重新实现QToolBar的paintEvent(QPaintEvent* e)
class MyToolBar : public QToolBar
{
  QObject
 public : // constructor and destructor 
public slots:
      void setTip(const QString& strTip)
      {
         m_strTip = strTip ;
         update();
      }
protected:
    void paintEvent(QPaintEvent* e); // reimplementing it 
private:
   QString m_strTip;
};
void MyToolBar::paintEvent(QPaintEvent* e)
{
        QPainter painter(this);
       qreal textWidth = fontMetrics().width(m_strTip);
       qreal textHeight =fontMetrics().height();
       qreal thisHeight= height();
       qreal topSpace = (thisHeight-textHeight)/2;
       QPointF textPot(width()-textWidth,topSpace+textHeight);
       painter.drawText(textPot,m_strTip);
  
      // 
      QToolBar::paintEvent(e);
}