Read-only client interface to receive notifications when a System Wide Property's value is changed.

The implementation is similar to RProperty so the general pitfalls of RProperties applies. I.e. if the value changes several times before being read, only the last set value will be available. And to avoid missing a second notification, subscribtion must be reissued the first thing when you get a change notification, not after the change have been handled.

CSsmSystemWideProperty wraps this class in an active object.

CSsmSystemWideProperty