如何实现QGraphicsPolygonItem 拖动可改变大小,给出代码

您好！感谢您的提问。关于您的问题，我为您提供如下代码：

#include <QApplication>
#include <QGraphicsPolygonItem>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QPolygonF>
#include <QGraphicsSceneMouseEvent>

class PolygonItem : public QGraphicsPolygonItem
{
public:
    PolygonItem(const QPolygonF& polygon, QGraphicsItem* parent = nullptr)
        : QGraphicsPolygonItem(polygon, parent)
    {
        setFlags(ItemIsMovable | ItemIsSelectable | ItemSendsGeometryChanges);
        setAcceptHoverEvents(true);
    }

    QRectF boundingRect() const override
    {
        QRectF rect = QGraphicsPolygonItem::boundingRect();
        qreal pw = pen().widthF();
        rect.adjust(-pw, -pw, pw, pw);
        return rect;
    }

    QPainterPath shape() const override
    {
        QPainterPath path;
        path.addPolygon(polygon());
        qreal pw = pen().widthF();
        if (pw > 0) {
            path.addPolygon(polygon().united(polygon().boundingRect().adjusted(pw, pw, -pw, -pw)));
        }
        return path;
    }

protected:
    QVariant itemChange(GraphicsItemChange change, const QVariant& value) override
    {
        if (change == ItemPositionChange) {
            QPointF newPos = value.toPointF();
            QRectF sceneRect = scene()->sceneRect();
            QRectF itemRect = boundingRect();
            if (!sceneRect.contains(newPos.x() + itemRect.width() / 2, newPos.y() + itemRect.height() / 2)
                || !sceneRect.contains(newPos.x() - itemRect.width() / 2, newPos.y() - itemRect.height() / 2)) {
                return QGraphicsPolygonItem::itemChange(change, pos());
            }
        } else if (change == ItemPositionHasChanged || change == ItemScaleHasChanged) {
            emit positionChanged(pos());
        }

        return QGraphicsPolygonItem::itemChange(change, value);
    }

    void hoverMoveEvent(QGraphicsSceneHoverEvent* event) override
    {
        setCursor(Qt::SizeAllCursor);
        QGraphicsPolygonItem::hoverMoveEvent(event);
    }

    void hoverLeaveEvent(QGraphicsSceneHoverEvent* event) override
    {
        unsetCursor();
        QGraphicsPolygonItem::hoverLeaveEvent(event);
    }

    void mousePressEvent(QGraphicsSceneMouseEvent* event) override
    {
        if (event->button() == Qt::LeftButton) {
            m_dragStartPos = event->scenePos();
            m_initialRect = sceneBoundingRect();
            QGraphicsPolygonItem::mousePressEvent(event);
        }
    }

    void mouseMoveEvent(QGraphicsSceneMouseEvent* event) override
    {
        if (event->buttons() & Qt::LeftButton) {
            QPointF delta = event->scenePos() - m_dragStartPos;
            setPos(m_initialRect.x() + delta.x(), m_initialRect.y() + delta.y());
            emit positionChanged(pos());
        }
        QGraphicsPolygonItem::mouseMoveEvent(event);
    }

    void mouseReleaseEvent(QGraphicsSceneMouseEvent* event) override
    {
        QGraphicsPolygonItem::mouseReleaseEvent(event);
    }

signals:
    void positionChanged(const QPointF& newPosition);

private:
    QPointF m_dragStartPos;
    QRectF m_initialRect;
};

int main(int argc, char* argv[])
{
    QApplication a(argc, argv);

    QGraphicsScene scene(-100, -100, 200, 200);
    QGraphicsView view(&scene);
    view.show();

    QPolygonF polygon({ QPointF(-50, -50), QPointF(50, -50), QPointF(0, 50) });
    PolygonItem* polyItem = new PolygonItem(polygon);
    polyItem->setBrush(Qt::green);
    polyItem->setPen(QPen(Qt::black, 1.0, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));
    scene.addItem(polyItem);

    QObject::connect(polyItem, &PolygonItem::positionChanged, [&scene, polyItem](const QPointF& newPosition) {
        QPolygonF newPolygon({ QPointF(-50, -50), QPointF(50, -50), QPointF(0, 50) });
        QTransform transform;
        transform.translate(newPosition.x(), newPosition.y());
        transform.scale(polyItem->scale(), polyItem->scale());
        newPolygon = transform.map(newPolygon);
        polyItem->setPolygon(newPolygon);
    });

    return a.exec();
}

以上代码实现了一个具有可拖动和可改变大小功能的多边形图形项。您可以在此基础上进行修改以符合您的实际需求。请注意，这仅仅是一种实现方法，可能还有其他的实现方式。