How can I set the display range of a QGraphicItemGroup? - c++

Question

I have a QGraphicsItemGroup aggregating several child items, and I want to show only part of the group.(not the numbers of child items, area). Just like the image here.
I want to show the display area.
To do that, I have tried the override the QGraphicsItemGroup::boundingRect(). However, nothing have happened. And i find this in QT docs, maybe this is the reason why doesn't work.
The boundingRect() function of QGraphicsItemGroup returns the bounding rectangle of all items in the item group.
Also, I know I can change the size of QGraphicsView to make it work. However I put the View as CentralWidget, as I also need to display other object in the View, I can not change the size of the View.
How can I set the display range of a QGraphicItemGroup?

Answer 1

To perform this task we can overwrite shape() by returning a QPainterPath that defines the visible region, so that it spreads to its children we enable the flag ItemClipsChildrenToShape:
class GraphicsItemGroup: public QGraphicsItemGroup{
public:
GraphicsItemGroup(QGraphicsItem * parent = 0):QGraphicsItemGroup(parent){
setFlag(QGraphicsItem::ItemClipsChildrenToShape, true);
}
QPainterPath shape() const
{
if(mShape.isEmpty())
return QGraphicsItemGroup::shape();
return mShape;
}
void setShape(const QPainterPath &shape){
mShape = shape;
update();
}
private:
QPainterPath mShape;
};
Example:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QWidget w;
w.setLayout(new QVBoxLayout);
QGraphicsView view;
QPushButton button("click me");
w.layout()->addWidget(&view);
w.layout()->addWidget(&button);
view.setScene(new QGraphicsScene);
GraphicsItemGroup group;
view.scene()->addItem(&group);
auto ellipse = new QGraphicsEllipseItem(QRectF(0, 0, 100, 100));
ellipse->setBrush(Qt::red);
auto rect = new QGraphicsRectItem(QRect(150, 150, 100, 100));
rect->setBrush(Qt::blue);
group.addToGroup(ellipse);
group.addToGroup(rect);
QObject::connect(&button, &QPushButton::clicked, [&group](){
QPainterPath shape;
if(group.shape().boundingRect() == group.boundingRect()){
shape.addRect(0, 50, 250, 150);
}
group.setShape(shape);
});
w.show();
return a.exec();
}
Output:
The complete example can be found in the following link.