associated type enums

Enum.swift

As brilliantly described here, in Swift4 you can now use Codable protocol to store enum with associated values in NSUserDefaults. You can use something like the following

enum Example {
  case caseOne(a: DownloadState, b: Route)
  case caseTwo(a: String, b: Int, c: Data?)
}

extension Example: Codable {
  private enum CodingKeys: String, CodingKey {
    case base, caseOneParams, caseTwoParams
  }

  private enum Base: String, Codable {
    case caseOne, caseTwo
  }

  private struct CaseOneParams: Codable {
    let a: DownloadState
    let b: Route
  }

  private struct CaseTwoParams: Codable {
    let a: String
    let b: Int
    let c: Data?
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)

    switch self {
    case .caseOne(let a, let b):
      try container.encode(Base.caseOne, forKey: .base)
      try container.encode(CaseOneParams(a: a, b: b), forKey: .caseOneParams)
    case .caseTwo(let a, let b, let c):
      try container.encode(Base.caseTwo, forKey: .base)
      try container.encode(CaseTwoParams(a: a, b: b, c: c), forKey: .caseTwoParams)
    }
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    let base = try container.decode(Base.self, forKey: .base)

    switch base {
    case .caseOne:
      let caseOneParams = try container.decode(CaseOneParams.self, forKey: .caseOneParams)
      self = .caseOne(a: caseOneParams.a, b: caseOneParams.b)
    case .caseTwo:
      let caseTwoParams = try container.decode(CaseTwoParams.self, forKey: .caseTwoParams)
      self = .caseTwo(a: caseTwoParams.a, b: caseTwoParams.b, c: caseTwoParams.c)
    }
  }
}
DownloadState and Route in the example code above have to be Codable types.

And then to write in NSUserDefaults

let example: Example = ... 
UserDefaults.standard.set(try? PropertyListEncoder().encode(example), forKey: "yourExampleKey")
to read

if let data = UserDefaults.standard.value(forKey:"yourExampleKey") as? Data {
    let example = try? PropertyListDecoder().decode(Example.self, from: data)
}