用qt实现一个可以用鼠标改变大小，方向和位置的矩形

可以通过继承QWidget或QFrame，并重写paintEvent、mousePressEvent、mouseMoveEvent和mouseReleaseEvent函数，实现一个自定义的矩形控件。

首先，在头文件中定义一个类，例如MyRect：

class MyRect : public QFrame
{
    Q_OBJECT

public:
    explicit MyRect(QWidget *parent = nullptr);

protected:
    void paintEvent(QPaintEvent *event) override;
    void mousePressEvent(QMouseEvent *event) override;
    void mouseMoveEvent(QMouseEvent *event) override;
    void mouseReleaseEvent(QMouseEvent *event) override;

private:
    bool m_isResizing;
    bool m_isMoving;
    QPoint m_dragStart;
    QRect m_rect;
};

在构造函数中初始化一些变量：

MyRect::MyRect(QWidget *parent)
    : QFrame(parent)
    , m_isResizing(false)
    , m_isMoving(false)
{
    setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    setCursor(Qt::SizeAllCursor);
    setMouseTracking(true);
}

重写paintEvent函数，绘制矩形：

void MyRect::paintEvent(QPaintEvent *event)
{
    Q_UNUSED(event);

    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);

    painter.setPen(QPen(Qt::black, 2));
    painter.setBrush(QBrush(Qt::lightGray));

    painter.drawRect(m_rect);
}

重写mousePressEvent函数，检查鼠标是否在矩形边框内或矩形内部，以及是否在矩形边框上进行大小调整：

void MyRect::mousePressEvent(QMouseEvent *event)
{
    if (event->button() == Qt::LeftButton) {
        if (m_rect.contains(event->pos())) {
            m_isMoving = true;
            m_dragStart = event->pos() - m_rect.topLeft();
        } else {
            QList<QRect> resizeRects = {
                QRect(m_rect.left() - 2, m_rect.top() - 2, 4, 4),
                QRect(m_rect.right() - 2, m_rect.top() - 2, 4, 4),
                QRect(m_rect.left() - 2, m_rect.bottom() - 2, 4, 4),
                QRect(m_rect.right() - 2, m_rect.bottom() - 2, 4, 4)
            };

            foreach (const QRect &resizeRect, resizeRects) {
                if (resizeRect.contains(event->pos())) {
                    m_isResizing = true;
                    m_dragStart = event->pos();
                    break;
                }
            }
        }
    }
}

重写mouseMoveEvent函数，实现矩形大小调整和位置移动：

void MyRect::mouseMoveEvent(QMouseEvent *event)
{
    if (m_isMoving) {
        m_rect.moveTopLeft(event->pos() - m_dragStart);
        update();
    } else if (m_isResizing) {
        int dx = event->pos().x() - m_dragStart.x();
        int dy = event->pos().y() - m_dragStart.y();

        if (m_dragStart.x() == m_rect.left()) {
            m_rect.setLeft(m_rect.left() + dx);
        } else {
            m_rect.setRight(m_rect.right() + dx);
        }

        if (m_dragStart.y() == m_rect.top()) {
            m_rect.setTop(m_rect.top() + dy);
        } else {
            m_rect.setBottom(m_rect.bottom() + dy);
        }

        m_dragStart = event->pos();
        update();
    } else {
        QList<QRect> resizeRects = {
            QRect(m_rect.left() - 2, m_rect.top() - 2, 4, 4),
            QRect(m_rect.right() - 2, m_rect.top() - 2, 4, 4),
            QRect(m_rect.left() - 2, m_rect.bottom() - 2, 4, 4),
            QRect(m_rect.right() - 2, m_rect.bottom() - 2, 4, 4)
        };

        bool cursorChanged = false;

        foreach (const QRect &resizeRect, resizeRects) {
            if (resizeRect.contains(event->pos())) {
                setCursor(getResizeCursor(resizeRect));
                cursorChanged = true;
                break;
            }
        }

        if (!cursorChanged) {
            setCursor(Qt::SizeAllCursor);
        }
    }
}

重写mouseReleaseEvent函数，重置一些变量：

void MyRect::mouseReleaseEvent(QMouseEvent *event)
{
    Q_UNUSED(event);

    m_isResizing = false;
    m_isMoving = false;
}

getResizeCursor函数用于根据鼠标位置返回正确的光标类型：

Qt::CursorShape MyRect::getResizeCursor(const QRect &rect) const
{
    if (rect.top() == m_rect.top()) {
        if (rect.left() == m_rect.left()) {
            return Qt::SizeFDiagCursor;
        } else {
            return Qt::SizeBDiagCursor;
        }
    } else {
        if (rect.left() == m_rect.left()) {
            return Qt::SizeBDiagCursor;
        } else {
            return Qt::SizeFDiagCursor;
        }
    }
}

最后，在主窗口中使用MyRect控件即可：

MyRect *rect = new MyRect(this);
rect->setGeometry(50, 50, 100, 100);

这样就可以通过鼠标改变矩形的大小、方向和位置了。