filter on a form - how?

[paucer]

I need add some functionality to a form, but only when filtering is enabled.
How can i check this?
Form's datasource has mathod filter(), but it seems this method isn't invoked, when user press filter shortcut button or press CTRL+F3 or filter from context menu. I tried to use prompt() method, but it isn't invoked when filtering is started via context menu. And i need do additional calculation, when the filter is enabled or is being disabled - removeFilter() works ok.
So how can I manage filtering?
At least i must know if fitering is turned on or off.