Swift 初始化Initialization

在Swift中初始化,可以是对一个类,结构体或是枚举.不像OC那样,Swift的初始化没有返回值.

初始化的基本表达式:

init() {    // perform some initialization here}

1.结构体的初始化

struct Fahrenheit {    var temperature: Double    init() {        temperature = 32.0    }}var f = Fahrenheit()print("The default temperature is \(f.temperature)° Fahrenheit")// Prints "The default temperature is 32.0° Fahrenheit"

上面的初始化实在init方法里面给属性temperature设置的初始值32.0.

也可以不在init里面设置初始值:

struct Fahrenheit {    var temperature = 32.0<p>}</p>

2.自定义初始化

truct Celsius {    var temperatureInCelsius: Double    init(fromFahrenheit fahrenheit: Double) {        temperatureInCelsius = (fahrenheit - 32.0) / 1.8    }    init(fromKelvin kelvin: Double) {        temperatureInCelsius = kelvin - 273.15    }}let boilingPointOfWater = Celsius(fromFahrenheit: 212.0)// boilingPointOfWater.temperatureInCelsius is 100.0let freezingPointOfWater = Celsius(fromKelvin: 273.15)// freezingPointOfWater.temperatureInCelsius is 0.0

上面的初始化定义了,华氏温度和摄氏温度的转换公式.

3.利用外部参数来给属性设置值

struct Color {    let red, green, blue: Double    init(red: Double, green: Double, blue: Double) {        self.red   = red        self.green = green        self.blue  = blue    }    init(white: Double) {        red   = white        green = white        blue  = white    }}

两个方法都可以在提供值得时候创建一个对象

let magenta = Color(red: 1.0, green: 0.0, blue: 1.0)let halfGray = Color(white: 0.5)

4.不使用外部参数名

struct Celsius {    var temperatureInCelsius: Double    init(fromFahrenheit fahrenheit: Double) {        temperatureInCelsius = (fahrenheit - 32.0) / 1.8    }    init(fromKelvin kelvin: Double) {        temperatureInCelsius = kelvin - 273.15    }    init(<span style="color:#cc0000;">_</span> celsius: Double) {        temperatureInCelsius = celsius    }}let bodyTemperature = Celsius(37.0)// bodyTemperature.temperatureInCelsius is 37.0

上面是通过写一个下划线"_"来代替明确的外部参数名.

5.可选属性类型/初始化的时候指定常亮属性

class SurveyQuestion {    var text: String    var response: String?    init(text: String) {        self.text = text    }    func ask() {        print(text)    }}let cheeseQuestion = SurveyQuestion(text: "Do you like cheese?")cheeseQuestion.ask()// Prints "Do you like cheese?"cheeseQuestion.response = "Yes, I do like cheese."

6.默认值初始化/缺省值

class ShoppingListItem {    var name: String?    var quantity = 1    var purchased = false}var item = ShoppingListItem()

这个类中,我们设置了quantity初始的默认值 = 1.

7.逐个成员的初始化

下面看看Rect的初始化过程.

先初始化size和point

struct Size {    var width = 0.0, height = 0.0}struct Point {    var x = 0.0, y = 0.0}

再初始化Rect

struct Rect {    var origin = Point()    var size = Size()    init() {}    init(origin: Point, size: Size) {        self.origin = origin        self.size = size    }    init(center: Point, size: Size) {        let originX = center.x - (size.width / 2)        let originY = center.y - (size.height / 2)        self.init(origin: Point(x: originX, y: originY), size: size)    }}

let originRect = Rect(origin: Point(x: 2.0, y: 2.0),                      size: Size(width: 5.0, height: 5.0))// originRect's origin is (2.0, 2.0) and its size is (5.0, 5.0)

let centerRect = Rect(center: Point(x: 4.0, y: 4.0),                      size: Size(width: 3.0, height: 3.0))// centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0)

8.类继承和初始化

在这里提到了两个构造器,指定构造器和方便构造器,没看懂.上截图吧

从这两张图中,我们似乎也可以看出什么来.D-->D ,C-->D/C.

这个部分感觉这个东西最好玩Unnamed

class Food {    var name: String    init(name: String) {        self.name = name    }    convenience init() {        self.init(name: "[Unnamed]")    }}

这个类的初始化

let namedMeat = Food(name: "Bacon")// namedMeat's name is "Bacon"

看,这里就显示了Unnamed的作用可,可以自定把对象的名字获取.

方便构造器的初始化有个关键字:convenience.

下面这类,继承自Food

class RecipeIngredient: Food {    var quantity: Int    init(name: String, quantity: Int) {        self.quantity = quantity        super.init(name: name)    }    override convenience init(name: String) {        self.init(name: name, quantity: 1)    }}

它的初始化过程:

有继承有C也有D了.

9.枚举的初始化

enum TemperatureUnit {    case Kelvin, Celsius, Fahrenheit    init?(symbol: Character) {        switch symbol {        case "K":            self = .Kelvin        case "C":            self = .Celsius        case "F":            self = .Fahrenheit        default:            return nil        }    }}

let fahrenheitUnit = TemperatureUnit(symbol: "F")if fahrenheitUnit != nil {    print("This is a defined temperature unit, so initialization succeeded.")}// Prints "This is a defined temperature unit, so initialization succeeded." let unknownUnit = TemperatureUnit(symbol: "X")if unknownUnit == nil {    print("This is not a defined temperature unit, so initialization failed.")}// Prints "This is not a defined temperature unit, so initialization failed."

10.枚举带初始值的初始化

enum TemperatureUnit: Character {    case Kelvin = "K", Celsius = "C", Fahrenheit = "F"} let fahrenheitUnit = TemperatureUnit(rawValue: "F")if fahrenheitUnit != nil {    print("This is a defined temperature unit, so initialization succeeded.")}// Prints "This is a defined temperature unit, so initialization succeeded." let unknownUnit = TemperatureUnit(rawValue: "X")if unknownUnit == nil {    print("This is not a defined temperature unit, so initialization failed.")}// Prints "This is not a defined temperature unit, so initialization failed."