Swift 4 Decodable with keys not known until decoding time

Question

How does the Swift 4 Decodable protocol cope with a dictionary containing a key whose name is not known until runtime? For example:

  [
    {
      "categoryName": "Trending",
      "Trending": [
        {
          "category": "Trending",
          "trailerPrice": "",
          "isFavourit": null,
          "isWatchlist": null
        }
      ]
    },
    {
      "categoryName": "Comedy",
      "Comedy": [
        {
          "category": "Comedy",
          "trailerPrice": "",
          "isFavourit": null,
          "isWatchlist": null
        }
      ]
    }
  ]

Here we have an array of dictionaries; the first has keys categoryName and Trending, while the second has keys categoryName and Comedy. The value of the categoryName key tells me the name of the second key. How do I express that using Decodable?

Answer 1

The key is in how you define the CodingKeys property. While it's most commonly an enum it can be anything that conforms to the CodingKey protocol. And to make dynamic keys, you can call a static function:

struct Category: Decodable {
    struct Detail: Decodable {
        var category: String
        var trailerPrice: String
        var isFavorite: Bool?
        var isWatchlist: Bool?
    }

    var name: String
    var detail: Detail

    private struct CodingKeys: CodingKey {
        var intValue: Int?
        var stringValue: String

        init?(intValue: Int) { self.intValue = intValue; self.stringValue = "\(intValue)" }
        init?(stringValue: String) { self.stringValue = stringValue }

        static let name = CodingKeys.make(key: "categoryName")
        static func make(key: String) -> CodingKeys {
            return CodingKeys(stringValue: key)!
        }
    }

    init(from coder: Decoder) throws {
        let container = try coder.container(keyedBy: CodingKeys.self)
        self.name = try container.decode(String.self, forKey: .name)
        self.detail = try container.decode([Detail].self, forKey: .make(key: name)).first!
    }
}

Usage:

let jsonData = """
  [
    {
      "categoryName": "Trending",
      "Trending": [
        {
          "category": "Trending",
          "trailerPrice": "",
          "isFavourite": null,
          "isWatchlist": null
        }
      ]
    },
    {
      "categoryName": "Comedy",
      "Comedy": [
        {
          "category": "Comedy",
          "trailerPrice": "",
          "isFavourite": null,
          "isWatchlist": null
        }
      ]
    }
  ]
""".data(using: .utf8)!

let categories = try! JSONDecoder().decode([Category].self, from: jsonData)

(I changed isFavourit in the JSON to isFavourite since I thought it was a mispelling. It's easy enough to adapt the code if that's not the case)