QT5双缓冲绘图简单代码

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);
    pix = new QPixmap(640, 480);
}

void Widget::mousePressEvent(QMouseEvent *e)
{
    m_startPos = e->pos();
}

void Widget::mouseMoveEvent(QMouseEvent *e)
{
    QPainter *painter = new QPainter;

    QPen pen;
   // pen.setStyle((Qt::PenStyle)style);
    pen.setWidth(2);
    QColor color(Qt::red);
    pen.setColor(color);

    painter->begin(pix);
    painter->setPen(pen);
    painter->drawLine(m_startPos,e->pos());
    painter->end();
    m_startPos =e->pos();
    update();
}

void Widget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
    painter.drawPixmap(QPoint(0,0),*pix);
}