Swift5.1 - 枚举enum（4）

1. 枚举的基本用法

• 使用 enum 关键词来定义 枚举；

enum Score {
    case a, b, c, d
}
// 等价于
enum Score {
    case a
    case b
    case c
    case d
}

var score = Score.a
score = .b
print(score) // b

switch score {
case .a:
    print("优秀")
case .b:
    print("良好")
case .c:
    print("及格")
case .d:
    print("不及格")
}

2. 关联值

• 有时会将枚举的成员值跟 其他类型 的关联存在一起；

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

var score = Score.point(90)
score = .grade("A")

switch score {
case let .point(s):      // 获取关联值
    print("score = \(s)")
case let .grade(g):      // 获取关联值
    print("core = \(g)")
}

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

var date = Date.digit(year: 2019, month: 6, day: 25)
date = .string("2019-06-25")

switch date {
case .digit(let year,let month, let day):  // 获取关联值
    print("year = \(year) , month = \(month), day = \(day)")
case .string(let dateStr):  // 获取关联值
    print(dateStr)
}

• 必要的时候，可以将 let 改为 var；

• 关联值实际使用场景；

  
  

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

var pw = Password.number(2, 4, 6, 8)
pw = .gesture("2468")

switch pw {
//case .number(let n1, let n2, let n3, let n4):
case let .number(n1, n2, n3, n4):  // 或者这样写，两种写法等价
    print("n1 = \(n1), n2 = \(n2), n3 = \(n3), n4 = \(n4)")
case let .gesture(g):
    print("gesture = \(g)")
}

• 关联值的底层存储原理（类似 共用体）：
  1. 1个字节存储成员值；
  2. N个字节存储关联值（N取占用内存最大的关联值），任何一个case的关联值都共用这N个字节；
  3. 所以整形关联值的枚举占用的内存大小为：(1 + n) * 8个字节, n 为最大关联数；

 enum TestEnum {
        case test1(Int, Int, Int)
        case test2(Int, Int)
        case test3(Int)
        case test4(Bool)
        case test5
    }
    
    // 1个字节存储成员值
    // N个字节存储关联值（N取占用内存最大的关联值），任何一个case的关联值都共用这N个字节
    // 共用体
    
    // 小端：高高低低
    // 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
    // 00 00 00 00 00 00 00
    var e = TestEnum.test1(1, 2, 3)
    print(Mems.ptr(ofVal: &e))
    
    // 04 00 00 00 00 00 00 00
    // 05 00 00 00 00 00 00 00
    // 00 00 00 00 00 00 00 00
    // 01
    // 00 00 00 00 00 00 00
    e = .test2(4, 5)
    print(Mems.memStr(ofVal: &e))
    
    // 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
    // 00 00 00 00 00 00 00
    e = .test3(6)
    
    // 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
    // 00 00 00 00 00 00 00
    e = .test4(true)
    
    // 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
    // 00 00 00 00 00 00 00
    e = .test5

3. 原始值 (rawValue)

• 枚举成员可以使用相同类型的默认值预先关联，这个默认值叫做 原始值(rawValue)；
• 原始值 不占用枚举变量内存；

enum Direction: String {
    case east = "东"
    case west = "西"
    case south = "南"
    case north = "北"
}

var direction = Direction.east
print(direction.rawValue)  // 东
direction = .west
print(direction.rawValue)  // 西

4. 隐式原始值

• 如果枚举的原始值类型为Int、String，swift将会自动设置原始值，如下:

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

// 等价于
enum Direction: String {
    case east, west, south, north
}

print(Direction.east.rawValue)  // east
print(Direction.west.rawValue)  // west

5. 递归枚举

• 使用 indirect 关键词，实现 enum 递归；

indirect enum Calculation {
    case number(Int)
    case sum(Calculation, Calculation)
}

// 也可以这样写
//enum Calculation {
//    case number(Int)
//    indirect case sum(Calculation, Calculation)
//}

let five = Calculation.number(5)
let four = Calculation.number(4)
let sum = Calculation.sum(five, four)

func calculation(_ cal: Calculation) -> Int {
    switch cal {
    case let .number(value):
        return value
    case let .sum(cal1, cal2):
        return calculation(cal1) + calculation(cal2)
    }
}

print("sum = \( calculation(sum) )")  // 9

6. MemoryLayout

• 可以使用 MemoryLayout 来获取 数据类型 占用内存的大小；

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

print( MemoryLayout<Date>.size )      // 实际占用的内存大小  25 = 3 * 8 + 1   三个整形（每个占用8个字节） + 1个字节（成员值类型 1个字节）
print( MemoryLayout<Date>.stride )    // 系统分配的内存  32 = 4 * 内存对齐参数
print( MemoryLayout<Date>.alignment ) // 内存对齐参数  8

let date = Date.digit(year: 2019, month: 6, day: 25)
print( MemoryLayout.size(ofValue: date) )      // 25
print( MemoryLayout.stride(ofValue: date) )    // 32
print( MemoryLayout.alignment(ofValue: date) ) // 8

let date = Date.other
print( MemoryLayout.size(ofValue: date) )      // 25
print( MemoryLayout.stride(ofValue: date) )    // 32
print( MemoryLayout.alignment(ofValue: date) ) // 8

源代码demo