Why doesn't child QGraphicsObject repaint its background correctly - qt

Question

The QGraphicsObject in green rectangle, it is the parent of the QGraphicsObject in red rectangle.
childInRed->setParentItem(this);
When I drag the parent object in green rect and move it fast, the background of the child object in red rect is not repainted correctly.
I know I can use update in the parent's mouseMoveEvent force the child to repaint. But this is not good, because I don't need to repaint the parent at all.
#include "asdf.h"
#include <QtWidgets/QGraphicsScene>
#include <QtWidgets/QGraphicsView>
#include <QtWidgets>
class CTestGraphicsObject : public QGraphicsObject
{
public:
QColor m_c;
CTestGraphicsObject(QColor c)
: QGraphicsObject(NULL)
, m_c(c)
{
setFlag(QGraphicsItem::ItemIsMovable, true);
setFlag(QGraphicsItem::ItemIsFocusable, true);
setFlag(QGraphicsItem::ItemIsSelectable, true);
auto effect = new QGraphicsDropShadowEffect;
effect->setOffset(4, 4);
effect->setBlurRadius(20);
setGraphicsEffect(effect);
}
virtual QRectF boundingRect() const override
{
auto rc = QRectF(0, 0, 100, 100);
return rc;
}
virtual void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override
{
painter->setPen(QPen(m_c));
painter->drawRect(this->boundingRect());
}
};
asdf::asdf(QWidget *parent)
: QMainWindow(parent)
{
ui.setupUi(this);
auto s = new QGraphicsScene(this);
auto v = new QGraphicsView;
v->setScene(s);
CTestGraphicsObject* pParent = new CTestGraphicsObject(Qt::green);
CTestGraphicsObject* pChild = new CTestGraphicsObject(Qt::red);
pChild->setParentItem(pParent);
pChild->setPos(0, 100);
s->addItem(pParent);
s->addItem(pChild);
QVBoxLayout* l = new QVBoxLayout(this->centralWidget());
l->addWidget(v);
}
asdf::~asdf()
{
}
The QGraphicsDropShadowEffect causes this problem, It seems I'm not using it in right way.

Answer 1

According to the Qt documentation, the scene uses the bounding rect and region to define the area to repainted when an item is updated (moved in your case).
If you child is outside its parent, the scene will miss some part when repainting...
Extend the bouding rect/region to cover its children.
If you do something like that, it will work:
virtual QRectF boundingRect() const override
{
if (this->childrenBoundingRect().isEmpty()) // No children
return QRectF(0, 0, 100, 100);
return QRectF(0, 0, 100, 100).united(this->childrenBoundingRect());
}
virtual void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override
{
painter->setPen(QPen(m_c));
painter->drawRect(QRectF(0, 0, 100, 100));
}