Handling JSON Array Containing Multiple Types - Swift 4 Decodable

Question

I am trying to use Swift 4 Decodable to parse an array that contains two different types of objects. The data looks something like this, with the included array being the one that contains both Member and ImageMedium objects:

{
  "data": [{
    "id": "8f7cbbac-c133-4b5e-a2ec-1f32353018fa",
    "type": "post",
    "title": "Test Post 1",
    "owner-id": "8986563c-438c-4d77-8115-9e5de2b6e477",
    "owner-type": "member"
  }, {
    "id": "f6b3c640-a58b-449f-93c7-f6cb7b569a9c",
    "type": "post",
    "title": "Test Post 2",
    "owner-id": "38d845a4-db66-48b9-9c15-d857166e255e",
    "owner-type": "member"
  }],
  "included": [{
    "id": "8986563c-438c-4d77-8115-9e5de2b6e477",
    "type": "member",
    "first-name": "John",
    "last-name": "Smith"
  }, {
    "id": "d7218ca1-de53-4832-bb8f-dbceb6747e98",
    "type": "image-medium",
    "asset-url": "https://faketest.com/fake-test-1.png",
    "owner-id": "f6b3c640-a58b-449f-93c7-f6cb7b569a9c",
    "owner-type": "post"
  }, {
    "id": "c59b8c72-13fc-44fd-8ef9-4b0f8fa486a0",
    "type": "image-medium",
    "asset-url": "https://faketest.com/fake-test-2.png",
    "owner-id": "8f7cbbac-c133-4b5e-a2ec-1f32353018fa",
    "owner-type": "post"
  }, {
    "id": "38d845a4-db66-48b9-9c15-d857166e255e",
    "type": "member",
    "first-name": "Jack",
    "last-name": "Doe"
  }]
}

I have tried a bunch of different ways to solve this cleanly using Decodable, but so far the only thing that has worked for me is making one struct for Included that contains all the properties of both objects as optionals, like this:

struct Root: Decodable {
    let data: [Post]?
    let included: [Included]?
}

struct Post: Decodable {
    let id: String?
    let type: String?
    let title: String?
    let ownerId: String?
    let ownerType: String?

    enum CodingKeys: String, CodingKey {
        case id
        case type
        case title
        case ownerId = "owner-id"
        case ownerType = "owner-type"
    }
}

struct Included: Decodable {
    let id: String?
    let type: String?
    let assetUrl: String?
    let ownerId: String?
    let ownerType: String?
    let firstName: String?
    let lastName: String?

    enum CodingKeys: String, CodingKey {
        case id
        case type
        case assetUrl = "asset-url"
        case ownerId = "owner-id"
        case ownerType = "owner-type"
        case firstName = "first-name"
        case lastName = "last-name"
    }
} 

This can work by implementing a method to create Member and ImageMedium objects from the Included struct based off what its type property is, however it's obviously less than ideal. I'm hoping there is a way to accomplish this using a custom init(from decoder: Decoder), but I haven't gotten it to work yet. Any ideas?

Answer 1

I figured out how to decode the mixed included array into two arrays of one type each. Using two Decodable structs is easier to deal with, and more versatile, than having one struct to cover multiple types of data.

This is what my final solution looks like for anyone who's interested:

struct Root: Decodable {
    let data: [Post]?
    let members: [Member]
    let images: [ImageMedium]

    init(from decoder: Decoder) throws {

        let container = try decoder.container(keyedBy: CodingKeys.self)

        data = try container.decode([Post].self, forKey: .data)

        var includedArray = try container.nestedUnkeyedContainer(forKey: .included)
        var membersArray: [Member] = []
        var imagesArray: [ImageMedium] = []

        while !includedArray.isAtEnd {

            do {
                if let member = try? includedArray.decode(Member.self) {
                    membersArray.append(member)
                }
                else if let image = try? includedArray.decode(ImageMedium.self) {
                    imagesArray.append(image)
                }
            }
        }
        members = membersArray
        images = imagesArray
    }

    enum CodingKeys: String, CodingKey {
        case data
        case included
    }
}

struct Post: Decodable {
    let id: String?
    let type: String?
    let title: String?
    let ownerId: String?
    let ownerType: String?

    enum CodingKeys: String, CodingKey {
        case id
        case type
        case title
        case ownerId = "owner-id"
        case ownerType = "owner-type"
    }
}

struct Member: Decodable {
    let id: String?
    let type: String?
    let firstName: String?
    let lastName: String?

    enum CodingKeys: String, CodingKey {
        case id
        case type
        case firstName = "first-name"
        case lastName = "last-name"
    }
}

struct ImageMedium: Decodable {
    let id: String?
    let type: String?
    let assetUrl: String?
    let ownerId: String?
    let ownerType: String?

    enum CodingKeys: String, CodingKey {
        case id
        case type
        case assetUrl = "asset-url"
        case ownerId = "owner-id"
        case ownerType = "owner-type"
    }
}