onChange -> find WHERE it changed

Hi, I recently started to use onChange() and observe(), and I don't totally get them...

I want to observe my screen, and when it detects a change, move the cursor to that position.

For now the only thing I made is :
#############################
def Fleche_Trouve(event):
            sleep(4)
            print ("ok")

onChange(50,Fleche_Trouve)
observe(2,background=False)
stopObserver()
#############################

It just prints "ok" when something change... but I have to add a "sleep(4)" inside because without it will print multiple "ok"

Thanks