Retrieve X11 window ID of a just-launched GUI-program

I want to retrieve the X id of GUI programs I launch in background, in order to work on their windows properties. I've been so far using this workaround: myprogram & sleep 1 winID=$(wmctrl -l | awk '/./{line=$0} END{print $1;}') But this relies on three heavy assumptions: - the program will take less than 1 second to get its window opened (not to mention than 1 second can be far too long) - the window will be appened at the end of the wmctrl list - no other window will be opened meanwhile Is there any signal myprogram will send once it has opened a window? How can I intercept it?