swift - 扩展

// 扩展为现有的类、结构体、枚举类型、或协议添加了新功能。这也包括了为无访问权限的源代码扩展类型的能力（即所谓的逆向建模）。扩展和 Objective-C 中的分类类似。（与 Objective-C 的分类不同的是，Swift 的扩展没有名字。）

扩展的语法(extension)

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

// 扩展可以使已有的类型遵循一个或多个协议。在这种情况下，协议名的书写方式与类或结构体完全一样
extension SomeType: SomeProtocol, AnotherProtocol {
    // implementation of protocol requirements goes here
}

计算属性

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"

初始化器

// 扩展可向已有的类型添加新的初始化器。这允许你扩展其他类型以使初始化器接收你的自定义类型作为形式参数，或提供该类型的原始实现中未包含的额外初始化选项。
// 扩展能为类添加新的便捷初始化器，但是不能为类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 必须由原来类的实现提供。 (无需写 convenience 因为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!

异变实例方法(允许在方法内修改值的方法)

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

下标

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

内嵌类型

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
        }
    }
}

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 +"