Codable class does not conform to protocol Decodab

问题:

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.

回答1:

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.

回答2:

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)
    }
}

回答3:

On the hindsight, I received a similar error when trying to set Codeable 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

回答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.

回答5:

insort, While implementing Codable, all properties which are none primitive data type (mean class type or may it can be objective-c class) must be Codable.

   weak var publication: Publication?

in this case publication is of type class so Publication must have implemented Codable

回答6:

I had a similar issue which I stumbled upon this fix to. As I am new to Swift, I am unsure as to why it works! If anyone knows I would appreciate the knowledge.

I changed this:

let id, type: Int

to this:

let id: Int
let type: Int