《Swift从入门到精通》（三）：枚举及内存布局

• 枚举的基本用法

enum Direction {
    case north
    case south
    case east
    case west
}
enum Direction {
    case north, south, east, west
}
// 以上两种写法是等价
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(90)
score = .grade("A")
switch score {
case .points(let i):
    print("points", i)
case let .grade(i):
   print("grade", i)

}
enum Date {
    case digit(year: Int, mouth: Int, day: Int)
    case string(String)
}
var date = Date.digit(year: 2021, mouth: 9, day: 27)
//date = .string("2021-09-27")
switch date {
case let .digit(a, b, c):
    print(a, b, c)
case .string(let a):
    print(a)
}

• 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， 原始值不占用枚举变量的内存

//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)
print(Grade.great.rawValue)
print(Grade.good.rawValue)
print(Grade.bad.rawValue)

• 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 = "east"
    case west = "west"
}
// 上面的枚举变量的原始值等价下面
enum Direction : String {
    case north, south, east, west
}
print(Direction.north) // north
print(Direction.north.rawValue) // north
enum Season : Int {
    case spring, summer, autumn = 4, winter
}
print(Season.spring.rawValue) // 0
print(Season.summer.rawValue) // 1
print(Season.autumn.rawValue) // 4
print(Season.winter.rawValue) // 5

• 递归枚举（Recursive Enumeration）

indirect enum ArithExpr {
    case number(Int)
    case sum(ArithExpr, ArithExpr)
    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 diff = ArithExpr.difference(sum, two)
func cal(_ expr: ArithExpr) -> Int {
    switch expr {
    case let .number(value):
        return value
    case let .sum(left, right):
        return cal(left) + cal(right)
    case let .difference(left, right)
        return cal(left) - cal(right)
    }
}
cal(difference) // 7

• 枚举的内存布局

enum TestEnum {
case test0(Int, Int, Int)
case test1(Int, Int)
case test2(Int)
case test3(Bool)
case test4
}
print(MemoryLayout.size)      // 25 实际用到的内存
print(MemoryLayout.stride)    // 32 申请分配的内存
print(MemoryLayout.alignment) // 8 内存对其参数
// 一共申请32个字节，实际用到25个字节，实际要用到的内存要按关联值占用到的最大的内存，在本例中是test0最大，关联3个Int类型，在64位系统中每个Int占用8个字节
// 因此要用24个字节，另外要一个字节即第25个字节用来标识枚举成员值，后面的7个字节用来内存补齐，以满足内存对齐
// 小端模式
// 01 00 00 00 00 00 00 00
// 02 00 00 00 00 00 00 00
// 03 00 00 00 00 00 00 00
// 00 (这一个字节标识枚举的成员值，0代表test0）
// 00 00 00 00 00 00 00
var t = TestEnum.test0(1, 2, 3)
// 小端模式
// 04 00 00 00 00 00 00 00
// 04 00 00 00 00 00 00 00
// 00 00 00 00 00 00 00 00
// 01 (这一个字节标识枚举的成员值，1代表test1）
// 00 00 00 00 00 00 00
t = .test1(4, 5)
// 小端模式
// 06 00 00 00 00 00 00 00
// 00 00 00 00 00 00 00 00
// 00 00 00 00 00 00 00 00
// 02 (这一个字节标识枚举的成员值，2代表test2）
// 00 00 00 00 00 00 00
t = .test2(6)
// 小端模式
// 01 00 00 00 00 00 00 00
// 00 00 00 00 00 00 00 00
// 00 00 00 00 00 00 00 00
// 03 (这一个字节标识枚举的成员值，3代表test3）
// 00 00 00 00 00 00 00
t = .test3(true)
// 小端模式
// 00 00 00 00 00 00 00 00
// 00 00 00 00 00 00 00 00
// 00 00 00 00 00 00 00 00
// 04 (这一个字节标识枚举的成员值，4代表test4）
// 00 00 00 00 00 00 00
t = .test4