Swift （關鍵字詳解）

Swift

• 計算型屬性（computed property）

  var valueT: Double {
         get {／／get方法在讀值的時候調用
             return     NSNumberFormatter().
             numberFromString(labelText.text!)!.doubleValue
         }
         set {／／set方法在賦值的時候調用
             labelText.text = 「\(newValue)」／／newValue即get中return的值
         }
     }
     Declaration var valueT: Double { get set }
     Declared In ViewController.swift

• 閉包（closure）: 用來接收參數和返回參數，相似於函數，可是沒有函數名

  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 })的極簡形式
     case "-": answer = operatorOnTheNum  {$0 - $1}
     case "*": answer = operatorOnTheNum  {$0 * $1}
     case "/": answer = operatorOnTheNum  {$0 / $1}
     default: break
     }

• mutating：在結構體和枚舉這兩種類型中，實例方法只有經過mutating才能夠屬性的。

  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)
     
     **當咱們經過上述代碼調用simpleAssert函數是，咱們不得不每次都須要調用someExpensiveComputation ()！＝42的值是真是假，那麼怎麼樣能作到延遲求值。**
     
     func simpleAssert(condition: () ->Bool, y: Bool, message: String) {
         if (y && !condition()) {//當爲False時，那麼整個式子的式子的值也爲False那麼在這種狀況下是不會調用condition的
             println(message)
         } else {
             println("!= 42")
         }
     }
     func someExpensiveComputation() -> Int {
         return 41
     }
     simpleAssert(false, {someExpensiveComputation() != 42},"==42")