如何在 swift 4 中使枚举可解码？

Question

enum PostType: Decodable {

    init(from decoder: Decoder) throws {

        // What do i put here?
    }

    case Image
    enum CodingKeys: String, CodingKey {
        case image
    }
}

我要做什么来完成这个？ 另外，假设我将case 更改为：

case image(value: Int)

我如何使它符合 Decodable ？

编辑这是我的完整代码（不起作用）

let jsonData = """
{
    "count": 4
}
""".data(using: .utf8)!

        do {
            let decoder = JSONDecoder()
            let response = try decoder.decode(PostType.self, from: jsonData)

            print(response)
        } catch {
            print(error)
        }
    }
}

enum PostType: Int, Codable {
    case count = 4
}

最终编辑 另外，它将如何处理这样的枚举？

enum PostType: Decodable {
    case count(number: Int)
}

Answer 1

这里有很多好的方法，但我还没有看到一个讨论具有多个值的枚举，尽管它可以从示例中推断出来——也许有人可以找到这个方法的用途：

import Foundation

enum Tup {
  case frist(String, next: Int)
  case second(Int, former: String)
  
  enum TupType: String, Codable {
    case first
    case second
  }
  enum CodingKeys: String, CodingKey {
    case type
    
    case first
    case firstNext
    
    case second
    case secondFormer
  }
  
}

extension Tup: Codable {
  init(from decoder: Decoder) throws {
    let values = try decoder.container(keyedBy: CodingKeys.self)
    let type = try values.decode(TupType.self, forKey: .type)
    switch type {
    case .first:
      let str = try values.decode(String.self, forKey: .first)
      let next = try values.decode(Int.self, forKey: .firstNext)
      self = .frist(str, next: next)
    case .second:
      let int = try values.decode(Int.self, forKey: .second)
      let former = try values.decode(String.self, forKey: .secondFormer)
      self = .second(int, former: former)
    }
  }
  
  func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    
    switch self {
    case .frist(let str, next: let next):
      try container.encode(TupType.first, forKey: .type)
      try container.encode(str, forKey: .first)
      try container.encode(next, forKey: .firstNext)
    case .second(let int, former: let former):
      try container.encode(TupType.second, forKey: .type)
      try container.encode(int, forKey: .second)
      try container.encode(former, forKey: .secondFormer)
    }
    
  }
}

let example1 = Tup.frist("123", next: 90)
do {
  let encoded = try JSONEncoder().encode(example1)
  print(encoded)
  
  let decoded = try JSONDecoder().decode(Tup.self, from: encoded)
  print("decoded 1 = \(decoded)")
}
catch {
  print("errpr = \(error.localizedDescription)")
}


let example2 = Tup.second(10, former: "dantheman")

do {
  let encoded = try JSONEncoder().encode(example2)
  print(encoded)
  
  let decoded = try JSONDecoder().decode(Tup.self, from: encoded)
  print("decoded 2 = \(decoded)")
}
catch {
  print("errpr = \(error.localizedDescription)")
}

Answer 2

特点

• 使用简单。 Decodable 实例中的一行：行 eg let enum: DecodableEnum<AnyEnum>
• 使用标准映射机制解码：JSONDecoder().decode(Model.self, from: data)
• 涵盖接收未知数据的情况（例如，如果您收到意外数据，映射Decodable 对象不会失败）
• 处理/传递mapping 或decoding 错误

详情

• Xcode 12.0.1 (12A7300)
• 斯威夫特 5.3

解决方案

import Foundation

enum DecodableEnum<Enum: RawRepresentable> where Enum.RawValue == String {
    case value(Enum)
    case error(DecodingError)

    var value: Enum? {
        switch self {
        case .value(let value): return value
        case .error: return nil
        }
    }

    var error: DecodingError? {
        switch self {
        case .value: return nil
        case .error(let error): return error
        }
    }

    enum DecodingError: Error {
        case notDefined(rawValue: String)
        case decoding(error: Error)
    }
}

extension DecodableEnum: Decodable {
    init(from decoder: Decoder) throws {
        do {
            let rawValue = try decoder.singleValueContainer().decode(String.self)
            guard let layout = Enum(rawValue: rawValue) else {
                self = .error(.notDefined(rawValue: rawValue))
                return
            }
            self = .value(layout)
        } catch let err {
            self = .error(.decoding(error: err))
        }
    }
}

使用示例

enum SimpleEnum: String, Codable {
    case a, b, c, d
}

struct Model: Decodable {
    let num: Int
    let str: String
    let enum1: DecodableEnum<SimpleEnum>
    let enum2: DecodableEnum<SimpleEnum>
    let enum3: DecodableEnum<SimpleEnum>
    let enum4: DecodableEnum<SimpleEnum>?
}

let dictionary: [String : Any] = ["num": 1, "str": "blablabla", "enum1": "b", "enum2": "_", "enum3": 1]

let data = try! JSONSerialization.data(withJSONObject: dictionary)
let object = try JSONDecoder().decode(Model.self, from: data)
print("1. \(object.enum1.value)")
print("2. \(object.enum2.error)")
print("3. \(object.enum3.error)")
print("4. \(object.enum4)")

