NSIndexSet (and its mutable counterpart,
NSMutableIndexSet) represents a sorted collection of unique unsigned integers--think of it like an
NSRange that supports non-contiguous series. It has wicked fast operations to find indexes in ranges or set intersections, and all of the convenience methods you'd expect in a Foundation collection class.
NSIndexSet all throughout the Foundation framework. Anytime a method gets multiple elements from a sorted collection, like an array or a table view, you can be sure that an
NSIndexSet parameter is in the mix.
If you look hard enough, you may start to find aspects of your data model that could be represented with
NSIndexSet. For example AFNetworking uses an index set to represent HTTP response status codes: the user defines a set of "acceptable" codes (in the 2XX range, by default), and the response is checked by using
- List of user preference switches to toggle? You could replace that with a single
- Table view with a dynamic number of pre-defined sections? Same as before:
enum, along with a giant
- Filtering a list of items by a set of composable conditions? Ditch the
NSPredicate; instead, cache the indexes of objects that fulfill each condition, and then get the union of those indexes as conditions are added and removed.
NSIndexSet is a solid class. A fair bit nerdier than its collection class siblings, but it has its place. It's at least a good reminder the useful things that you find by paying attention to what Foundation uses in its own APIs.