If the value of NSKeyValueObservingOptionPrior was specified at observer registration time, and this notification is sent prior to a change as a result.
The change dictionary contains an NSKeyValueChangeNotificationIsPriorKey entry whose value is an NSNumber wrapping YES.
Available in Mac OS X v10.5 and later.
Declaration Syntax Version Information- Available in Monobjc Bridge: 10.6 (For Mac OS X 10.6 and later), 10.5 (For Mac OS X 10.5 and later)