newKey
If the value of the Kindkey entry is Setting, and New was specified when the observer was registered, the value of this key is the new value for the attribute.
Declaration
static let newKey: NSKeyValueChangeKeyDiscussion
For NSKeyValueChange.insertion or NSKeyValueChange.replacement, if new was specified when the observer was registered, the value for this key is an NSArray instance that contains the objects that have been inserted or replaced other objects, respectively.