oldKey
If the value of the Kindkey entry is Setting, and Old was specified when the observer was registered, the value of this key is the value before the attribute was changed.
Declaration
static let oldKey: NSKeyValueChangeKeyDiscussion
For NSKeyValueChange.removal or NSKeyValueChange.replacement, if old 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the observer was registered, the value is an NSArray instance that contains the objects that have been removed or have been replaced by other objects, respectively.