How do I make an enum Decodable in swift 4?

Question

enum PostType: Decodable {

    init(from decoder: Decoder) throws {

        // What do i put here?
    }

    case Image
    enum CodingKeys: String, CodingKey {
          


        

Answer 1

To extend on @Toka's answer, you may too add a raw representable value to the enum, and use the default optional constructor to build the enum without a switch:

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
  }
}

It may be extended using a custom protocol that allows to refactor the constructor:

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
}

It can also be easily extended for throwing an error if an invalid enum value was specified, rather than defaulting on a value. Gist with this change is available here: https://gist.github.com/stephanecopin/4283175fabf6f0cdaf87fef2a00c8128.
The code was compiled and tested using Swift 4.1/Xcode 9.3.