qt qgraphicsview 移动圆弧

要在Qt的QGraphicsView中移动圆弧，可以创建一个自定义的QGraphicsItem子类，重写它的paint()和boundingRect()函数，并在其中实现圆弧的绘制和移动逻辑。

以下是一个简单的示例代码，它创建了一个圆弧，并通过鼠标拖动来移动它：

#include <QtWidgets>

class ArcItem : public QGraphicsItem
{
public:
    ArcItem(qreal x, qreal y, qreal width, qreal height, qreal startAngle, qreal spanAngle, QGraphicsItem *parent = nullptr)
        : QGraphicsItem(parent),
          m_rect(x, y, width, height),
          m_startAngle(startAngle),
          m_spanAngle(spanAngle),
          m_mousePressed(false)
    {
        setFlags(ItemIsMovable | ItemSendsGeometryChanges);
    }

    QRectF boundingRect() const override
    {
        return m_rect;
    }

    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override
    {
        painter->drawArc(m_rect, m_startAngle * 16, m_spanAngle * 16);
    }

protected:
    QVariant itemChange(GraphicsItemChange change, const QVariant &value) override
    {
        if (change == ItemPositionChange && scene()) {
            QPointF newPos = value.toPointF();
            QRectF rect = scene()->sceneRect();
            if (!rect.contains(newPos)) {
                // Keep the item inside the scene rect.
                newPos.setX(qMin(rect.right(), qMax(newPos.x(), rect.left())));
                newPos.setY(qMin(rect.bottom(), qMax(newPos.y(), rect.top())));
                return newPos;
            }
        }
        return QGraphicsItem::itemChange(change, value);
    }

    void mousePressEvent(QGraphicsSceneMouseEvent *event) override
    {
        m_mousePressed = true;
        QGraphicsItem::mousePressEvent(event);
    }

    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override
    {
        m_mousePressed = false;
        QGraphicsItem::mouseReleaseEvent(event);
    }

    void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override
    {
        if (m_mousePressed) {
            QPointF delta = event->pos() - event->lastPos();
            setPos(pos() + delta);
        }
        QGraphicsItem::mouseMoveEvent(event);
    }

private:
    QRectF m_rect;
    qreal m_startAngle;
    qreal m_spanAngle;
    bool m_mousePressed;
};

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

    QGraphicsScene scene;
    scene.setSceneRect(-100, -100, 200, 200);

    ArcItem *arc = new ArcItem(-50, -50, 100, 100, 30, 120);
    scene.addItem(arc);

    QGraphicsView view(&scene);
    view.setRenderHint(QPainter::Antialiasing);
    view.setDragMode(QGraphicsView::RubberBandDrag);
    view.show();

    return a.exec();
}

这个示例创建了一个圆弧，定义了它的位置、大小和起始角度、跨越角度。在重写的mousePressEvent()、mouseReleaseEvent()和mouseMoveEvent()函数中，实现了通过鼠标拖动来移动圆弧的逻辑。