[Swift 官方教程]08 枚举

枚举语法 (Enumeration Syntax)

🕐 定义:

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

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

🕑 获取:

var d = CompassPoint.west

🕒 若变量的类型已知, 则可以省略枚举类型:

d = .east

在 switch 中使用枚举值 (Matching Enumeration Values with a Switch Statement)

switch d {
case .north:
    print("north")
case .south:
    print("south")
case .east:
    print("east")
case .west:
    print("west")
}

遍历枚举的 case (Iterating over Enumeration Cases)

使用 CaseIterable 协议:

enum Beverage: CaseIterable {
    case coffee, tea, juice
}

🕐 查看枚举数:

let n = Beverage.allCases.count
print(n)

🕑 遍历:

for i in Beverage.allCases {
    print(i)
}

关联值 (Associated Values)

给枚举值设置类型:

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

获取枚举值并为其设定类型值:

var code = Barcode.upc(8, 85909, 51226, 3)
code = .qrCode("ABCDEFGHIJKLMNOP")

在 switch 中提取枚举值的类型值:

switch code {
case .upc(let a, let b, let c, let d):
    print("UPC: \(a), \(b), \(c), \(d).")
case .qrCode(let a):
    print("QR code: \(a).")
}

原始值 (Raw Values)

给枚举添加类型, 随后设置原始值:

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

每个原始值必须是同一类型且唯一

隐式原始值赋值 (Implicitly Assigned Raw Values)

🕐 Int 类枚举给首个原始值赋值, 后续的原始值会自动赋值并自增:

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

🕑 String 类枚举的原始值为其本身转换为字符串类型:

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

🕒 获取原始值:

let e = Planet.earth.rawValue
let w = CompassPoint.west.rawValue

从原始值进行初始化 (Initializing from a Raw Value)

从原始值获取对应的枚举值:

let p = Planet(rawValue: 7)

返回的是可选值

递归枚举 (Recursive Enumerations)

使用 indirect 定义递归枚举:

enum Expression {
    case num(Int)
    indirect case add(Expression, Expression)
    indirect case multi(Expression, Expression)
}

可以被改写为:

indirect enum Expression {
    case num(Int)
    case add(Expression, Expression)
    case multi(Expression, Expression)
}

创建对应的枚举常量:

let five = Expression.num(5)
let four = Expression.num(4)
let sum = Expression.add(five, four)
let product = Expression.multi(sum, Expression.num(2))

运用它们:

func evaluate(_ e: Expression) -> Int {
    switch e {
    case let .num(v):
        return v
    case let .add(left, right):
        return evaluate(left) + evaluate(right)
    case let .multi(left, right):
        return evaluate(left) * evaluate(right)
    }
}
print(evaluate(product))