Swift 扩展（Extensions译文）

扩展可添加新的函数或是提供默认实现，但是无法覆盖已有的函数

语法

为类型 SomeType 扩展多个协议的实现

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

协议扩展  Adding Protocol Conformance with an Extension

范型扩展 Extending a Generic Type

有条件的通过扩展范型类型添加功能 Extensions with a Generic Where Clause

计算属性

可以给存在类型添加计算实例属性和计算类型属性，下面给内置的 Double 类型添加计算实例属性：

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"

初始化方法

扩展可以给存在的类型添加新的便捷初始化方法，用来使用自定义类型作为已有类型的初始化方法参数，是适配器模式的使用。但无法给类添加指定初始化方法和反初始化方法。详情可参考 Initializer Delegation for Value Types.

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

通过扩展给已有类型添加实例方法和类型方法。下面给 Int 添加实例方法：

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

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

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

下标

可以通过扩展给已有类型添加下标。下面给内置的 Int 添加一个下标，返回对应位置的数，如：

• 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
    }
}
746381295[0]
// returns 5
746381295[1]
// returns 9
746381295[2]
// returns 2
746381295[8]
// returns 7

越界会返回 0 ：

746381295[9]
// returns 0, as if you had requested:
0746381295[9]

嵌套类型

可以通过扩展，给已有到类、结构、枚举，添加新的嵌套类型：

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
        }
    }
}

使用扩展给 Int 添加一个新的嵌套枚举类型 Kind，用来划分整数为三个类别，即正、零和负；添加一个新的计数属性 kind 。

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 + "

资料