Swift4 枚舉，自學筆記

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枚舉

蘋果官方文檔 枚舉蘋果官方文檔中文翻譯 枚舉

枚舉文法

enum SomeEnumeration {    // enumeration definition goes here}

enum CompassPoint {    case north    case south    case east    case west}var directionToHead = CompassPoint.westdirectionToHead = .east

enum Planet {    case mercury, venus, earth, mars, jupiter, saturn, uranus, neptune}

使用Switch語句來匹配枚舉值

directionToHead = .southswitch directionToHead {    case .north:        print("Lots of planets have a north")    case .south:        print("Watch out for penguins")    case .east:        print("Where the sun rises")    case .west:        print("Where the skies are blue")}// prints "Watch out for penguins"

如果不能為所有枚舉成員都提供一個 case，那你也可以提供一個==default==情況來包含那些不能被明確寫出的成員：

let somePlanet = Planet.earthswitch somePlanet {    case.earth:        print("Mostly harmless")    default:        print("Not a safe place for humans")}// Prints "Mostly harmless"

關聯值

enum Barcode {    case upc(Int, Int, Int, Int)    case qrCode(String)}var productBarcode = Barcode.upc(8, 85909, 51226, 3)productBarcode = .qrCode("ABCDEFGHIJKLMNOP")switch productBarcode {    case .upc(let numberSystem, let manufacturer, let product, let check):        print("UPC: \(numberSystem), \(manufacturer), \(product), \(check).")    case .qrCode(let productCode):        print("QR code: \(productCode).")}// Prints "QR code: ABCDEFGHIJKLMNOP."

如果對於一個枚舉成員的所有的相關值都被提取為常量，或如果都被提取為變數，為了簡潔，你可以用一個單獨的 var或 let在成員名稱前標註：

switch productBarcode {case let .upc(numberSystem, manufacturer, product, check):    print("UPC : \(numberSystem), \(manufacturer), \(product), \(check).")case let .qrCode(productCode):    print("QR code: \(productCode).")}// Prints "QR code: ABCDEFGHIJKLMNOP."

原始值

枚舉成員可以用相同類型的預設值預先填充（稱為原始值）。

enum ASCIIControlCharacter: Character {    case tab = "\t"    case lineFeed = "\n"    case carriageReturn = "\r"}

隱式指定的原始值

當你在操作儲存整數或字串原始值枚舉的時候，你不必顯式地給每一個成員都分配一個原始值。當你沒有分配時，Swift 將會自動為你分配值。

enum Planet: Int {    case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune}enum CompassPoint: String {    case north, south, east, west}let earthsOrder = Planet.Earth.rawValue    // earthsOrder is 3 let sunsetDirection = CompassPoint.west.rawValue    // sunsetDirection is "west"

從原始值初始化

用原始實值型別來定義一個枚舉，那麼枚舉就會自動收到一個可以接受原始實值型別的值的初始化器（叫做==rawValue==的形式參數）然後返回一個枚舉成員或者 nil

let possiblePlanet = Planet(rawValue: 7)    // possiblePlanet is of type Planet? and equals Planet.Uranus

原始值初始化器是一個可失敗初始化器

let positionToFind = 11if let somePlanet = Planet(rawValue: positionToFind) {    switch somePlanet {    case .earth:        print("Mostly harmless")    default:        print("Not a safe place for humans")    }} else {    print("There isn‘t a planet at position \(positionToFind)")}// Prints "There isn‘t a planet at position 11"

遞迴枚舉

遞迴枚舉是擁有另一個枚舉作為枚舉成員關聯值的枚舉。在聲明枚舉成員之前使用==indirect==關鍵字來明確它是遞迴的。

enum ArithmeticExpression {    case number(Int)    indirect case addition(ArithmeticExpression, ArithmeticExpression)    indirect case multiplication(ArithmeticExpression, ArithmeticExpression)}

可以在枚舉之前寫==indirect==來讓整個枚舉成員在需要時可以遞迴：

indirect enum ArithmeticExpression {    case number(Int)    case addition(ArithmeticExpression, ArithmeticExpression)    case multiplication(ArithmeticExpression, ArithmeticExpression)}

let five = ArithmeticExpression.number(5)let four = ArithmeticExpression.number(4)let sum = ArithmeticExpression.addition(five, four)let product = ArithmeticExpression.multiplication(sum, ArithmeticExpression.number(2))

func evaluate(_ expression: ArithmeticExpression) -> Int {    switch expression {    case let .number(value):        return value    case let .addition(left, right):        return evaluate(left) + evaluate(right)    case let .multiplication(left, right):        return evaluate(left) * evaluate(right)    }} print(evaluate(product))// Prints "18"

Swift4 枚舉，自學筆記