Is there a way to tell when KVO starts/ends for a particular value?

Question

I have some values that are computed over collections, and may or may not be displayed (and thus may or may not have an observer) at any given time. I would rather not have to track all the members of the collection if nobody is observing my computed values.

Can I tell if anyone is currently observing a value, and can I tell when they start observing?

I know for a given object foo I can use [foo observationInfo] to get a list of observers with key paths registered with a root at foo, but that doesn't automatically get all paths TO foo (in fact it only gets ones registered to observe foo's self key).

Answer 1

That’s not a good idea from the design point of view. If you really insist on not updating the contents when nobody needs them (which could be a legitimate case, for example if the updates are expensive), you can introduce methods to start/stop the updates:

- (void) beginUpdatingContents;
- (void) endUpdatingContents;

These should be tied to a counter inside the class and if the counter is > 0, you know somebody wants to keep the contents updated. This solution is explicit and therefore better than silent magic with KVO.