swift 学习（9）枚举（Enumerations）

An enumeration is a list of related values that define a common type and let you work with values in a type-safe way.
枚举是定义一个公共类型的相关值的列表，可让您以类型安全的方式使用值。

声明枚举

enum Month {
  case january
  case february
  case march
  case april
  case may
  case june
  case july
  case august
  case september
  case october
  case november
  case december
}

enum Month {
  case january, february, march, april, may, june, july, august,
  september, october, november, december
}

声明带有计算属性的枚举

enum Month {
    case january, february, march, april, may, june, july, august,
    september, october, november, december
    //let semester = month.semester // "Autumn"
    var semester : String {
        switch self {
        case .august, .september, .october, .november, .december:
            return "Autumn"
        case .january, .february, .march, .april, .may:
            return "Spring"
        case .june, .july:
            return "Summer"
        }
    }
    
}
var month = Month.april
month = .september
let semester = month.semester // "Autumn"

Raw values

Unlike enumeration values in C, Swift enum values are not backed by integers as a default.
与C中的枚举值不同，Swift枚举值默认不支持整数。

需要手动设定

enum Month: Int {
  case january = 1, february, march, april, may, june, july,
  august, september, october, november, december
}

Month.october.rawValue // 10

Initializing with the raw value

let fifthMonth = Month(rawValue: 5)
monthsUntilWinterBreak(from: fifthMonth) // Error: value not unwrapped

 let fifthMonth = Month(rawValue: 5)!
monthsUntilWinterBreak(from: fifthMonth)  // 7

String raw values

enum Icon: String {
  case music
  case sports
  case weather
  var filename: String {
    // 2
    return "\(rawValue.capitalized).png"
  }
}
let icon = Icon.weather
icon.filename // Weather.png

Associated values

• 每个枚举值都有0个或者多个关联值
• 每个枚举值的关联值都有自己的数据类型
• 你可以像定义方法参数一样定义关联值

enum WithdrawalResult {
  case success(newBalance: Int)
  case error(message: String)
}

let result = WithdrawalResult.success(newBalance: 100)
//let result = WithdrawalResult.error(message: "测试错误数据")

switch result {
case .success(let newBalance):
    print("Your new balance is: \(newBalance)")
case .error(let message):
    print(message)
}


switch result {
case .success(let newBalance):
  print("Your new balance is: \(newBalance)")
case .error(let message):
  print(message)
}

//多个关联值
enum WithdrawalResult {
    case success(newBalance: Int,parameters: Int)
    case error(message: String)
}

特殊的条件判断

enum HTTPMethod {
  case get
  case post(body: String)
}

let request = HTTPMethod.post(body: "Hi there")
/* 
 In this code, guard case checks to see 
 if request contains the post enumeration case and if so,
 reads and binds the associated value.
 guard case 判断 request 是否是post ，如果是post，读取并且关联值到body
 */
guard case .post(let body) = request else {
  fatalError("No message was posted")
}
print(body)

//与上面语句判断等同
if case .post(let body) = request  {
    print(body)

}

Optionals
Optionals 的底层实现就是枚举，只是编译器隐藏了实现细节，包括可选值绑定 ？ 和 ！ 操作符，以及nil的实现。

var age: Int?
age = 17
age = nil

switch age {
case .none:
  print("No value")
case .some(let value):
  print("Got a value: \(value)")
}

if let age = age {
    print("\(age)")//17

}

if (age != nil)  {
    print("\(age)")//Optional(17)

}

error
//Optional type 'Int?' cannot be used as a boolean; test for '!= nil' instead
if age {
    print("\(age)")

}

Key points

• An enumeration is a list of related values that define a common type. 枚举是定义公共类型的相关值的列表。

• Enumerations provide an alternative to old-fashioned integer values.枚举提供了一种替代老式整数值的方法

• You can use enumerations to handle responses, store state and encapsulate values.您可以使用枚举来处理响应，存储状态和封装值。

• Case-less enumerations prevent the creation of instances.枚举里枚举值为空不允许创建实例。