[Qt-interest] How can I make a Page Turning by mouse drag?

[Kermit Mei]

Hello, I want to create a stackedWidget, if I press the mouse on it, and
drag for a given distance, then the stackedWidget will change it current
widget. 

Then how can I implement it? 
In 
void QWidget::mouseMoveEvent ( QMouseEvent * event );
I don't know how to get the distance that I moved by mouse.

Thanks.