## Properties Computed properties are provided by classes, structures, and enumerations. Stored properties are provided only by classes and structures. ~~~ let rangeOfFourItems = FixedLengthRange(firstValue: 0, length: 4) // this range represents integer values 0, 1, 2, and 3 rangeOfFourItems.firstValue = 6 // this will report an error, even thought firstValue is a variable property ~~~ Because rangeOfFourItems is declared as a constant (with the let keyword), it is not possible to change its firstValue property, even though firstValue is a variable property. This behavior is due to structures being value types. When an instance of a value type is marked as a constant, so are all of its properties. The same is not true for classes, which are reference types. If you assign an instance of a reference type to a constant, you can still change that instance’s variable properties. ~~~ class DataImporter { /* DataImporter is a class to import data from an external file. The class is assumed to take a non-trivial amount of time to initialize. */ var fileName = "data.txt" // the DataImporter class would provide data importing functionality here } class DataManager { @lazy var importer = DataImporter() var data = String[]() // the DataManager class would provide data management functionality here } let manager = DataManager() manager.data += "Some data" manager.data += "Some more data" // the DataImporter instance for the importer property has not yet been created println(manager.importer.fileName) // the DataImporter instance for the importer property has now been created // prints "data.txt" ~~~ In addition to stored properties, classes, structures, and enumerations can define computed properties, which do not actually store a value. Instead, they provide a getter and an optional setter to retrieve and set other properties and values indirectly. ~~~ struct Point { var x = 0.0, y = 0.0 } struct Size { var width = 0.0, height = 0.0 } struct Rect { var origin = Point() var size = Size() var center: Point { // center是computed property 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))") // prints "square.origin is now at (10.0, 10.0)" ~~~ You must declare computed properties—including read-only computed properties—as variable properties with the var keyword, because their value is not fixed. ~~~ struct Cuboid { var width = 0.0, height = 0.0, depth = 0.0 var volume: Double { return width * height * depth } } let fourByFiveByTwo = Cuboid(width: 4.0, height: 5.0, depth: 2.0) println("the volume of fourByFiveByTwo is \(fourByFiveByTwo.volume)") // prints "the volume of fourByFiveByTwo is 40.0” ~~~ willSet and didSet observers are not called when a property is first initialized. They are only called when the property’s value is set outside of an initialization context. ~~~ class StepCounter { var totalSteps: Int = 0 { willSet(newTotalSteps) { println("About to set totalSteps to \(newTotalSteps)") } didSet { if totalSteps > oldValue { println("Added \(totalSteps - oldValue) steps") } } } } let stepCounter = StepCounter() stepCounter.totalSteps = 200 // About to set totalSteps to 200 // Added 200 steps stepCounter.totalSteps = 360 // About to set totalSteps to 360 // Added 160 steps stepCounter.totalSteps = 896 // About to set totalSteps to 896 // Added 536 steps ~~~ If you assign a value to a property within its own didSet observer, the new value that you assign will replace the one that was just set. Global constants and variables are always computed lazily, in a similar manner to Lazy Stored Properties. Unlike lazy stored properties, global constants and variables do not need to be marked with the @lazy attribute. Local constants and variables are never computed lazily. For value types (that is, structures and enumerations), you can define stored and computed type properties. For classes, you can define computed type properties only. Unlike stored instance properties, you must always give stored type properties a default value. This is because the type itself does not have an initializer that can assign a value to a stored type property at initialization time. ~~~ struct SomeStructure { static var storedTypeProperty = "Some value." static var computedTypeProperty: Int { // return an Int value here } } enum SomeEnumeration { static var storedTypeProperty = "Some value." static var computedTypeProperty: Int { // return an Int value here } } class SomeClass { class var computedTypeProperty: Int { // return an Int value here } } println(SomeClass.computedTypeProperty) // prints "42" println(SomeStructure.storedTypeProperty) // prints "Some value." SomeStructure.storedTypeProperty = "Another value." println(SomeStructure.storedTypeProperty) // prints "Another value." ~~~ ~~~ struct AudioChannel { static let thresholdLevel = 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 { // store this as the new overall maximum input level AudioChannel.maxInputLevelForAllChannels = currentLevel } } } } var leftChannel = AudioChannel() var rightChannel = AudioChannel() leftChannel.currentLevel = 7 println(leftChannel.currentLevel) // prints "7" println(AudioChannel.maxInputLevelForAllChannels) // prints “7" rightChannel.currentLevel = 11 println(rightChannel.currentLevel) // prints "10" println(AudioChannel.maxInputLevelForAllChannels) // prints "10" ~~~