Generics

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Generics

Functions

You can follow along with this section by creating a blank iOS playground in Xcode. Replace the boilerplate with this code:

import UIKit

func swapTwoValues(a: Int, b: Int) -> (a: Int, b: Int) {
  let temp = a
  let newA = b
  let newB = temp
  return (newA, newB)
}

swapTwoValues(a: 42, b: 5)

func swapTwoValues(a: String, b: String) -> (a: String, b: String) {
  let temp = a
  let newA = b
  let newB = temp
  return (newA, newB)
}

swapTwoValues(a: "begin", b: "finish")

// Don't copy this in your playground
func swapTwoValues(a: Any, b: Any) -> (a: Any, b: Any) {
  let temp = a
  let newA = b
  let newB = temp
  return (newA, newB)
}

var name = "Tim Cook"
var phoneNumber = 5141111111
swapTwoValues(a: name, b: phoneNumber)

func swapValues<T>(a: T, b: T) -> (a: T, b: T) {
  let temp = a
  let newA = b
  let newB = temp
  return (newA, newB)
}

swapValues(a: 42, b: 5)
swapValues(a: "begin", b: "finish")

Generic Types

All Swift collections are generic types. For example, this array initializer creates an array containing the elements of a sequence:

init<S>(_ s: S) where Element == S.Element, S : Sequence

protocol Sequence<Element>

Creating a Generic Type

Here’s a generic struct you could use to store and retrieve any type from UserDefaults:

struct Setting<T> {
  let key: String

  var value: T? {
    get {
      UserDefaults.standard.value(forKey: key) as? T
    } set {
      UserDefaults.standard.setValue(newValue, forKey: key)
    }
  }
}

var volume = Setting<Float>(key: "audioVolume")
volume.value = 0.5

let setting: Setting<Int> = .init(key: "myKey")

Extending a Generic Type

You can extend generic types the same way you can extend a normal class, struct, enum, or protocol. In the extension of a generic type, you have access to its type parameters:

extension Setting {
  mutating func save(from untypedValue: Any) {
    if let value = untypedValue as? T {
      self.value = value
    }
  }
}
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