How can I set the display range of a QGraphicItemGroup?

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.