Swift属性

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属性的存储

属性的主要作用是存储数据，可以常量属性和变量属性；

struct FixedLengthRange {var firstValue: Int let length: Int}var rangeOfThreeItems =FixedLengthRange(firstValue: 0,length: 3) // the range represents integer values 0, 1, and2 rangeOfThreeItems.firstValue = 6// the range now represents integer values 6, 7, and 8

但是 rangeOfFourItems 实例为常量属性也是不可以修改的。

et rangeOfFourItems = FixedLengthRange(firstValue: 0, length: 4)// this range represents integer values 0, 1, 2, and 3 rangeOfFourItems.firstValue = 6

延时存储属性

延时存储属性是初始化时候不分配值,直到第一次使用它。

属性@lazy 声明。

class DataImporter {/*DataImporter is a class to import data from anexternalfile.The   class  is assumed     to  take  a  non-trivial amount of time toinitialize.*/var fileName = "data.txt"// the  DataImporter   class  would   provide  dataimporting functionality here}class DataManager {@lazy varimporter= DataImporter()var data = ""// the DataManager class would provide data management functionality here}let manager= DataManager() manager.data += "Some data" manager.data += "Some more data"println(manager.importer.fileName)

计算属性

有的时候一些属性是通过其他的属性计算得出的，通过 get 和 set 访问器对其访问。

//定义 Point struct Point {var x =0.0, y = 0.0}//定义 Size struct Size {var width = 0.0, height = 0.0}//定义 Rect struct Rect {var origin = Point()var size = Size()var center: Point {get {let centerX = origin.x+ (size.width / 2)let centerY = origin.y + (size.height / 2)return Point(x: centerX, y: centerY)}set(newCenter) {origin.x = newCenter.x - (size.width / 2)origin.y = newCenter.y - (size.height / 2) }}}var square =Rect(origin: Point(x: 0.0, y: 0.0), size: Size(width: 10.0,height: 10.0))let initialSquareCenter =square.center square.center = Point(x: 15.0, y: 15.0) println("square.origin is  now    at  (\(square.origin.x),\(square.origin.y))")

属性观察者

为了监听属性的变化，swift 通过了属性观察者。

• willSet 观察者是在存储之前调用。

• didSet 新值存储后调用。

class StepCounter {var totalSteps: Int = 0{willSet(newTotalSteps) {println("About    to    set    totalSteps    to\(newTotalSteps)")}didSet {if totalSteps >oldValue   {steps")}println("Added   \(totalSteps  - oldValue)}let stepCounter = StepCounter()stepCounter.totalSteps = 200// About to set totalStepsto 200// Added200steps stepCounter.totalSteps = 360// About to set totalStepsto 360// Added160steps stepCounter.totalSteps = 896// About to set totalStepsto 896// Added536steps

静态属性

静态属性在结构体中使用 static 定义，类中使用 class

定义。

struct SomeStructure {static var storedTypeProperty = "Some value."static var computedTypeProperty: Int{// return anInt value here }}class SomeClass {class varcomputedTypeProperty: Int {// return anInt value here}}

调用的时候可以直接使用类和结构体名调用。实例：

struct AudioChannel {static letthresholdLevel= 10static var maxInputLevelForAllChannels= 0 var currentLevel:Int = 0 {didSet {if               currentLevel                 > AudioChannel.thresholdLevel {// cap   the  new   audio   level  to  the threshold levelcurrentLevel                             = AudioChannel.thresholdLevelif               currentLevel                 > AudioChannel.maxInputLevelForAllChannels {// storethis as the new overall maximum input levelAudioChannel.maxInputLevelForAllChannels          =currentLevel}}}}var leftChannel =AudioChannel()var rightChannel =AudioChannel()leftChannel.currentLevel = 7println(leftChannel.currentLevel)// prints "7"println(AudioChannel.maxInputLevelForAllChannels)

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