Returns an ordered set containing the results of invoking
value using key on each of the ordered set’s objects.
- iOS 5.0+
- macOS 10.7+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
The key to retrieve.
The ordered set of the values for the retrieved key. The returned ordered set might not have the same number of members as the receiver.
The returned ordered set will not contain any elements corresponding to instances of
nil, nor will it contain duplicates.