Enumerations in Swift are flexible, and do not have to provide a value for each case of the enumeration. If a value(known as "raw" value) if provied for each enumeration case, the value can be a string, a character, or a value of any integer for floating-point type.

枚举的语法

enum CompassPoint {
	case north
	case south
	case east
	case west
}

多个case可以写在一行，用逗号分隔

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

简单使用

var directionToHead = CompassPoint.west

directionToHead = .east

关联值(Associated Values)

enum Barcode {
	case upc(Int, Int, Int, Int)
	case qrCode(String)
}

var productBarcode = Barcode.upc(8, 85909, 51226, 3)
productBarcode = .qrCode("ABCDEFGHIJKLMNOPQRSTUVWXYZ")

提取关联的值:

switch productBarcode {
case .upc(let numberSystem, let manufacturer, var product, let check):
    break
case .qrCode(let productCode):
    break
}

如果关联的值都要提取为常量或者都要提取为变量，可以这样写:

switch productBarcode {
case let .upc(numberSytem, manufacturer, product, check):
    break
case let .qrCode(productCode):
    break
}

Raw Values

//raw value可以是字符串、字符、任何整型、浮点型
enum ASCIIControlCharacter: Character {
    case tab = "\t"
    case lineFeed = "\n"
    case carriageReturn = "\r"
}

Implicitly Assigned Raw Values

当你使用枚举存储整型或者字符串类型的raw value时，没有必要给每一个case赋值。整型从0开始，字符串的raw value是case的名字。

enum RefinementPlanet: Int {
    case mercury = 1, vanus, earth, mars, jupiter, saturn, uranus, neptune
}

enum RefinementCompassPoint: String {
    case north, south, east, west
}

let earthsOrder = Planet.earth.rawValue
// earthsOrder is 3
 
let sunsetDirection = CompassPoint.west.rawValue
// sunsetDirection is "west"

Initializing from a Raw Value

If you define an enumeration with a raw-value type, the enumeration automatically receives an initializer that takes a value of the raw value’s type (as a parameter called rawValue) and returns either an enumeration case or nil.

let possiblePlanet = RefinementPlanet(rawValue: 7)
//possiblePlanet的类型是 RefinementPlanet?

Recursive Enumerations

递归枚举是一个枚举的一个或者多个case使用另一个枚举的实例作为关联值。通过的case前写indirect来表示这个case是递归的。

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