Codable class does not conform to protocol Decodable

Why am I getting a "Type 'Bookmark' does not conform to protocol 'Decodable'" error message?

class Bookmark: Codable {
   weak var publication: Publication?
   var indexPath: [Int]
   var locationInText = 0

   enum CodingKeys: String, CodingKey {
      case indexPath
      case locationInText
   }

   init(publication: Publication?, indexPath: [Int]) {
      self.publication = publication
      self.indexPath = indexPath
   }
}

I do not wish to save the publication var since the Publication owns the Bookmark but the bookmark needs to know which Publication it belongs to. The decode init of Publication will set the bookmark reference to itself.

Solution 1:

The compiler cannot synthesise the required init(from:) method due to the weak reference, so you need to write it yourself.

class Bookmark: Codable {
    weak var publication: Publication?
    var indexPath: [Int]
    var locationInText = 0

    private enum CodingKeys: String, CodingKey {
        case indexPath
        case locationInText
    }

    init(publication: Publication?, indexPath: [Int]) {
        self.publication = publication
        self.indexPath = indexPath
    }

    required init(from decoder:Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        indexPath = try values.decode([Int].self, forKey: .indexPath)
        locationInText = try values.decode(Int.self, forKey: .locationInText)
    }
}

Solution 2:

Why am I getting a "Type 'Bookmark' does not conform to protocol 'Decodable'" error message

It's either because Publication isn't Decodable (you have not shown what it is, so it's hard to tell) or because of the weak designation on publication.

Either way, it's easy to fix: you just need to implement init(from:) to complete the implementation of Decodable; the compiler is simply telling you that this implementation cannot be synthesized.

Solution 3:

On the hindsight, I received a similar error when trying to set Codable to my class which consisted of NSNumber type variables. See image below:

Changing NSNumber to primitive data type Int resolved the issue. See below:

I'm guessing this might be true for other datatypes that require bridging to Swift Standard Library Value Types such as NSString, NSArray and so on

Solution 4:

Another reason you could get this message is if your CodingKeys enum isn't exhaustive. If you have three properties in the data type, then your CodingKeys enum needs to have three property/name cases as well.

Solution 5:

Simply because your CodingKeys enum is not exhaustive, add publication property to the enum to achieve that.

try this:

class Bookmark: Codable {
   weak var publication: Publication?
   var indexPath: [Int]
   var locationInText = 0

   // All your properties should be included
   enum CodingKeys: String, CodingKey {
      case indexPath
      case locationInText
      case publication   // this one was missing
   }
}

You wont need the init method anymore as the implementation now can be synthesized.