enum UserState {
    case LoggedIn(LoggedInState)
    case LoggedOut(LoggedOutState)
}

enum LoggedInState: String {
    case playing
    case paused
    case stopped
}

enum LoggedOutState: String {
    case Unregistered
    case Registered
}

extension UserState: Codable {
    enum CodingKeys: String, CodingKey {
        case loggedIn
        case loggedOut
    }

    enum CodingError: Error {
        case decoding(String)

    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        if let loggedIn = try? values.decode(String.self, forKey: .loggedIn) {
            self = .LoggedIn(LoggedInState(rawValue: loggedIn)!)
        }

        if let loggedOut = try? values.decode(String.self, forKey: .loggedOut) {
            self = .LoggedOut(LoggedOutState(rawValue: loggedOut)!)
        }

        throw CodingError.decoding("Decoding failed")
    }

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

        switch self {
        case let .LoggedIn(value):
            try container.encode(value.rawValue, forKey: .loggedIn)
        case let .LoggedOut(value):
            try container.encode(value.rawValue, forKey: .loggedOut)
        }
    }
}



class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let state = UserState.LoggedIn(.playing)
        UserDefaults.standard.set(encoded, forKey: "state")
    }
}

[User Defaults] Attempt to set a non-property-list object Codable.UserState.LoggedIn(Codable.LoggedInState.playing) as an NSUserDefaults/CFPreferences value for key state
2018-01-20 19:06:26.909349+0200 Codable[6291:789687]