如何用QPainter + QPixmap实现鼠标绘图？

#include <QtWidgets>

class DrawingWidget : public QWidget
{
public:
    DrawingWidget(QWidget *parent = nullptr) : QWidget(parent)
    {
        setFixedSize(400, 400);
        pixmap = QPixmap(size());
        pixmap.fill(Qt::white);
    }

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

    void mouseMoveEvent(QMouseEvent *event) override
    {
        if (event->buttons() & Qt::LeftButton)
        {
            QPainter painter(&pixmap);
            painter.setPen(Qt::black);
            painter.drawLine(lastPoint, event->pos());
            lastPoint = event->pos();
            update();
        }
    }

    void paintEvent(QPaintEvent *event) override
    {
        QPainter painter(this);
        painter.drawPixmap(rect(), pixmap);
    }

private:
    QPixmap pixmap;
    QPoint lastPoint;
};

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

    DrawingWidget widget;
    widget.show();

    return app.exec();
}