signals and windows

[zz excite it]

Hi to all,
I'm facing an apparently easy to solve problem but I'm lost now.

the flow of events I need to manage is this:

1) create a main window
        create a label and fill it  "bla"
        create a button
2) on button push 
        create a modal child window
        manipulate the data of the label  "blabla"
        close the child window
3) automatically refresh the label on the main window with the new data "bla" -> "blabla".

So, my question is:
        on which (if any) signal I can attach a callback to force the actualization of the label
        when I close the modal child window?

I tested a few but without the desired result.

Thanks in advance,
Tito