【Swift學習】Swift編程之旅---構造方法（十八）

　　初始化是為了使用某個類、結構體或枚舉類型的實例而進行的準備過程。這個過程包括為每個存儲的屬性設置一個初始值，然後執行新實例所需的任何其他設置或初始化。

init() {
    // perform some initialization here
}

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

struct Fahrenheit { 
    var temperature = 32.0 
} 

struct Celsius { 
    var temperatureInCelsius: Double = 0.0 
    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 是 100.0 
let freezingPointOfWater = Celsius(fromKelvin: 273.15) 
// freezingPointOfWater.temperatureInCelsius 是 0.0” 

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) 
// 報編譯時錯誤，需要外部名稱 

　　無外部名稱參數的構造器

　　如果你不希望為構造器的某個參數提供外部名字，你可以使用下劃線_來顯示描述它的外部名，以此覆蓋上面所說的預設行為。

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

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

class SurveyQuestion { 
    let text: String 
    var response: String? 
    init(text: String) { 
        self.text = text 
    } 
    func ask() { 
        println(text) 
    } 
} 
let beetsQuestion = SurveyQuestion(text: "How about beets?") 
beetsQuestion.ask() 
// 輸出 "How about beets?" 
beetsQuestion.response = "I also like beets. (But not with cheese.) 

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

struct Size { 
    var width = 0.0, height = 0.0 
} 
let twoByTwo = Size(width: 2.0, height: 2.0) 

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 的原點是 (0.0, 0.0)，尺寸是 (0.0, 0.0) 

let originRect = Rect(origin: Point(x: 2.0, y: 2.0), 
    size: Size(width: 5.0, height: 5.0)) 
// originRect 的原點是 (2.0, 2.0)，尺寸是 (5.0, 5.0) 

init(parameters) {
    statements
}

便利構造器實現通過在init關鍵字前面加convenience關鍵字

convenience init(parameters) {
    statements
}

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

let namedMeat = Food(name: "Bacon") 
// namedMeat 的名字是 "Bacon” 

let mysteryMeat = Food() 
// mysteryMeat 的名字是 [Unnamed] 

class RecipeIngredient: Food { 
    var quantity: Int 
    init(name: String, quantity: Int) { 
        self.quantity = quantity 
        super.init(name: name) 
    } 
    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.lowercaseString)" 
        output += purchased ? " ?" : " ?" 
        return output 
    } 
} 

var breakfastList = [ 
    ShoppingListItem(), 
    ShoppingListItem(name: "Bacon"), 
    ShoppingListItem(name: "Eggs", quantity: 6), 
] 
breakfastList[0].name = "Orange juice" 
breakfastList[0].purchased = true 
for item in breakfastList { 
    println(item.description) 
} 
// 1 x orange juice ? 
// 1 x bacon ? 
// 6 x eggs ? 

class SomeClass { 
    let someProperty: SomeType = { 
        // 在這個閉包中給 someProperty 創建一個預設值 
        // someValue 必須和 SomeType 類型相同 
        return someValue 
        }() 
} 

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 squareIsBlackAtRow(row: Int, column: Int) -> Bool {
        return boardColors[(row * 8) + column]
    }
}

let board = Chessboard()
print(board.squareIsBlackAtRow(0, column: 1))
// Prints "true"
print(board.squareIsBlackAtRow(7, column: 7))
// Prints "false”