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。
