Swift-----枚举enum

1 定义枚举时，如果不写后面等于的值，默认是从0开始的，依次加1.

enum Month: Int {
    case January = 1
    case February = 2
    case March = 3
    case April = 4
    case May = 5
    case June = 6
    case July = 7
    case August = 8
    case September = 9
    case October = 10
    case November = 11
    case December = 12
}

enum Season {
    case Spring, Summer, Autumn, Winter
}

func season(month: Month) -> Season {
    switch (month) {
    case .March, .April, .May:
        return .Spring
    case .June, .July, .August:
        return .Summer
    case .September, .October, .November:
        return .Autumn
    case .December, .January, .February:
        return .Winter
    }
}

func monthBeforeNewYear(month: Month) -> Int {
    return 12 - month.rawValue
}

let month = Month.April
monthBeforeNewYear(month: month)

let month1 = Month(rawValue: 4)
if let month1 = month1 {
    monthBeforeNewYear(month: month1)
}

2 枚举的类型，默认是Int类型，也可以是其他类型，例如字符串。

　　如果不写值，默认就是定义case时的字符串

enum pargramingLanguage: String {
    case Swift
    case ObjecticeC = "Objective-C"
    case JavaScript = "javaScript"
    case PHP
}

let language = pargramingLanguage.Swift
print("\(language.rawValue) is my favorite language.")　　// Swift is my favorite language.

3 Raw Value

enum Money: Int {
    case Yuan = 100
    case Jiao = 10
    case Fen = 1
}

3 Associate Value(关联值)

enum ATMState {
    case Success(Int)
    case Error(String)
    case Waiting
}

var balance = 1000

func withdraw(amount: Int) -> ATMState {
    if balance >= amount {
        return .Success(balance - amount)
    } else {
        return .Error("Not enough money.")
    }
}

let result = withdraw(amount: 400)
switch result {
case let .Success(newBalance):
    print("\(newBalance) yuan left in your amount.")
case let .Error(errorMessage):
    print("error: \(errorMessage)")
case .Waiting:
    print("Waiting")
}

// 也可以不用解析关联值
switch result {
case .Success:
    print("Success")
case .Error:
    print("Error")
case .Waiting:
    print("Waiting")
}

4 也可以关联多个值

enum Shape {
    case Square(side: Double)
    case Rectangle(width: Double, height: Double)
    case Circle(centerx: Double, centery: Double, radius: Double)
    case Point
    
    func area() -> Double {
        switch self {
        case let .Square(side):
            return side * side
        case let .Rectangle(width, height):
            return width * height
        case let .Circle(_, _, radius):
            return radius * radius
        case .Point:
            return 0
        }
    }
}

let square = Shape.Square(side: 20)
let rectangle = Shape.Rectangle(width: 10, height: 20)
let circle = Shape.Circle(centerx: 0, centery: 0, radius: 10)
let point = Shape.Point

square.area()
rectangle.area()
circle.area()
point.area()

5 可选型（Optional）实际上就是枚举

var string: Optional<String> = Optional.some("A string")　　// Optional<String>实际上就是String?

switch (string) {
case .none:
    print("String is nil.")
case let .some(value):
    print("String is \(value).")
}

// 解包简化为：
if let string = string {
    print("String is \(string).")
}

string = .none        // nil

 6 枚举递归（在enum前面加indirect关键字，或者是在需要递归的case语句的case前面加关键字）

indirect enum ArithmeticExpression {
    case Number(Int)
    case Addition(ArithmeticExpression, ArithmeticExpression)
    case Multiplication(ArithmeticExpression, ArithmeticExpression)
}
// 或者可以这样定义枚举
enum ArithmeticExpression {
　　case Number(Int)
　　indirect case Addition(ArithmeticExpression, ArithmeticExpression)
　　indirect case Multiplication(ArithmeticExpression, ArithmeticExpression)
}

// 计算这个表达式 (5 + 4) * 2
let five = ArithmeticExpression.Number(5)
let four = ArithmeticExpression.Number(4)
let add = ArithmeticExpression.Addition(five, four)
let two = ArithmeticExpression.Number(2)
let result = ArithmeticExpression.Multiplication(add, two)

func evaluate(expression: ArithmeticExpression) -> Int {
    switch expression {
    case let .Number(number):
        return number
    case let .Addition(num1, num2):
        return evaluate(expression: num1) + evaluate(expression: num2)
    case let .Multiplication(num1, num2):
        return evaluate(expression: num1) * evaluate(expression: num2)
    }
}

evaluate(expression: result)　　// 18　　　　

7 枚举中也可以定义方法

enum Shape {
    case Square(side: Double)
    case Rectangle(width: Double, height: Double)
    case Circle(centerx: Double, centery: Double, radius: Double)
    case Point
    
    func area() -> Double {
        switch (self) {
        case let .Square(side):
            return side * side
        case let .Rectangle(width, height):
            return width * height
        case let .Circle(_, _, radius):
            return M_PI * radius * radius
        case .Point:
            return 0
        }
    }
}

let square = Shape.Square(side: 20)
let rectangle = Shape.Rectangle(width: 10, height: 20)
let circle = Shape.Circle(centerx: 0, centery: 0, radius: 10)
let point = Shape.Point

square.area()
rectangle.area()
circle.area()
point.area()

8 枚举也是值类型

let circle = Shape.Circle(centerx: 0, centery: 0, radius: 10)
var point = circle
point = Shape.Point

point　　 // Point
circle　　// Circle(0.0, 0.0, 10.0)

9 如果要在一个枚举中定义一个方法来改变自身的状态，需要在方法前添加关键字mutating

enum Swich {
    case on
    case off
    
    mutating func changeState() {
        switch(self) {
        case .off:
            self = .on
        case .on:
            self = .off
        }
    }
}