Swift扩展（Extension）

在现有类和结构体的类型基础上，扩展新的功能。 语法：

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extension SomeType{

// new functionality to add to SomeType goes here

}

An extension can extend an existing type to make itadopt one or more protocols.Where this is the case,

the protocol names are written in exactly the sameway as for a class or structure:

extension SomeType: SomeProtocol,AnotherProtocol {

// implementation of protocol requirements goes here

}

计算属性

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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

println("One inch is \(oneInch) meters")

// prints "One inch is 0.0254 meters"

let threeFeet =3.ft

println("Three feet is \(threeFeet) meters")

// prints "Three feet is0.914399970739201 meters"

let aMarathon = 42.km + 195.m

println("Amarathon is \(aMarathon) meters long")

使用构造器

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struct Size {

varwidth = 0.0, height = 0.0

}

struct Point {

var x = 0.0, y = 0.0

}

struct Rect {

var origin = Point()

var size= Size()

}

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)//调用 Rect 结构体构造器}

}

let centerRect = Rect(center: Point(x: 4.0, y: 4.0), size: Size(width: 3.0, height: 3.0))

方法扩展

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struct Size {

varwidth = 0.0, height = 0.0

}

struct Point {

var x = 0.0, y = 0.0

}

struct Rect {

var origin = Point()

var size= Size()

}

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)//调用 Rect 结构体构造器

}

}

let centerRect = Rect(center: Point(x: 4.0, y: 4.0), size: Size(width: 3.0, height: 3.0))

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