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

Swift would throw a .dataCorrupted error if it encounters unknown enum value. If your data is coming from a server, it can send you an unknown enum value at any time (bug server side, new type added in an API version and you want the previous versions of your app to handle the case gracefully, etc), you'd better be prepared, and code "defensive style" to safely decode your enums.

Here is an example on how to do it, with or without associated value

    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)
          }
       }
    }

And how to use it in a enclosing struct:

    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)
      }
   }