Extensions Swift


익스텐션은 주어진 클래스, 구조체, 열거형, 또는 프로토콜 형식에 기능을 추가하는 것이다. 원본 소스코드에 접근할 수 없는 상황에서도 형식을 확장할 수 있다. (retroactive modeling). 익스텐션은 오브젝티브C의 카테고리와 유사하다. (오브젝티브 C의 카테고리와 달리 스위프트 익스텐션은 이름을 갖지 않는다.)

스위프트의 익스텐션은 다음을 할 수 있다.

- 계산된 인스턴스 프로퍼티와 계산된 형식 프로퍼티를 추가할 수 있다.
- 인스턴스 메소드와 타입 메소드를 정의한다
- 새로운 이니셜라이져를 제공한다.
- 서브스크립트를 정의한다
- 새로운 네스트드 형식을 정의하고 사용한다
- 주어진 형식이 프로토콜에 연결되게 한다

In Swift, you can even extend a protocol to provide implementations of its requirements or add additional functionality that conforming types can take advantage of. For more details, see Protocol Extensions.


Extensions can add new functionality to a type, but they cannot override existing functionality.

익스텐션 문법

extension 키워드를 사용해 익스센션을 지정한다.

extension SomeType {
  // new functionality to add to SomeType goes here

익스텐션은 한 개이상의 프로토콜을 확장하도록 주어진 형식을 확장할 수 있다.

extension SomeType: SomeProtocol, AnotherProtocol {
  // implementation of protocol requirements goes here

Adding protocol conformance in this way is described in Adding Protocol Conformance with an Extension.

An extension can be used to extend an existing generic type, as described in Extending a Generic Type. You can also extend a generic type to conditionally add functionality, as described in Extensions with a Generic Where Clause.


If you define an extension to add new functionality to an existing type, the new functionality will be available on all existing instances of that type, even if they were created before the extension was defined.

계산된 프로퍼티

익스텐션은 기존 형식에 계산된 인스턴스 프로퍼티와 계산된 타입 프로퍼티를 추가할 수 있다. This example adds five computed instance properties to Swift’s built-in Double type, to provide basic support for working with distance units:

extension Double {
  var km: Double { return self*1_000.0}
  var m: Double ( return self }
  var cm: Double { return self / 100.0 }
  var mm: Double { return self / 1_000.0 }
  var ft: Double { return self / 3.28084 }

let oneInch = 25.4.mm
print("One inch is \(oneInch) meters")
// Prints "One inch is 0.0254 meters"
let threeFeet = 3.ft
print("Three feet is \(threeFeet) meters")
// Prints "Three feet is 0.914399970739201 meters"

These computed properties express that a Double value should be considered as a certain unit of length. Although they are implemented as computed properties, the names of these properties can be appended to a floating-point literal value with dot syntax, as a way to use that literal value to perform distance conversions.

In this example, a Double value of 1.0 is considered to represent “one meter”. This is why the m computed property returns self—the expression 1.m is considered to calculate a Double value of 1.0.

Other units require some conversion to be expressed as a value measured in meters. One kilometer is the same as 1,000 meters, so the km computed property multiplies the value by 1_000.00 to convert into a number expressed in meters. Similarly, there are 3.28084 feet in a meter, and so the ft computed property divides the underlying Double value by 3.28084, to convert it from feet to meters.

These properties are read-only computed properties, and so they are expressed without the get keyword, for brevity. Their return value is of type Double, and can be used within mathematical calculations wherever a Double is accepted:

let aMarathon = 42.km + 195.m
print("A marathon is \(aMarathon) meters long")
// Prints "A marathon is 42195.0 meters long"

Extensions can add new computed properties, but they cannot add stored properties, or add property observers to existing properties.


익스텐션은 주어진 형식에 새로운 이니셜라이져를 추가할 수 있다. 이 것은 주어진 형식을 확장하여 자체적인 형식으로 이니셜라이져 파라미터로 사용할 수 있게 하든지, 추가적인 이니셜라이제이션 옵션을 제공할 수 있도록 한다.

익스텐션은 클래스에 새로운 편의 이니셜라이져를 추가할 수 있지만 새로운 지정된 이니셜라이져 나 디이니셜라이져는 추가살 수 없다. 지정된 이니셜라이져와 디이니셜라이져는 반드시 원본 클래스 구현에 의해 제공되어야 한다.

If you use an extension to add an initializer to a value type that provides default values for all of its stored properties and does not define any custom initializers, you can call the default initializer and memberwise initializer for that value type from within your extension’s initializer. This wouldn’t be the case if you had written the initializer as part of the value type’s original implementation, as described in Initializer Delegation for Value Types.

