Initialization 程式範例 — Apple’s The Swift Programming Language
Initializers
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"
Default Property Values
struct Fahrenheit { var temperature = 32.0}
Initialization Parameters
有參數的 init。
struct 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
Parameter Names and Argument Labels
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 }}let magenta = Color(red: 1.0, green: 0.0, blue: 1.0)let halfGray = Color(white: 0.5)let veryGreen = Color(0.0, 1.0, 0.0)// this reports a compile-time error - argument labels are required
Initializer Parameters Without Argument Labels
利用 _ 取消 argument label。
struct Celsius { var temperatureInCelsius: Double init(fromFahrenheit fahrenheit: Double) { temperatureInCelsius = (fahrenheit - 32.0) / 1.8 } init(fromKelvin kelvin: Double) { temperatureInCelsius = kelvin - 273.15 } init(_ celsius: Double) { temperatureInCelsius = celsius }}let bodyTemperature = Celsius(37.0)// bodyTemperature.temperatureInCelsius is 37.0
Optional Property Types
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."
Assigning Constant Properties During Initialization
class SurveyQuestion { let text: String var response: String? init(text: String) { self.text = text } func ask() { print(text) }}let beetsQuestion = SurveyQuestion(text: "How about beets?")beetsQuestion.ask()// Prints "How about beets?"beetsQuestion.response = "I also like beets. (But not with cheese.)"
Default Initializers
當 property 在宣告時都有設定初始值，而且沒有另外定義 init 時，Swift 將自動生成沒有參數的 Default Initializer，init()。利用 default Initializer 生成東西時，東西的 property 將為當初宣告時設定的初始值。
class ShoppingListItem { var name: String? var quantity = 1 var purchased = false}var item = ShoppingListItem()
Memberwise Initializers for Structure Types
struct Size { var width = 0.0, height = 0.0}let twoByTwo = Size(width: 2.0, height: 2.0)
Initializer Delegation for Value Types
struct Size { var width = 0.0, height = 0.0}struct Point { var x = 0.0, y = 0.0}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 basicRect = Rect()// basicRect's origin is (0.0, 0.0) and its size is (0.0, 0.0)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)
Initializer Inheritance and Overriding
class Vehicle { var numberOfWheels = 0 var description: String { return "\(numberOfWheels) wheel(s)" }}let vehicle = Vehicle()print("Vehicle: \(vehicle.description)")// Vehicle: 0 wheel(s)class Bicycle: Vehicle { override init() { super.init() numberOfWheels = 2 }}let bicycle = Bicycle()print("Bicycle: \(bicycle.description)")// Bicycle: 2 wheel(s)
Designated and Convenience Initializers in Action
class Food { var name: String init(name: String) { self.name = name } convenience init() { self.init(name: "[Unnamed]") }}let mysteryMeat = Food()// mysteryMeat's name is "[Unnamed]"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) }}let oneMysteryItem = RecipeIngredient()let oneBacon = RecipeIngredient(name: "Bacon")let sixEggs = RecipeIngredient(name: "Eggs", quantity: 6)class ShoppingListItem: RecipeIngredient { var purchased = false var description: String { var output = "\(quantity) x \(name)" output += purchased ? " ✔" : " ✘" return output }}var breakfastList = [ShoppingListItem(),ShoppingListItem(name: "Bacon"),ShoppingListItem(name: "Eggs", quantity: 6),]breakfastList[0].name = "Orange juice"breakfastList[0].purchased = truefor item in breakfastList {print(item.description)}// 1 x Orange juice ✔// 1 x Bacon ✘// 6 x Eggs ✘
Failable Initializers
inti? ，可能失敗的 init。生成的東西是 optional，失敗時會生成 nil。
let wholeNumber: Double = 12345.0let pi = 3.14159if let valueMaintained = Int(exactly: wholeNumber) {print("\(wholeNumber) conversion to Int maintains value of \(valueMaintained)")}// Prints "12345.0 conversion to Int maintains value of 12345"let valueChanged = Int(exactly: pi)// valueChanged is of type Int?, not Intif valueChanged == nil {print("\(pi) conversion to Int does not maintain value")}// Prints "3.14159 conversion to Int does not maintain value"
生成 Animal 時，當 species 是空字串時回傳 nil。
struct Animal { let species: String init?(species: String) { if species.isEmpty { return nil } self.species = species } } let someCreature = Animal(species: "Giraffe") // someCreature is of type Animal?, not Animal if let giraffe = someCreature { print("An animal was initialized with a species of \(giraffe.species)") } // Prints "An animal was initialized with a species of Giraffe" let anonymousCreature = Animal(species: "")// anonymousCreature is of type Animal?, not Animal if anonymousCreature == nil { print("The anonymous creature could not be initialized") } // Prints "The anonymous creature could not be initialized"
Failable Initializers for Enumerations
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."
Failable Initializers for Enumerations with Raw Values
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."
Propagation of Initialization Failure
呼叫父類別的 init?，也可能造成失敗，得到 nil。
class Product { let name: String init?(name: String) { if name.isEmpty { return nil } self.name = name }}class CartItem: Product { let quantity: Int init?(name: String, quantity: Int) { if quantity < 1 { return nil } self.quantity = quantity super.init(name: name) }}if let twoSocks = CartItem(name: "sock", quantity: 2) { print("Item: \(twoSocks.name), quantity: \(twoSocks.quantity)")}// Prints "Item: sock, quantity: 2"if let zeroShirts = CartItem(name: "shirt", quantity: 0) { print("Item: \(zeroShirts.name), quantity: \(zeroShirts.quantity)")} else { print("Unable to initialize zero shirts")}// Prints "Unable to initialize zero shirts"if let oneUnnamed = CartItem(name: "", quantity: 1) { print("Item: \(oneUnnamed.name), quantity: \(oneUnnamed.quantity)")} else { print("Unable to initialize one unnamed product")}// Prints "Unable to initialize one unnamed product"
Overriding a Failable Initializer
覆寫父類別的 Failable Initializer 時，也可以定義成 nonfailable initializer。不過如果在 nonfailable initializer 裡呼叫父類別的 Failable Initializer 時，要加 force-unwrap !。
class Document { var name: String? // this initializer creates a document with a nil name value init() {} // this initializer creates a document with a nonempty name value init?(name: String) { if name.isEmpty { return nil } self.name = name }}class AutomaticallyNamedDocument: Document { override init() { super.init() self.name = "[Untitled]" } override init(name: String) { super.init() if name.isEmpty { self.name = "[Untitled]" } else { self.name = name } }}class UntitledDocument: Document { override init() { super.init(name: "[Untitled]")! }}
The init! Failable Initializer
除了定義 init?，也可以定義 init!。
Required Initializers
加了 required 的 initializers，子類別也要定義(或繼承)。
class SomeClass { required init() { // initializer implementation goes here }}class SomeSubclass: SomeClass { required init() { // subclass implementation of the required initializer goes here }}
Setting a Default Property Value with a Closure or Function
利用 closure 或 global function 設定 property。
例子:
西洋棋的格子。true 代表黑色，false 代表白色。
struct Chessboard { let boardColors: [Bool] = { var temporaryBoard = [Bool]() var isBlack = false for i in 1...8 { for j in 1...8 { temporaryBoard.append(isBlack) isBlack = !isBlack } isBlack = !isBlack } return temporaryBoard }() func squareIsBlackAt(row: Int, column: Int) -> Bool { return boardColors[(row * 8) + column] }}let board = Chessboard()print(board.squareIsBlackAt(row: 0, column: 1))// Prints "true"print(board.squareIsBlackAt(row: 7, column: 7))// Prints "false"
