NSArray and its subclass
NSMutable manage ordered collections of objects called arrays.
NSArray creates static arrays, and
NSMutable creates dynamic arrays. You can use arrays when you need an ordered collection of objects.
Creating NSArray Objects Using Array Literals
In addition to the provided initializers, such as
init, you can create an
NSArray object using an array literal.
In Objective-C, the compiler generates code that makes an underlying call to the
You should not terminate the list of objects with
nil when using this literal syntax, and in fact
nil is an invalid value. For more information about object literals in Objective-C, see Working with Objects in Programming with Objective-C.
In Swift, the
NSArray class conforms to the ArrayLiteralConvertible protocol, which allows it to be initialized with array literals. For more information about object literals in Swift, see Literal Expression in The Swift Programming Language (Swift 3.1).
Accessing Values Using Subscripting
In addition to the provided instance methods, such as
object(at:), you can access
NSArray values by their indexes using subscripting.
There is typically little reason to subclass
NSArray. The class does well what it is designed to do—maintain an ordered collection of objects. But there are situations where a custom
NSArray object might come in handy. Here are a few possibilities:
NSArraystores the elements of its collection. You might do this for performance reasons or for better compatibility with legacy code.
Acquiring more information about what is happening to the collection (for example, statistics gathering).
Methods to Override
Any subclass of
NSArray must override the primitive instance methods
object(at:). These methods must operate on the backing store that you provide for the elements of the collection. For this backing store you can use a static array, a standard
NSArray object, or some other data type or mechanism. You may also choose to override, partially or fully, any other
NSArray method for which you want to provide an alternative implementation.
You might want to implement an initializer for your subclass that is suited to the backing store that the subclass is managing. If you do, your initializer must invoke one of the designated initializers of the
NSArray class, either
NSArray class adopts the
NSCoding protocols; custom subclasses of
NSArray should override the methods in these protocols as necessary.
NSArray is the public interface for a class cluster and what this entails for your subclass. You must provide the storage for your subclass and implement the primitive methods that directly act on that storage.
Alternatives to Subclassing
Before making a custom subclass of
NSPointer and the corresponding Core Foundation type, CFArray. Because
CFArray are “toll-free bridged,” you can substitute a
CFArray object for a
NSArray object in your code (with appropriate casting). Although they are corresponding types,
NSArray do not have identical interfaces or implementations, and you can sometimes do things with
CFArray that you cannot easily do with
NSArray. For example,
CFArray provides a set of callbacks, some of which are for implementing custom retain-release behavior. If you specify
NULL implementations for these callbacks, you can easily get a non-retaining array.
If the behavior you want to add supplements that of the existing class, you could write a category on
NSArray. Keep in mind, however, that this category will be in effect for all instances of
NSArray that you use, and this might have unintended consequences. Alternatively, you could use composition to achieve the desired behavior.