iOS development Swift-enumeration

Enum: A group of related values ​​that define a common type, allowing you to use these values ​​in your code in a type-safe manner.

1. Enumeration syntax

//枚举成员不会被赋予默认的整型值。成员本身就是完备的值，类型为CompassPoint。
enum CompassPoint {
    case north
    case south
    case east
    case west
}
//或者
enum Planet {
    case mercury, venus, earth
}

2. The use of enumeration

var direction = CompassPoint.west
//direction: 已被推断类型，所以下次给他赋值的时候：
direction = .east

3. Use the Switch statement to match the enumeration value

direction = .south
switch direction {
    case .north:
        print("north")
    case .south:
        print("south")
    case .east:
        print("east")
    case .west:
        print("west")
}
//强调判断枚举的全部成员，如果没有判断全部成员则报错。所以在不需要判断全部成员时使用default。
switch direction {
    case .north:
        print("north")
    default:
        print("其他")
}

4. Traversal of enumeration members

enum CompassPoint: CaseIterable {
//                         CaseIterable :  协议
    case coffee, tea, juice
}
let number = CompassPoint.allCases.count
//                                    allCases:包含所有枚举成员的集合方法
print("\(number)")

//或者
for com in CompassPoint.allCases{
    print(com)
}

5. Associated value

enum Barcode {
    case upc(Int, Int, Int, Int)
    case qrCode(String)
}
var product = Barcode.upc(8, 85909, 51226, 3)
product = .qrCode("ABCDEFG")

switch product {
    case .upc(let n, let m, let p, let c):
        print("upc:\(n), \(m), \(p), \(c)")
    case .qrCode(let pc):
        print("QR code:\(pc).")
}
//简洁后：
switch product {
    case let .upc(n, m, p, c):
        print("upc:\(n), \(m), \(p), \(c)")
    case let .qrCode(pc):
        print("QR code:\(pc).")
}

6. Raw value

Raw value: The value that was pre-populated when the enum was defined.

enum ASCIICharacter: Character {
    case tab = "\t"
    case lineFeed = "\h"
    case carriageReturn = "\r"
}

7. Implicit assignment of primitive values

(1) When the integer is the original value, the implicit assignment increments by 1. When the original value is not set, it defaults to 0 and increments by 1 thereafter.

enum Planet: Int {
    case m = 1, n, r, j   //只为m赋原始值1
}
var x = Planet.n     //创建枚举变量x，   x的值为n
var y = x.rawValue    //通过rawValue属性来获取枚举的原始值
print(y)    //y的值为2

(2) The string is the original value, and the implicit assignment is the name of the enumeration member.

enum CompassPoint: String {
    case north, south, east, west   //他们的原始值为"north", "south", "east", "west"
}

8. Initialize the enumeration instance with the original value

let poss = Planet(rawValue: 7)
//poss:返回名称。如果没有则返回nil。     7:要查找的值。

9. Recursive enumeration

enum Arith {
    case number(Int)
    indirect case add(Arith, Arith)   //indirect :可递归
    indirect case mul(Arith, Arith)
}
//简化
indirect enum Arith {    //indirect :所有成员可递归
    case number(Int)
    case add(Arith, Arith)
    case mul(Arith, Arith)
}
//使用
let five = Arith.number(5)
let four = Arith.number(4)
let sum = Arith.add(five, four)
let product = Arith.mul(sum, Arith.number(2))