用qgraphicsitem实现一个可用用鼠标调整大小，方向和位置的矩形

可以继承QGraphicsItem类，实现自己的可调整大小、方向和位置的矩形类。具体步骤如下：

1. 定义矩形类，继承QGraphicsItem类，重写boundingRect()和paint()函数。

class ResizableRectItem : public QGraphicsItem {
public:
    ResizableRectItem(QGraphicsItem *parent = nullptr);
    QRectF boundingRect() const override;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override;
};

2. 实现boundingRect()函数，返回矩形的边界矩形。

QRectF ResizableRectItem::boundingRect() const {
    return QRectF(-50, -50, 100, 100);
}

3. 实现paint()函数，绘制矩形和调整手柄。

void ResizableRectItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) {
    // 绘制矩形
    painter->drawRect(-50, -50, 100, 100);

    // 绘制调整手柄
    painter->setBrush(Qt::white);
    painter->drawEllipse(-55, -55, 10, 10);
    painter->drawEllipse(45, -55, 10, 10);
    painter->drawEllipse(-55, 45, 10, 10);
    painter->drawEllipse(45, 45, 10, 10);
}

4. 实现鼠标事件函数，处理鼠标按下、移动和释放事件，实现矩形的调整。

void ResizableRectItem::mousePressEvent(QGraphicsSceneMouseEvent *event) {
    if (event->button() == Qt::LeftButton) {
        // 记录鼠标按下的位置
        m_lastPos = event->pos();
    }
}

void ResizableRectItem::mouseMoveEvent(QGraphicsSceneMouseEvent *event) {
    // 计算鼠标移动的距离
    QPointF delta = event->pos() - m_lastPos;

    // 根据调整手柄的位置，调整矩形
    if (event->buttons() & Qt::LeftButton) {
        if (QRectF(-55, -55, 10, 10).contains(delta)) {
            // 调整左上角
            setPos(pos() + delta);
            QRectF rect = boundingRect();
            setRect(rect.x() + delta.x(), rect.y() + delta.y(), rect.width() - delta.x(), rect.height() - delta.y());
        } else if (QRectF(45, -55, 10, 10).contains(delta)) {
            // 调整右上角
            setPos(pos() + QPointF(0, delta.y()));
            QRectF rect = boundingRect();
            setRect(rect.x(), rect.y() + delta.y(), rect.width() + delta.x(), rect.height() - delta.y());
        } else if (QRectF(-55, 45, 10, 10).contains(delta)) {
            // 调整左下角
            setPos(pos() + QPointF(delta.x(), 0));
            QRectF rect = boundingRect();
            setRect(rect.x() + delta.x(), rect.y(), rect.width() - delta.x(), rect.height() + delta.y());
        } else if (QRectF(45, 45, 10, 10).contains(delta)) {
            // 调整右下角
            QRectF rect = boundingRect();
            setRect(rect.x(), rect.y(), rect.width() + delta.x(), rect.height() + delta.y());
        } else {
            // 移动矩形
            setPos(pos() + delta);
        }

        // 更新鼠标位置
        m_lastPos = event->pos();
    }
}

void ResizableRectItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event) {
    Q_UNUSED(event);
}

完整代码如下：

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsItem>
#include <QPainter>

class ResizableRectItem : public QGraphicsItem {
public:
    ResizableRectItem(QGraphicsItem *parent = nullptr);
    QRectF boundingRect() const override;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override;
    void mousePressEvent(QGraphicsSceneMouseEvent *event) override;
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override;
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override;

private:
    QPointF m_lastPos;
};

ResizableRectItem::ResizableRectItem(QGraphicsItem *parent)
    : QGraphicsItem(parent)
{
    setFlag(QGraphicsItem::ItemIsMovable, true);
    setFlag(QGraphicsItem::ItemIsSelectable, true);
}

QRectF ResizableRectItem::boundingRect() const {
    return QRectF(-50, -50, 100, 100);
}

void ResizableRectItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) {
    // 绘制矩形
    painter->drawRect(-50, -50, 100, 100);

    // 绘制调整手柄
    painter->setBrush(Qt::white);
    painter->drawEllipse(-55, -55, 10, 10);
    painter->drawEllipse(45, -55, 10, 10);
    painter->drawEllipse(-55, 45, 10, 10);
    painter->drawEllipse(45, 45, 10, 10);
}

void ResizableRectItem::mousePressEvent(QGraphicsSceneMouseEvent *event) {
    if (event->button() == Qt::LeftButton) {
        // 记录鼠标按下的位置
        m_lastPos = event->pos();
    }
}

void ResizableRectItem::mouseMoveEvent(QGraphicsSceneMouseEvent *event) {
    // 计算鼠标移动的距离
    QPointF delta = event->pos() - m_lastPos;

    // 根据调整手柄的位置，调整矩形
    if (event->buttons() & Qt::LeftButton) {
        if (QRectF(-55, -55, 10, 10).contains(delta)) {
            // 调整左上角
            setPos(pos() + delta);
            QRectF rect = boundingRect();
            setRect(rect.x() + delta.x(), rect.y() + delta.y(), rect.width() - delta.x(), rect.height() - delta.y());
        } else if (QRectF(45, -55, 10, 10).contains(delta)) {
            // 调整右上角
            setPos(pos() + QPointF(0, delta.y()));
            QRectF rect = boundingRect();
            setRect(rect.x(), rect.y() + delta.y(), rect.width() + delta.x(), rect.height() - delta.y());
        } else if (QRectF(-55, 45, 10, 10).contains(delta)) {
            // 调整左下角
            setPos(pos() + QPointF(delta.x(), 0));
            QRectF rect = boundingRect();
            setRect(rect.x() + delta.x(), rect.y(), rect.width() - delta.x(), rect.height() + delta.y());
        } else if (QRectF(45, 45, 10, 10).contains(delta)) {
            // 调整右下角
            QRectF rect = boundingRect();
            setRect(rect.x(), rect.y(), rect.width() + delta.x(), rect.height() + delta.y());
        } else {
            // 移动矩形
            setPos(pos() + delta);
        }

        // 更新鼠标位置
        m_lastPos = event->pos();
    }
}

void ResizableRectItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event) {
    Q_UNUSED(event);
}

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

    QGraphicsScene scene;
    QGraphicsView view(&scene);

    ResizableRectItem *rect = new ResizableRectItem;
    scene.addItem(rect);

    view.show();

    return a.exec();
}