If the value of the NSKeyValueChangeKindKey entry is NSKeyValueChangeSetting, and NSKeyValueObservingOptionOld was specified when the observer was registered, the value of this key is the value before the attribute was changed.
For NSKeyValueChangeRemoval or NSKeyValueChangeReplacement, if NSKeyValueObservingOptionOld was specified when the observer was registered, the value is an NSArray instance that contains the objects that have been removed or have been replaced by other objects, respectively.
Available in Mac OS X v10.3 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)