// 擴展為現有的類、結構體、枚舉類型、或協議添加了新功能。這也包括了為無訪問權限的源代碼擴展類型的能力（即所謂的逆向建模）。擴展和 Objective-C 中的分類類似。（與 Objective-C 的分類不同的是，Swift 的擴展沒有名字。）

擴展的語法(extension)

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

// 擴展可以使已有的類型遵循一個或多個協議。在這種情況下，協議名的書寫方式與類或結構體完全一樣
extension SomeType: SomeProtocol, AnotherProtocol {
    // implementation of protocol requirements goes here
}

計算屬性

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"

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

初始化器

// 擴展可向已有的類型添加新的初始化器。這允許你擴展其他類型以使初始化器接收你的自定義類型作為形式參數，或提供該類型的原始實現中未包含的額外初始化選項。
// 擴展能為類添加新的便捷初始化器，但是不能為類添加指定初始化器或反初始化器。指定初始化器和反初始化器 必須由原來類的實現提供。 (無需寫 convenience 因為只能添加新的便捷初始化器)
//

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))
// centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0)

方法

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

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

異變實例方法(允許在方法內修改值的方法)

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

下標

extension Int {
    subscript(digitIndex: Int) -> Int {
        var decimalBase = 1
        for _ in 0..<digitIndex {
            decimalBase *= 10
        }
        return (self / decimalBase) % 10
    }
}
746381295[0]
// returns 5
746381295[1]
// returns 9
746381295[2]
// returns 2
746381295[8]
// returns 7

內嵌類型

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("- ", terminator: "")
        case .Zero:
            print("0 ", terminator: "")
        case .Positive:
            print("+ ", terminator: "")
        }
    }
    print("")
}
printIntegerKinds([3, 19, -27, 0, -6, 0, 7])
// Prints "+ + - 0 - 0 +"