Swift屬性

屬性的存儲

屬性的主要作用是存儲數據，可以常量屬性和變量屬 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 實例為常量屬性也是不可以修改的。

l

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

延時存儲屬性

延時存儲屬性是初始化時候不分配值,直到第一次使 用它。

屬性@lazy 聲明。

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)

計算屬性

有的時候一些屬性是通過其他的屬性計算得出的，通 過 get 和 set 訪問器對其訪問。

//定義 Point struct Point {
var x =0.0, y = 0.0
}
//定義 Size struct Size {
var width = 0.0, height = 0.0
}
//定義 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))")
 

屬性觀察者

為了監聽屬性的變化，swift 通過了屬性觀察者。

? willSet 觀察者是在存儲之前調用。

? didSet 新值存儲後調用。

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

靜態屬性

靜態屬性在結構體中使用 static 定義，類中使用 class

定義。

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

調用的時候可以直接使用類和結構體名調用。 實例：

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

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