• 寫法(以enum關鍵字開頭,后跟名稱,{}內是枚舉內容,每一個枚舉值使用關鍵字case標志)

enum Week {
    case Monday
    case Tuesday
    case Wednesday
    case Thursday
    case Friday
    case Saturday
    case Sunday
}
enum anotherWeek {
    case Monday,Tuesday,Wednesday,Thursday,Friday,Saturday,Sunday
}

• Swift中,枚舉會定義一個全新的類型

//簡單寫法
enum Week {
    case Monday
    case Tuesday
    case Wednesday
    case Thursday
    case Friday
    case Saturday
    case Sunday
}
//合并寫法
enum anotherWeek {
    case Monday,Tuesday,Wednesday,Thursday,Friday,Saturday,Sunday
}
//定義枚舉變量
var first = Week.Monday
var second = anotherWeek.Saturday
//枚舉值匹配,匹配時,我們必須明確枚舉內的每個值都不會遺漏
switch first {
case .Monday,.Tuesday,.Wednesday,.Thursday,.Friday:
    print("工作日")
case .Saturday,.Sunday:
    print("休息")
}
//當不需要詳盡匹配時,我們也可以使用"default"來標識不需要明確處理的成員
switch second {
case .Monday,.Tuesday,.Wednesday,.Thursday,.Friday:
    print("工作日")
default:
    print("休息")
}

• 值關聯(lián)(我們可以在枚舉中提供相關類型的關聯(lián)值,在使用switch進行匹配時,我們可以用let或者var來提取每個變量,當整個分支語句的所有變量都需要提取時,我們可以把let或者var寫在成員變量前)

enum Request {
    case result(Int,String)
}
var success = Request.result(200, "Success")
switch success {
case .result(let code,let message):
    print("the resultCode is \(code) request \(message)")
}
switch success {
case let .result(code,message):
    print("the resultCode is \(code) request \(message)")
}

• 原始值:
  在Swift中,定義枚舉時,如果我們給枚舉設置類型,那么枚舉的每個成員都會相應的附上一個對應類型的原始值,此時我們可以根據這個原始值來生成枚舉中的一個變量.
  以下面代碼為例,Week(rawValue:str)會返回一個Week?的類型,所以我們對他進行枚舉時,必須確保其不為nil

enum Week:String {
    case Monday
    case Tuesday
    case Wednesday
    case Thursday
    case Friday
    case Saturday
    case Sunday
}
var str = "Monday"
if var strOfWeek = Week(rawValue: str) {
    switch strOfWeek {
    case .Monday,.Tuesday,.Wednesday,.Thursday,.Friday:
        print("工作日")
    default:
        print("休息")
    }
}

• 遞歸枚舉(在成員變量前,加indirect關鍵字,表明該成員可以被遞歸)

enum ArithmeticExpression {
    case number(Int)
    indirect case addition(ArithmeticExpression, ArithmeticExpression)
    indirect case multiplication(ArithmeticExpression, ArithmeticExpression)
}
let value1 = ArithmeticExpression.number(4)
let value2 = ArithmeticExpression.number(5)
let sum = ArithmeticExpression.addition(value1, value2)
let product = ArithmeticExpression.multiplication(sum, ArithmeticExpression.number(2))
func evalute(_ expression:ArithmeticExpression) ->Int {
    switch expression {
    case let .number(a):
        return a
    case let .addition(value1, value2):
        return evalute(value1) + evalute(value2)
    case let .multiplication(value1,value2):
        return evalute(value1) * evalute(value2)
    }
}
evalute(product)