why doesn't the button work?

[mattia]

First, you are mixing aproaches.
Either use the 'ui' as member OR derive it.
I recommend using it as member - more flexible.

I'd like to know how's the derive approach, are there any examples on the net?
Thanks

[high_flyer]

Its all in the docs.
Note that you can use the single or multiple inheritance approaches, see what best fits your case.