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(); 
}