void MyMainWindow::paintEvent(QPaintEvent *)
{
paint=new QPainter;
paint->begin(this);
paint->setPen(QPen(Qt::blue,4,Qt::DashLine)); //设置画笔形式
paint->setBrush(QBrush(Qt::red,Qt::SolidPattern)); //设置画刷形式
paint->drawEllipse(20,20,100,100);
paint->end();
}