        QGraphicsSceneMouseEvent * pMouseEvent = dynamic_cast<QGraphicsSceneMouseEvent*>(event);
        CGraphicsItem *pItem = new CGraphicsItem(pMouseEvent->scenePos().x(), pMouseEvent->scenePos().y());
        this->addItem(pItem);
        pItem->setPos(pMouseEvent->scenePos());
        m_vtItems.push_back(pItem);
        break;
    }
    default:
        break;
    }
    return QGraphicsScene::event(event);
}

// item
QRectF CGraphicsItem::boundingRect() const
{
    qreal penWidth = 1;
    return QRectF((-m_siWidth - penWidth) / 2, (-m_siHeight - penWidth) / 2, m_siWidth + penWidth,
                  m_siHeight + penWidth);
}

void CGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    painter->setRenderHint(QPainter::Antialiasing, true); 
    QPainterPath oEllipsePath;
    QRectF oEllipseRect = { -m_siWidth/2, -m_siHeight/2, m_siWidth, m_siHeight };
    oEllipsePath.addEllipse(oEllipseRect);
    painter->fillPath(oEllipsePath, Qt::black);
}

Running diagram：

Whenever I add an item, QGraphicsScene::drawBackground will be triggered to draw the item area.