Animating something by updating across irregular time intervals is not a problem: just calculate how much time has elapsed since the last update, and update using that time factor. If I am animating objects that interact with each other by colliding (in a physically realistic way), I may run into trouble if too many interactions occur in a short period of time, for the time needed to calculate the collisions may be longer than the ideal update interval length. In this case I want to compute the time of the collisions and update the locations over the whole sequence of collisions - but draw nothing until everything in the model is current. I will know when everything is current when I check for the next collision and find that it occurs in the (relatively) distant future. It is then that I will do the drawing (after one final update that coordinates the model with system time). I wouldn't want to wait to draw as I may have already waited to draw while sorting out the collisions.
It seems my answer does lie with CFRunLoopObserver, which is described in a link that was posted earlier:
http://developer.apple.com/library/mac/#documentation/CoreFoundation/Reference/CFRunLoopObserverRef/Reference/reference.html%23//apple_ref/doc/uid/20001442
In particular, the "The Run Loop Sequence of Events" is helpful (I tried to look for this earlier, but didn't know what to search for):
1. Notify observers that the run loop has been entered.
2. Notify observers that any ready timers are about to fire.
3. Notify observers that any input sources that are not port based are about to fire.
4. Fire any non-port-based input sources that are ready to fire.
5. If a port-based input source is ready and waiting to fire, process the event immediately. Go to step 9.
6. Notify observers that the thread is about to sleep.
7. Put the thread to sleep until one of the following events occurs:
* An event arrives for a port-based input source.
* A timer fires.
* The timeout value set for the run loop expires.
* The run loop is explicitly woken up.
8. Notify observers that the thread just woke up.
9. Process the pending event.
* If a user-defined timer fired, process the timer event and restart the loop. Go to step 2.
* If an input source fired, deliver the event.
* If the run loop was explicitly woken up but has not yet timed out, restart the loop. Go to step 2.
10. Notify observers that the run loop has exited.