Swift 属性

存储属性

import UIKit

// 存储属性
struct FixedLengthRange {
    var firstValue: Int
    var length: Int
}

var rangeOfThreeItems = FixedLengthRange(firstValue: 0, length: 3)

rangeOfThreeItems.firstValue = 6

let rangeOfFourItems = FixedLengthRange(firstValue: 0, length: 4)

// 结构体常量的属性值是不可以修改的，会报错，如下
// rangeOfFourItems.firstValue = 6

懒加载的存储属性

// 懒加载的存储属性
class DataImporter {
    var fileName = "data.text"
}

class DataManager{
    lazy var importer = DataImporter()
    var data = [String]()
}

let manager = DataManager()
manager.data.append("Some data")
manager.data.append("Some more data")

print(manager.importer.fileName)

console log 如下

计算属性

// 计算属性
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 {
        get {
            let centerX = origin.x + size.width / 2.0
            let centerY = origin.y + size.height / 2.0
            return Point(x: centerX, y: centerY)
        }
        
        set(newCenter) {
            origin.x = newCenter.x - size.width / 2.0
            origin.y = newCenter.y - size.height / 2.0
        }
    }
}


var square = Rect(origin: Point(x: 0, y: 0), size: Size(width: 10, height: 10))
let initialSquareCenter = square.center
print("square center is now at (\(initialSquareCenter.x), \(initialSquareCenter.y))")

square.center = Point(x: 15, y: 15)
print("square origin is now at (\(square.origin.x), \(square.origin.y))")

// 计算属性，设置方法使用默认参数
struct AlternativeRect {
    var origin = Point()
    var size = Size()
    var center: Point {
        get {
            let centerX = origin.x + size.width / 2.0
            let centerY = origin.y + size.height / 2.0
            return Point(x: centerX, y: centerY)
        }
        
        set {
            origin.x = newValue.x - size.width / 2.0
            origin.y = newValue.y - size.height / 2.0
        }
    }
}

console log 如下

只读的计算属性

// 只读的计算属性
struct Cuboid {
    var width = 0.0, height = 0.0, depth = 0.0
    var volume: Double{
        return width * height * depth
    }
}

let fourByFiveByTwo = Cuboid(width: 4, height: 5, depth: 2)
print("the volume of fourByFiveByTwo is \(fourByFiveByTwo.volume)")

console log如下

观察属性

// 观察属性
class StepCounter {
    var totalSteps: Int = 0 {
        willSet (newTotalSteps) {
            print("About to set totalSteps to \(newTotalSteps)")
        }
        didSet (oldTotalSteps) {
            if totalSteps > oldTotalSteps {
                print("Add \(totalSteps - oldTotalSteps) steps")
            }
        }
    }
}

let stepCounter = StepCounter()
stepCounter.totalSteps = 100
stepCounter.totalSteps = 80

// 观察属性使用默认参数
class AnotherStepCounter {
    var totalSteps: Int = 0 {
        willSet {
            print("About to set totalSteps to \(newValue)")
        }
        didSet {
            if totalSteps > oldValue {
                print("Add \(totalSteps - oldValue) steps")
            }
        }
    }
}

let anotherStepCounter = AnotherStepCounter()
anotherStepCounter.totalSteps = 85
anotherStepCounter.totalSteps = 520

console log 如下

static 关键字

// static 关键字
struct SomeStructure {
    static var storedTypeProperty = "Some value."
    static var computedTypeProperty: Int {
        return 1
    }
}

print(SomeStructure.storedTypeProperty)
SomeStructure.storedTypeProperty = "Another Value."
print(SomeStructure.storedTypeProperty)


struct AudioChannel {
    static let thresholdLevel = 10
    static var maxInputLevelForAllChannels = 0
    var currentLevel: Int = 0 {
        didSet {
            if currentLevel > AudioChannel.thresholdLevel {
                currentLevel = AudioChannel.thresholdLevel
            }
            
            if currentLevel > AudioChannel.maxInputLevelForAllChannels {
                AudioChannel.maxInputLevelForAllChannels = currentLevel
            }
        }
    }
}

var leftChannel = AudioChannel()
var rightChannel = AudioChannel()

leftChannel.currentLevel = 7
print(leftChannel.currentLevel)
print(AudioChannel.maxInputLevelForAllChannels)

rightChannel.currentLevel = 11
print(rightChannel.currentLevel)
print(AudioChannel.maxInputLevelForAllChannels)

console log 如下