catch event when xprop "change"

[Akbar]

Hi,

I don't know if this the right mailing list to ask, so:
this is the command:
xprop -root _NET_NUMBER_OF_DESKTOPS
xprop -root _NET_CURRENT_DESKTOP
xprop -root _NET_DESKTOP_VIEWPORT
or you can replace with
xprop -root blablabla

I need to find a way to catch the event when return value of these
commands change (c api).

Do you know how?

Thank you.