If the value of the kindKey entry is NSKeyValueChange.insertion, NSKeyValueChange.removal, or NSKeyValueChange.replacement, the value of this key is an NSIndexSet object that contains the indexes of the inserted, removed, or replaced objects.
If the value of the kindKey entry is NSKeyValueChange.setting, and new was specified when the observer was registered, the value of this key is the new value for the attribute.
If the value of the kindKey entry is NSKeyValueChange.setting, and old was specified when the observer was registered, the value of this key is the value before the attribute was changed.