Is there a window notification mechanism in gnome?

[Kala B <kalabalun gmail com>]

Hi,

Do all GNOME applications set the _NET_WM_PID property on their windows? 
If not, how is the relation between a window and its owning process
established?

Is there a library wherein I could get callbacks for window open
events? ( Along with the pid ).

Thanks & Regards
Kala.