QWidget继承自QPaintDevice，这样就可以直接把QWidget传入QPainter的构造函数，比如QPainter(mylabel)，然后设置QWidget的长宽后直接进行作画了

比如用QLabel在主界面上画两条虚线：

bool ContentWidget::eventFilter(QObject *obj, QEvent *event)
{
 if(obj == line_label_1 || obj == line_label_2)
 {
  if(event->type() == QEvent::Paint)
  {
   int label_height_1 = line_label_1->height();
   int label_width_1 = line_label_1->width();
   QPainter painter(line_label_1);
   painter.setPen(QPen(QColor(220, 220, 220), 1, Qt::DashLine));
   painter.drawLine(label_width_1/2, 0, label_width_1/2, label_height_1); 
   int label_height_2 = line_label_2->height();
   int label_width_2 = line_label_2->width();
   QPainter painter2(line_label_2);
   painter2.setPen(QPen(QColor(220, 220, 220), 1, Qt::DashLine));
   painter2.drawLine(label_width_2/2, 0, label_width_2/2, label_height_2);
  }
 }
 return QWidget::eventFilter(obj, event);
}

http://blog.sina.com.cn/s/blog_a6fb6cc90101e8lv.html