別のCodingKeysを用意して、複数キーを使ってプロパティを組み立てる例 #CodePiece

CodePiece.swift

import UIKit

func decodeJSON<T: Decodable>(_ t: T.Type, from string: String) {
    let data = string.data(using: .utf8)!

    let decoder = JSONDecoder()
    let s = try! decoder.decode(t, from: data)
    print(s)
}

struct KeyedWithComposed: Decodable {

    enum Kind {
        case image(UIImage)
        case url(URL)
        case text(String)
    }
    let id: Int
    let kind: Kind

    private enum CodingKeys: CodingKey {
        case id
        case kind
    }
    private enum KindCodingKeys: CodingKey {
        case kind
        case image
        case url
        case text
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(Int.self, forKey: .id)
        kind = try type(of: self).makeKind(from: decoder)
    }

    private static func makeKind(from decoder: Decoder) throws -> Kind {
        let composingContainer = try decoder.container(keyedBy: KindCodingKeys.self)

        switch try composingContainer.decode(String.self, forKey: .kind) {
        case "image":
            let imageData = try composingContainer.decode(Data.self, forKey: .image)
            guard let image = UIImage(data: imageData) else {
                throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: [KindCodingKeys.image],
                                                                        debugDescription: "imageのデータがおかしい"))
            }
            return .image(image)
        case "url":
            return .url(try composingContainer.decode(URL.self, forKey: .url))
        case "text":
            return .text(try composingContainer.decode(String.self, forKey: .text))
        case _:
            throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: [CodingKeys.kind],
                                                                        debugDescription: "kindの値が想定外"))
        }
    }
}

decodeJSON(KeyedWithComposed.self, from: """
{
    "id": 100,
    "kind": "text",
    "image": null,
    "url": null,
    "text": "imageやurlはないけどtextはあるよ"
}
""")