qt创建矩形的graphicsitem使得它可以被拖动，被拖拽缩放，被拖动旋转点旋转

可以通过继承 QGraphicsItem 实现这个功能。以下是一个简单的示例：

#include <QGraphicsItem>
#include <QPainter>

class RectangleItem : public QGraphicsItem
{
public:
    RectangleItem(QGraphicsItem *parent = nullptr)
        : QGraphicsItem(parent)
        , m_rect(0, 0, 100, 100)
        , m_rotationPoint(m_rect.center())
    {
        setFlags(ItemIsMovable | ItemIsSelectable | ItemSendsGeometryChanges);
        setAcceptHoverEvents(true);
    }

    QRectF boundingRect() const override
    {
        return m_rect.adjusted(-5, -5, 5, 5);
    }

    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override
    {
        Q_UNUSED(option);
        Q_UNUSED(widget);

        painter->setRenderHint(QPainter::Antialiasing);
        painter->setPen(Qt::black);
        painter->setBrush(Qt::lightGray);
        painter->drawRect(m_rect);

        // 画旋转点
        painter->setBrush(Qt::black);
        painter->drawEllipse(m_rotationPoint, 5, 5);
    }

    QVariant itemChange(GraphicsItemChange change, const QVariant &value) override
    {
        if (change == ItemPositionChange || change == ItemRotationChange) {
            // 更新旋转点的位置
            m_rotationPoint = mapToScene(m_rect.center());
        }
        return QGraphicsItem::itemChange(change, value);
    }

    void hoverMoveEvent(QGraphicsSceneHoverEvent *event) override
    {
        // 鼠标在旋转点上时，更改鼠标形状
        if (QLineF(event->pos(), m_rotationPoint).length() < 10) {
            setCursor(Qt::PointingHandCursor);
        } else {
            setCursor(Qt::ArrowCursor);
        }
    }

    void mousePressEvent(QGraphicsSceneMouseEvent *event) override
    {
        if (QLineF(event->pos(), m_rotationPoint).length() < 10) {
            // 点击旋转点，记录下初始角度
            QPointF center = mapToScene(m_rect.center());
            m_initialAngle = QLineF(center, event->scenePos()).angle();
        } else {
            QGraphicsItem::mousePressEvent(event);
        }
    }

    void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override
    {
        if (event->buttons() & Qt::LeftButton && isSelected()) {
            if (QLineF(event->pos(), m_rotationPoint).length() < 10) {
                // 拖动旋转点，计算角度差并旋转矩形
                QPointF center = mapToScene(m_rect.center());
                qreal angle = QLineF(center, event->scenePos()).angle();
                setRotation(rotation() + angle - m_initialAngle);
                m_initialAngle = angle;
            } else {
                QGraphicsItem::mouseMoveEvent(event);
            }
        }
    }

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

private:
    QRectF m_rect; // 矩形的区域
    QPointF m_rotationPoint; // 旋转点的位置
    qreal m_initialAngle = 0; // 拖拽旋转点时记录下来的初始角度
};

在 `QGraphicsView` 中添加该 item：

QGraphicsScene *scene = new QGraphicsScene(this);
ui->graphicsView->setScene(scene);

RectangleItem *item = new RectangleItem;
scene->addItem(item);

这样就可以通过鼠标拖拽、缩放、旋转点旋转来操作该矩形了。