If you use an extension to add an initializer to a structure that was declared in another module, the new initializer can’t access self until it calls an initializer from the defining module.

The example below defines a custom Rect structure to represent a geometric rectangle. The example also defines two supporting structures called Size and Point, both of which provide default values of 0.0 for all of their properties:

struct Size {
    var width = 0.0, height = 0.0
struct Point {
    var x = 0.0, y = 0.0
struct Rect {
    var origin = Point()
    var size = Size()

Because the Rect structure provides default values for all of its properties, it receives a default initializer and a memberwise initializer automatically, as described in Default Initializers. These initializers can be used to create new Rect instances:

let defaultRect = Rect()
let memberwiseRect = Rect(origin: Point(x: 2.0, y: 2.0),
   size: Size(width: 5.0, height: 5.0))

You can extend the Rect structure to provide an additional initializer that takes a specific center point and size:

extension Rect {
  init(center: Point, size: Size) {
    let originX = center.x - (size.width / 2)
    let originY = center.y - (size.height / 2)
    self.init(origin: Point(x:originX, y:originY), size: size)

This new initializer starts by calculating an appropriate origin point based on the provided center point and size value. The initializer then calls the structure’s automatic memberwise initializer init(origin:size:), which stores the new origin and size values in the appropriate properties:

let centerRect = Rect(center: Point(x:4.0,y:4.0), size: Size(width: 3.0, height: 3.0))
// centerRect's origin is (2.5, 2.5) and its size is (3.0,3.0)

익스텐션으로 새로운 이니셜라이져를 제공하면 각 인스턴스가 이니셜라이져 완료한번에 완전히 초기화되게 하는 책임을 가진다.


익스텐션은 주어진 형식에 새로운 인스턴스 메소드와 타입 메소드를 추가할 수 있다. 다음 예시는 repetitions를 Int 형식에 추가함을 보여준다.

extension Int {
  func repetitions(task: ()->Void) {
    for _ in 0..<self {

The repetitions(task:) method takes a single argument of type () -> Void, which indicates a function that has no parameters and does not return a value.

After defining this extension, you can call the repetitions(task:) method on any integer to perform a task that many number of times:

3.repetitions {
// Hello!
// Hello!
// Hello!

뮤테이팅 인스턴스 메소드

익스텐션으로 추가될 인스턴스 메소드는 인스턴스 자체도 변경할 수 있다. 자신을 변경할 메소드나 프로퍼티는 원본 구현에서처럼 mutating 으로 마크되어야 한다.

The example below adds a new mutating method called square to Swift’s Int type, which squares the original value:

extension Int {
    mutating func square() {
        self = self * self
var someInt = 3
// someInt is now 9


Extensions can add new subscripts to an existing type. This example adds an integer subscript to Swift’s built-in Int type. This subscript [n] returns the decimal digit n places in from the right of the number:

123456789[0] returns 9
123456789[1] returns 8
…and so on:

extension Int {
  subscript(digitIndex: Int) -> Int {
    var decimalBase = 1
    for _ in 0..<digitIndex {
      decimalBase *= 10
    return (self / decimalBase) % 10
// returns 5
// returns 9
// returns 2
// returns 7

If the Int value does not have enough digits for the requested index, the subscript implementation returns 0, as if the number had been padded with zeros to the left:

// returns 0, as if you had requested:

네스트 형식

Extensions can add new nested types to existing classes, structures, and enumerations:

extension Int {
  enum Kind {
    case negative, zero, positive
  var kind: Kind {
    switch self {
    case 0:
      return .zero
    case let x where x > 0:
      return .positive
      return .negative

This example adds a new nested enumeration to Int. This enumeration, called Kind, expresses the kind of number that a particular integer represents. Specifically, it expresses whether the number is negative, zero, or positive.

This example also adds a new computed instance property to Int, called kind, which returns the appropriate Kind enumeration case for that integer.

The nested enumeration can now be used with any Int value:

func printIntegerKinds(_ numbers: [Int]) {
  for number in numbers {
    switch number.kind {
    case .negative:
      print("- ", terminator: "")
    case .zero:
      print("0 ", terminator: "")
    case .positive:
      print("+ ", terminator: "")
printIntegerKinds([3, 19, -27, 0, -6, 0, 7])
// Prints "+ + - 0 - 0 + "
This function, printIntegerKinds(_:), takes an input array of Int values and iterates over those values in turn. For each integer in the array, the function considers the kind computed property for that integer, and prints an appropriate description.


number.kind is already known to be of type Int.Kind. Because of this, all of the Int.Kind case values can be written in shorthand form inside the switch statement, such as .negative rather than Int.Kind.negative.


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