Swift 关键字详解

Swift

• 计算型属性（computed property）

  var valueT: Double {
         get {／／get方法在读值的时候调用
             return     NSNumberFormatter().
             numberFromString(labelText.text!)!.doubleValue
         }
         set {／／set方法在赋值的时候调用
             labelText.text = “\(newValue)”／／newValue即get中return的值
         }
     }
     Declaration var valueT: Double { get set }
     Declared In ViewController.swift

• 闭包（closure）: 用来接收参数和返回参数，类似于函数，但是没有函数名

  var operators: String = “/“,num1 = 2.0,num2 = 1.0,answer: Double!
     func operatorOnTheNum(operators: (Double,Double) -> Double ) -> Double {
         return operators(num1,num2)
     }
     switch operators {
     case "+": answer = operatorOnTheNum  {$0 + $1}／／opaeratorOnTheNum({(opt1: Double,opt2: Double) -> Double in return opt1 + opt2 })的极简形式
     case "-": answer = operatorOnTheNum  {$0 - $1}
     case "*": answer = operatorOnTheNum  {$0 * $1}
     case "/": answer = operatorOnTheNum  {$0 / $1}
     default: break
     }

• mutating：在结构体和枚举这两种类型中，实例方法只有通过mutating才可以属性的。

  protocol Togglable {
         mutating func toogle() 
     }
     enumeration OnOffSwitch: Togglable {
         case Off,On
         mutating func toogle() {
             switch self {
             case On:
                 self = On
             case Off:
                 self = Off
             }
     }

• @auto_closure

  func simpleAssert(x: Bool) {
         let a ＝ 0
         if a & x {
             ……
         } else {
             ……
         }
     }
     simpleAssert(someExpensivecomputation() !=42)
     
     **当我们通过上述代码调用simpleAssert函数是，我们不得不每次都需要调用someExpensivecomputation ()！＝42的值是真是假，那么怎么样能做到延迟求值。**
     
     func simpleAssert(condition: () ->Bool,y: Bool,message: String) {
         if (y && !condition()) {//当为False时，那么整个式子的式子的值也为False那么在这种情况下是不会调用condition的
             println(message)
         } else {
             println("!= 42")
         }
     }
     func someExpensivecomputation() -> Int {
         return 41
     }
     simpleAssert(false,{someExpensivecomputation() != 42},"==42")