How to refresh a Drawingarea?

[Luciano Ponte <l ponte pronet it>]

I have written a small application that draws a line on a drawingarea by 
clicking  on a button.
The problem is that this line remain invisible until I hide the 
drawingarea by another window and then unhide it. How can I force the 
redraw of such a drawing area in order to shows its contents?

Thanks

Luciano