使QGraphicsLineItem拖拽起来不那么麻烦

class DraggableLineItem : public QGraphicsLineItem
{
public:
    DraggableLineItem(QGraphicsItem* parent = nullptr) : QGraphicsLineItem(parent)
    {
        setFlag(QGraphicsItem::ItemIsMovable); // 允许拖拽移动
        setFlag(QGraphicsItem::ItemSendsGeometryChanges); // 发送几何变化信号
    }

    QVariant itemChange(GraphicsItemChange change, const QVariant& value) override
    {
        if (change == QGraphicsItem::ItemPositionChange && scene()) {
            // 限制拖拽范围在场景内
            QPointF newPos = value.toPointF();
            QRectF sceneRect = scene()->sceneRect();
            qreal x = qMin(qMax(newPos.x(), sceneRect.left()), sceneRect.right());
            qreal y = qMin(qMax(newPos.y(), sceneRect.top()), sceneRect.bottom());
            return QPointF(x, y);
        }
        return QGraphicsItem::itemChange(change, value);
    }
};

QGraphicsScene scene;
DraggableLineItem* lineItem = new DraggableLineItem();
scene.addItem(lineItem);