qgraphicsitem选中出现边框可以拉伸的代码例程

QGraphicsItem是Qt中的一个重要的图形项类，用于在QGraphicsScene中显示图形元素。要实现选中QGraphicsItem后出现可拉伸的边框，可以通过以下代码示例实现：

#include <QGraphicsItem>
#include <QPainter>
#include <QStyleOptionGraphicsItem>
#include <QKeyEvent>

class ResizableItem : public QGraphicsItem
{
public:
    ResizableItem(QGraphicsItem *parent = nullptr) : QGraphicsItem(parent),
        m_isSelected(false), m_borderColor(Qt::red), m_borderWidth(2), m_resizeHandleSize(6)
    {
        setFlags(QGraphicsItem::ItemIsSelectable);
    }

    QRectF boundingRect() const override
    {
        return QRectF(0, 0, 100, 100);
    }

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

        painter->setRenderHint(QPainter::Antialiasing);

        // 绘制选中时的边框
        if (m_isSelected) {
            painter->setPen(QPen(m_borderColor, m_borderWidth));
            painter->setBrush(Qt::NoBrush);
            painter->drawRoundedRect(boundingRect(), 5, 5);
            
            // 绘制调整大小的手柄
            painter->setBrush(Qt::SolidPattern);
            painter->setPen(Qt::NoPen);
            painter->drawRect(boundingRect().topRight().x() - m_resizeHandleSize / 2,
                              boundingRect().topRight().y() - m_resizeHandleSize / 2,
                              m_resizeHandleSize, m_resizeHandleSize);
        }

        // 绘制图形元素内容
        painter->setPen(Qt::black);
        painter->setBrush(QColor(192, 192, 192));
        painter->drawRoundedRect(0, 0, 100, 100, 5, 5);
    }

    void setSelected(bool selected)
    {
        m_isSelected = selected;
        update();
    }

protected:
    void keyPressEvent(QKeyEvent *event) override
    {
        if (event->key() == Qt::Key_Delete) {
            scene()->removeItem(this);
            delete this;
        }
    }

    void mousePressEvent(QGraphicsSceneMouseEvent *event) override
    {
        // 仅在点击图形元素时选中
        if (event->button() == Qt::LeftButton) {
            setSelected(true);
            event->accept();
        } else {
            setSelected(false);
            event->ignore();
        }
    }

private:
    bool m_isSelected;
    QColor m_borderColor;
    int m_borderWidth;
    int m_resizeHandleSize;
};

以上示例代码定义了一个基于QGraphicsItem的可拉伸项（ResizableItem），并重写了paint()函数以绘制图形和选中边框，重写了mousePressEvent()函数以实现点击图形元素时选中的功能，以及重写了keyPressEvent()函数以实现删除选中项的功能。
你可以将这段代码插入进自己的Qt项目中，然后在场景中添加ResizableItem即可实现选中时出现可拉伸的边框的效果。