Answer 3

这很简单，只需使用隐式分配的 String 或 Int 原始值。

enum PostType: Int, Codable {
    case image, blob
}

image 编码为0 和blob 编码为1

或者

enum PostType: String, Codable {
    case image, blob
}

image 编码为"image" 和blob 编码为"blob"

---

这是一个如何使用它的简单示例：

enum PostType : Int, Codable {
    case count = 4
}

struct Post : Codable {
    var type : PostType
}

let jsonString = "{\"type\": 4}"

let jsonData = Data(jsonString.utf8)

do {
    let decoded = try JSONDecoder().decode(Post.self, from: jsonData)
    print("decoded:", decoded.type)
} catch {
    print(error)
}

Answer 4

如何使关联类型的枚举符合Codable

此答案类似于@Howard Lovatt 的答案，但避免创建PostTypeCodableForm 结构，而是使用KeyedEncodingContainer 类型provided by Apple 作为Encoder 和Decoder 的属性，从而减少样板。

enum PostType: Codable {
    case count(number: Int)
    case title(String)
}

extension PostType {

    private enum CodingKeys: String, CodingKey {
        case count
        case title
    }

    enum PostTypeCodingError: Error {
        case decoding(String)
    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        if let value = try? values.decode(Int.self, forKey: .count) {
            self = .count(number: value)
            return
        }
        if let value = try? values.decode(String.self, forKey: .title) {
            self = .title(value)
            return
        }
        throw PostTypeCodingError.decoding("Whoops! \(dump(values))")
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        switch self {
        case .count(let number):
            try container.encode(number, forKey: .count)
        case .title(let value):
            try container.encode(value, forKey: .title)
        }
    }
}

此代码适用于 Xcode 9b3。

import Foundation // Needed for JSONEncoder/JSONDecoder

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let decoder = JSONDecoder()

let count = PostType.count(number: 42)
let countData = try encoder.encode(count)
let countJSON = String.init(data: countData, encoding: .utf8)!
print(countJSON)
//    {
//      "count" : 42
//    }

let decodedCount = try decoder.decode(PostType.self, from: countData)

let title = PostType.title("Hello, World!")
let titleData = try encoder.encode(title)
let titleJSON = String.init(data: titleData, encoding: .utf8)!
print(titleJSON)
//    {
//        "title": "Hello, World!"
//    }
let decodedTitle = try decoder.decode(PostType.self, from: titleData)

Answer 5

实际上，上面的答案确实很棒，但是它们缺少许多人在不断开发的客户端/服务器项目中需要的一些细节。我们开发一个应用程序，而我们的后端随着时间的推移不断发展，这意味着一些枚举案例将改变这种演变。因此，我们需要一种能够解码包含未知情况的枚举数组的枚举解码策略。否则解码包含数组的对象就会失败。

我做的很简单：

enum Direction: String, Decodable {
    case north, south, east, west
}

struct DirectionList {
   let directions: [Direction]
}

extension DirectionList: Decodable {

    public init(from decoder: Decoder) throws {

        var container = try decoder.unkeyedContainer()

        var directions: [Direction] = []

        while !container.isAtEnd {

            // Here we just decode the string from the JSON which always works as long as the array element is a string
            let rawValue = try container.decode(String.self)

            guard let direction = Direction(rawValue: rawValue) else {
                // Unknown enum value found - ignore, print error to console or log error to analytics service so you'll always know that there are apps out which cannot decode enum cases!
                continue
            }
            // Add all known enum cases to the list of directions
            directions.append(direction)
        }
        self.directions = directions
    }
}

奖励：隐藏实施 > 将其设为集合

隐藏实现细节总是一个好主意。为此，您只需要多一点代码。诀窍是使DirectionsList 符合Collection 并使您的内部list 数组私有：

struct DirectionList {

    typealias ArrayType = [Direction]

    private let directions: ArrayType
}

extension DirectionList: Collection {

    typealias Index = ArrayType.Index
    typealias Element = ArrayType.Element

    // The upper and lower bounds of the collection, used in iterations
    var startIndex: Index { return directions.startIndex }
    var endIndex: Index { return directions.endIndex }

    // Required subscript, based on a dictionary index
    subscript(index: Index) -> Element {
        get { return directions[index] }
    }

    // Method that returns the next index when iterating
    func index(after i: Index) -> Index {
        return directions.index(after: i)
    }
}

您可以在 John Sundell 的这篇博文中了解更多关于遵守自定义集合的信息：https://medium.com/@johnsundell/creating-custom-collections-in-swift-a344e25d0bb0

