Swift属性

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。

l

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( 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= 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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