swfit-扩展语法

import UIKit

extension Double {

    var km: Double { return self * 1_000.}
    var m : Double { return self}
    var cm: Double { return self / 100.0}
    var mm: Double { return self / 1_000.}
    var ft: Double { return self / 3.28084}
}

extension Int {
    mutating func square() {
        self = self * self
    }
}

extension Int {
    subscript(digitIndex: Int) -> Int {
        var decimalBase =
        for _ in ..<digitIndex {
            decimalBase *=
        }
        return (self / decimalBase) %
    }
}

extension Int {
    enum Kind {
        case Negative, Zero, Positive
    }
    var kind: Kind {
        switch self {
        case :
            return .Zero
        case let x where x > :
            return .Positive
        default:
            return .Negative
        } }
}

class First_Demo4: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        self.navigationItem.title = "扩展语法"
        self.view.backgroundColor = UIColor.white

        let oneInch = 25.4.mm
        print("one inch is \(oneInch) meters")
        // 打印 “One inch is 0.0254 meters”
        let threeFeet = .ft
        print("Three feet is \(threeFeet) meters")
        // 打印 “Three feet is 0.914399970739201 meters”
        let aMarathon = .km + .m
        print("A marathon is \(aMarathon) meters long")
        // 打印 “A marathon is 42195.0 meters long”

        /*
         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可变方法一样。
         */
        var someInt =
        someInt.square()
        print(someInt)
        // someInt 的值现在是 9

        print([]) // 返回 5
        print([]) // 返回 9
        print("-------------我是分割线----------------")

        //判断正负
        /*
         函数 printIntegerKinds(_:) 接受一个 Int 数组,然后对该数组进行迭代。
         在每次迭代过程中,对当前整数 的计算型属性 kind 的值进行评估,并打印出适当的描述。
         */
        printIntegerKinds([, , -, , -, , ])

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    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("")
    }

}