在現有類和結構體的類型基礎上，擴展新的功能。

語法：

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
}

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
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")

使用構造器

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)//調用 Rect 結構體構造器} 
}
let centerRect = Rect(center: Point(x: 4.0, y: 4.0), size: Size(width: 3.0, height: 3.0))

方法擴展

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)//調用 Rect 結構體構造器
}
}
let centerRect = Rect(center: Point(x: 4.0, y: 4.0), size: Size(width: 3.0, height: 3.0))

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Swift擴展（Extension）