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Working with Foundation Types

Use bridged Foundation types in your Swift codebase to work with dates, times, and other values.

Overview

When importing the Foundation framework, the Swift overlay provides value types for many bridged reference types. Many other types are renamed or nested to clarify relationships.

Prefer Swift Value Types to Bridged Objective-C Reference Types

The value types in the table below have the same functionality as their corresponding reference types. Class clusters that include immutable and mutable subclasses, like NSArray and NSMutableArray, are bridged to a single value type.

Swift value type

Objective-C reference type

AffineTransform

NSAffineTransform

Array

NSArray

Calendar

NSCalendar

CharacterSet

NSCharacterSet

Data

NSData

DateComponents

NSDateComponents

DateInterval

NSDateInterval

Date

NSDate

Decimal

NSDecimalNumber

Dictionary

NSDictionary

IndexPath

NSIndexPath

IndexSet

NSIndexSet

Locale

NSLocale

Measurement

NSMeasurement

Notification

NSNotification

Swift numeric types (Int, Float, and so on)

NSNumber

PersonNameComponents

NSPersonNameComponents

Set

NSSet

String

NSString

TimeZone

NSTimeZone

URL

NSURL

URLComponents

NSURLComponents

URLQueryItem

NSURLQueryItem

URLRequest

NSURLRequest

UUID

NSUUID

When Swift code imports Objective-C APIs, the importer replaces Foundation reference types with their corresponding value types. For this reason, you should almost never need to use a bridged reference type directly in your own code.

If you need the reference semantics that come with the Foundation reference type, you can access it with its original NS class name prefix. Cast between a Swift value type and its corresponding reference type by using the as keyword.

let dataValue = Data(base64Encoded: myString)
let dataReference = dataValue as NSData?

Note Renamed Reference Types

For Foundation types that aren’t bridged to value types, the Swift overlay renames classes and protocols, as well as related enumerations and constants. These types and protocols drop their NS prefix, with the following exceptions:

  • Classes specific to Objective-C or inherently tied to the Objective-C runtime, like NSObject, NSAutoreleasePool, NSException, and NSProxy

  • Platform-specific classes, like NSBackgroundActivity, NSUserNotification, and NSXPCConnection

Foundation classes often declare enumeration or constant types. When importing these types, Swift moves them to be nested types of their related types. For example, the NSJSONReadingOptions option set is imported as JSONSerialization.ReadingOptions.

See Also

Cocoa Frameworks