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")

（编辑：李大同）

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