NSSortDescriptor

NSSortDescriptor is a foundation class used to describe how to sort a collection of objects. It encapsulates a single sorting criterion and can be combined with other descriptors to perform multi-level sorting. The descriptor is immutable and commonly applied to arrays via sorting methods that accept a list of sort descriptors.

Key properties and initializers

- key: the key path used to extract the value from each element, typically a property name or dictionary key. If the key is nil, the object itself is compared.

- ascending: a boolean indicating whether the sort is in ascending or descending order.

- selector: an optional Objective-C selector that specifies how to compare values, allowing custom comparison logic.

- comparator: an optional block or closure that directly compares two values and returns a comparison result.

Usage overview

Sort descriptors are created with a key and an order, and optionally a comparator or selector. They are supplied to sorting methods on arrays, such as those that accept an array of descriptors. When applied, the collection is sorted by the first descriptor, and ties are broken by subsequent descriptors if provided. The key path must be compatible with key-value coding, so dictionaries and custom objects can be sorted by their corresponding values.

Behavior and notes

NSSortDescriptor supports sorting by values retrieved via KVC, and can use a default comparison method if no custom comparator or selector is provided. It is commonly used in Cocoa and Cocoa Touch to implement stable, multi-level sorts for user interfaces and data models. The class conforms to standard Foundation protocols for copying and coding.

See also

Sort descriptors can be combined to form complex sorting rules, and are frequently used with NSArray and NSMutableArray sorting methods. Related concepts include NSSortDescriptor reverse descriptors and the mechanics of key-value coding.