Swift教程之扩展

import Foundation
//扩展就是添加新功能。扩展和 Objective-C 中的分类（categories）类似。【不过与Objective-C不同的是，Swift 的扩展没有名字。】
//扩展可以添加新的计算属性，但是不可以添加存储属性，也不可以向已有属性添加属性观测器(property observers)。

//MARK:----------计算型属性（Computed Properties）-----
//单位转换
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("Oneinch is \(oneInch) meters")
// 打印输出："One inch is 0.0254 meters"
let threeFeet = 3.ft
print("Three feet is \(threeFeet) meters")
// 打印输出："Three feet is 0.914399970739201 meters"
let aMarathon = 42.km + 195.m
print("A marathon is \(aMarathon) meters long")
// 打印输出："A marathon is 42195.0 meters long"

//MARK:----------构造器（Initializers）--------
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的原点是 (2.5, 2.5)，大小是 (3.0, 3.0)

//MARK:----------方法（Methods）---------
//向Int类型添加一个名为repetitions的新实例方法，方法使用了一个() -> ()类型的单参数（single argument），表明函数没有参数而且没有返回值。
extension Int
{
func repetitions(task: () -> ())
{
for _ in 0..<self
{
task()
}
}
}
3.repetitions({
print("Hello!")
})
// Hello!
// Hello!
// Hello!

//MARK:----------修改实例方法（Mutating Instance Methods）---------
//通过扩展添加的实例方法也可以修改该实例本身。结构体和枚举类型中修改self或其属性的方法必须将该实例方法标注为mutating，正如来自原始实现的修改方法一样。

extension Int
{
mutating func square()
{
self = self * self
}
}
var someInt = 3
someInt.square()
// someInt 现在值是 9

//MARK:---------嵌套类型（Nested Types）-----------
extension Character
{
enum Kind
{
case Vowel, Consonant, Other
}
var kind: Kind
{
switch String(self).lowercaseString
{
case "a", "e", "i", "o", "u":
return .Vowel
case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",
"n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":
return .Consonant
default:
return .Other
}
}
}

func printLetterKinds(word: String)
{
print("'\(word)'由如下字母类型组成:")
//appendNewline换行打印
for character in word.characters
{
switch character.kind
{
//元音
case .Vowel:
print("元音字母 ", appendNewline: false)
//辅音
case .Consonant:
print("辅音字母 ", appendNewline: false)
case .Other:
print("其他 ", appendNewline: false)
}
}
print("\n", appendNewline: false)
}
printLetterKinds("Hello")