帮我写一个自定义QScrollArea，可以实现鼠标拖动

好的，下面是一个自定义的 `DraggableScrollArea` 类，它继承自 `QScrollArea`，并实现了鼠标拖动功能：

#include <QMouseEvent>
#include <QScrollBar>
#include <QScrollArea>

class DraggableScrollArea : public QScrollArea
{
public:
    DraggableScrollArea(QWidget *parent = nullptr) : QScrollArea(parent), m_lastPos(QPoint(-1, -1)) {}

protected:
    void mousePressEvent(QMouseEvent *event) override
    {
        if (event->button() == Qt::LeftButton) {
            m_lastPos = event->pos();
            event->accept();
        } else {
            QScrollArea::mousePressEvent(event);
        }
    }

    void mouseMoveEvent(QMouseEvent *event) override
    {
        if (event->buttons() & Qt::LeftButton && m_lastPos != QPoint(-1, -1)) {
            QPoint delta = event->pos() - m_lastPos;
            m_lastPos = event->pos();
            horizontalScrollBar()->setValue(horizontalScrollBar()->value() - delta.x());
            verticalScrollBar()->setValue(verticalScrollBar()->value() - delta.y());
            event->accept();
        } else {
            QScrollArea::mouseMoveEvent(event);
        }
    }

    void mouseReleaseEvent(QMouseEvent *event) override
    {
        if (event->button() == Qt::LeftButton) {
            m_lastPos = QPoint(-1, -1);
            event->accept();
        } else {
            QScrollArea::mouseReleaseEvent(event);
        }
    }

private:
    QPoint m_lastPos;
};

这个自定义的 `DraggableScrollArea` 类重写了鼠标按下、鼠标移动和鼠标释放事件，捕获并处理鼠标拖动事件，实现了鼠标拖动功能。你可以像使用普通的 `QScrollArea` 一样使用这个类，只需要将 `QScrollArea` 替换为 `DraggableScrollArea` 即可。