Why does @Published attribute cause this error: Type 'Booking' does not conform to protocol 'Decodable'

Question

I ran into a weird problem. Just adding the attribute @Published causes this error: "Type 'Booking' does not conform to protocol 'Decodable'"

THIS compiles fine:

class Booking: ObservableObject, Codable {
    private (set) var module    : Module   
    private (set) var lecturer  : Lecturer? = nil
    
    init(withModul module : Module) {
        self.module = module
    }
}

WHILE here comes the error:

class Booking: ObservableObject, Codable {
    private (set) var module : Module   
    @Published private (set) var lecturer : Lecturer? = nil

    init(withModul module : Module) {
        self.module = module
    }
}

And of course type 'Lecturer' and 'Module' (which are structs) conform both to 'Codable' and also all its properties.

Never had that problem before, any suggestions.

Answer 1

Because you've added the @Published property wrapper to lecturer, the type of that property is now Published<Lecturer> rather than Lecturer. Published doesn't conform to Codable even if it's wrapped Value conforms to Codable.

If you don't want the wrapped property to be part of the encoding/decoding, you can simply omit it from the Codable conformance by creating a CodingKey conformant enum for Booking and omitting the lecturer property from there.

class Booking: ObservableObject, Codable {
    private (set) var module : Module
    @Published private (set) var lecturer : Lecturer? = nil

    init(withModul module : Module) {
        self.module = module
    }

    enum CodingKeys: String, CodingKey {
      case module
    }
}

Alternatively, if you do want encode/decode lecturer, you need to manually implement the Codable conformance.

class Booking: ObservableObject, Codable {
  private (set) var module : Module
  @Published private (set) var lecturer : Lecturer? = nil

  init(withModul module : Module) {
    self.module = module
  }

  enum CodingKeys: String, CodingKey {
    case module
    case lecturer
  }

  required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    self.lecturer = try container.decodeIfPresent(Lecturer.self, forKey: .lecturer)
    self.module = try container.decode(Module.self, forKey: .module)
  }

  func encode(to encoder: Encoder) throws {
    try lecturer?.encode(to: encoder)
    try module.encode(to: encoder)
  }
}