Answer 6

要扩展@Toka 的答案，您也可以向枚举添加原始可表示值，并使用默认的可选构造函数来构建没有switch 的枚举：

enum MediaType: String, Decodable {
  case audio = "AUDIO"
  case multipleChoice = "MULTIPLE_CHOICES"
  case other

  init(from decoder: Decoder) throws {
    let label = try decoder.singleValueContainer().decode(String.self)
    self = MediaType(rawValue: label) ?? .other
  }
}

它可以使用允许重构构造函数的自定义协议进行扩展：

protocol EnumDecodable: RawRepresentable, Decodable {
  static var defaultDecoderValue: Self { get }
}

extension EnumDecodable where RawValue: Decodable {
  init(from decoder: Decoder) throws {
    let value = try decoder.singleValueContainer().decode(RawValue.self)
    self = Self(rawValue: value) ?? Self.defaultDecoderValue
  }
}

enum MediaType: String, EnumDecodable {
  static let defaultDecoderValue: MediaType = .other

  case audio = "AUDIO"
  case multipleChoices = "MULTIPLE_CHOICES"
  case other
}

如果指定了无效的枚举值，它也可以很容易地扩展为抛出错误，而不是默认值。此处提供了此更改的要点：https://gist.github.com/stephanecopin/4283175fabf6f0cdaf87fef2a00c8128。
代码使用 Swift 4.1/Xcode 9.3 编译和测试。

Answer 7

@proxpero 的一个更简洁的响应变体是将解码器表述为：

public init(from decoder: Decoder) throws {
    let values = try decoder.container(keyedBy: CodingKeys.self)
    guard let key = values.allKeys.first else { throw err("No valid keys in: \(values)") }
    func dec<T: Decodable>() throws -> T { return try values.decode(T.self, forKey: key) }

    switch key {
    case .count: self = try .count(dec())
    case .title: self = try .title(dec())
    }
}

func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    switch self {
    case .count(let x): try container.encode(x, forKey: .count)
    case .title(let x): try container.encode(x, forKey: .title)
    }
}

这允许编译器详尽地验证这些情况，并且在编码值与键的预期值不匹配的情况下也不会抑制错误消息。

Answer 8

如果遇到未知的枚举值，Swift 会抛出 .dataCorrupted 错误。如果您的数据来自服务器，它可以随时向您发送未知的枚举值（错误服务器端、API 版本中添加的新类型以及您希望应用程序的先前版本能够优雅地处理这种情况等），你最好做好准备，编写“防御式”代码来安全地解码你的枚举。

这里是一个关于如何做的例子，有或没有相关值

    enum MediaType: Decodable {
       case audio
       case multipleChoice
       case other
       // case other(String) -> we could also parametrise the enum like that

       init(from decoder: Decoder) throws {
          let label = try decoder.singleValueContainer().decode(String.self)
          switch label {
             case "AUDIO": self = .audio
             case "MULTIPLE_CHOICES": self = .multipleChoice
             default: self = .other
             // default: self = .other(label)
          }
       }
    }

以及如何在封闭结构中使用它：

    struct Question {
       [...]
       let type: MediaType

       enum CodingKeys: String, CodingKey {
          [...]
          case type = "type"
       }


   extension Question: Decodable {
      init(from decoder: Decoder) throws {
         let container = try decoder.container(keyedBy: CodingKeys.self)
         [...]
         type = try container.decode(MediaType.self, forKey: .type)
      }
   }

Answer 9

你可以做你想做的事，但它有点复杂:(

import Foundation

enum PostType: Codable {
    case count(number: Int)
    case comment(text: String)

    init(from decoder: Decoder) throws {
        self = try PostTypeCodableForm(from: decoder).enumForm()
    }

    func encode(to encoder: Encoder) throws {
        try PostTypeCodableForm(self).encode(to: encoder)
    }
}

struct PostTypeCodableForm: Codable {
    // All fields must be optional!
    var countNumber: Int?
    var commentText: String?

    init(_ enumForm: PostType) {
        switch enumForm {
        case .count(let number):
            countNumber = number
        case .comment(let text):
            commentText = text
        }
    }

    func enumForm() throws -> PostType {
        if let number = countNumber {
            guard commentText == nil else {
                throw DecodeError.moreThanOneEnumCase
            }
            return .count(number: number)
        }
        if let text = commentText {
            guard countNumber == nil else {
                throw DecodeError.moreThanOneEnumCase
            }
            return .comment(text: text)
        }
        throw DecodeError.noRecognizedContent
    }

    enum DecodeError: Error {
        case noRecognizedContent
        case moreThanOneEnumCase
    }
}

let test = PostType.count(number: 3)
let data = try JSONEncoder().encode(test)
let string = String(data: data, encoding: .utf8)!
print(string) // {"countNumber":3}
let result = try JSONDecoder().decode(PostType.self, from: data)
print(result) // count(3)