This method allows to explicitly update the instance itself and nested observers. If notifications are disabled a call to this method ensures an update of such nested observers. It should be implemented in derived classes whenever applicable
register with all observables of a given observer. Note that this does not include registering with the observer itself.
This method must be implemented in derived classes. An instance of Observer does not call this method directly: instead, it will be called by the observables the instance registered with when they need to notify any changes.
Object that gets notified when a given observable changes