枚举的基本用法

• 定义：枚举简单的说也是一种数据类型，只不过是这种数据类型只包含自定义的特定数据，它是一组有共同特性的数据的集合。

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

enum Direction {
    case north,south,east,west
}

var dir = Direction.west
dir = Direction.east
dir = .north
print(dir) // north

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

关联值（Associated Values）

• 有时将枚举的成员值跟其他类型的值关联存储在一起，会非常有用

enum Score {
    case points(Int)
    case grade(Character)
}

var score = Score.points(96)
score = Score.grade("A")

switch score {
case let .points(i):
    print(i, "points")
case let .grade(i):
    print("grade", i)
} // grade A

enum Date {
    case digit(year: Int, month: Int, day: Int)
    case string(String)
}

var date = Date.digit(year: 2022, month: 10, day: 14)
date = .string("2022-10-14")
switch date {
case .digit(let year, let month, let day):
    print(year, month, day)
case let .string(value):
    print(value)
}

• 必要时 let 也可以改为 var

enum Password {
    case number(Int, Int, Int, Int)
    case gesture(String)
}

var pwd = Password.number(3, 5, 7, 8)
pwd = .gesture("123456")

switch pwd {
case let .number(n1, n2, n3, n4):
    print("number is", n1, n2, n3, n4)
case let .gesture(str):
    print("gesture is", str)
}

原始值（Raw Values）

• 枚举成员可以使用相同类型的默认值预先对应，这个默认值叫做：原始值

enum PokerSuit : Character {
    case spade = "♠️"
    case heart = "♥️"
    case diamond = "♦️"
    case club = "♣️"
}

var suit = PokerSuit.spade
print(suit)
print(suit.rawValue) // ♠️
print(PokerSuit.club.rawValue) // ♣️

enum Grade : String {
    case perfect = "A"
    case great = "B"
    case good = "C"
    case bad = "D"
}
print(Grade.perfect.rawValue) //A
print(Grade.great.rawValue) //B
print(Grade.good.rawValue) //C
print(Grade.bad.rawValue) //D

注意：原始值不占用枚举变量的内存

隐式原始值（Implicitly Assigned Raw Values）

• 如果枚举的原始值类型是 Int、String ，Swift会自动分配原始值

enum Direction : String {
    case north = "north"
    case south = "south"
    case east = "ease"
    case west = "west"
}

//等价于
enum Direction :String {
    case north, south, east, west
}
print(Direction.north) //north
print(Direction.north.rawValue) //north

enum Season : Int {
    case spring, summer, autumn, winter
}
print(Season.spring.rawValue) //0
print(Season.summer.rawValue) //1
print(Season.autumn.rawValue) //2
print(Season.winter.rawValue) //3

enum Season : Int {
    case spring = 1, summer, autumn = 4, winter
}
print(Season.spring.rawValue) //1
print(Season.summer.rawValue) //2
print(Season.autumn.rawValue) //4
print(Season.winter.rawValue) //5

递归枚举（Recursive Enumeration）

递归枚举必须前面添加 indirect

indirect enum ArithExpr {
    case number(Int)
    case sum(ArithExpr, ArithExpr)
    case difference(ArithExpr, ArithExpr)
}
//或者
enum ArithExpr {
    case number(Int)
    indirect case sum(ArithExpr, ArithExpr)
    indirect case difference(ArithExpr, ArithExpr)
}

let five = ArithExpr.number(5)
let four = ArithExpr.number(4)
let two = ArithExpr.number(2)
let sum = ArithExpr.sum(five, four)
let difference = ArithExpr.difference(sum, two)

func calculate (_ expr: ArithExpr) -> Int {
    switch expr {
    case let .number(value):
        return value
    case let .sum(left, right):
        return calculate(left) + calculate(right)
    case let .difference(left, right):
        return calculate(left) - calculate(right)
    }
}

calculate(difference)

MemoryLayout

• 可以使用MemoryLayout获取数据类型占用的内存大小

enum Password {
    case number (Int, Int, Int, Int)
    case other
}

MemoryLayout<Password>.stride // 40, 分配占用的空间大小
MemoryLayout<Password>.size // 33, 时机用到的空间大小
MemoryLayout<Password>.alignment // 8, 对齐参数

var pwd = Password.number(9, 8, 6, 4)
pwd = .other
MemoryLayout.stride(ofValue: pwd) // 40
MemoryLayout.size(ofValue: pwd) // 33
MemoryLayout.alignment(ofValue: pwd) // 8