Extension - gpeegpee/learn-swift GitHub Wiki

Extension

Contents

Details

  • Syntax
// Extensions
/*
extension SomeType {
    // new functionality to add to SomeType goes here
}
extension SomeType: SomeProtocol, AnotherProtocol {
    // implementation of protocol requirements goes here
}
*/
  • Computed Properties
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")

let threeFeet = 3.ft
print("Three feet is \(threeFeet) meters")

let aMarathon = 42.km + 195.m
print("A marathon is \(aMarathon) meters long")
//  Extensions can add new computed properties, but they cannot add stored properties, or add property observers to existing properties.
  • Initializers
// Initializers
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()
}
let defaultRect = Rect()
let memberwiseRect = Rect(origin: Point(x: 2.0, y:2.0), size: Size(width: 5.0, height: 5.0))

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)
    }
}
let centerRect = Rect(center: Point(x: 4.0, y: 4.0), size: Size(width: 3.0, height:3.0))
  • method
// Methods
extension Int {
    func repetitions(task: () -> Void) {
        for _ in 0..<self {
            task()
        }
    }
}

3.repetitions {
    print("Hello!")
}

// Mutating Instance Methods
extension Int {
    mutating func square() {
        self = self * self
    }
}

var someInt = 3
someInt.square()
someInt
  • subsripts
// Subscripts
// 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:
extension Int {
    subscript(digitIndex: Int) -> Int {
        var decimalBase = 1
        for _ in 0..<digitIndex {
            decimalBase *= 10
        }
        return (self / decimalBase) % 10
    }
}

746381295[0]
746381295[1]
746381295[2]
746381295[8]

// 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:
746381295[9]
  • Nested Types
// Nested Types
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
        default:
            return .negative
        }
    }
}


func printIntegerKinds(_ numbers: [Int]) {
    for number in numbers {
        switch number.kind {
        case .negative:
            print("-")
            //print("- ", terminator: false)
        case .zero:
            print("0")
            //print("0 ", terminator: false)
        case .positive:
            print("+")
            //print("+ ", terminator: false)
        }
    }
    print("")
}
printIntegerKinds([3, 19, -27, 0, -6, 0, 7])

Reference