在Swift中解码嵌套JSON对象的便捷方法？

Question

Let's say you have some JSON:

{
    "status": "error",
    "data": {
        "errormessage": "Could not get user with ID: -1.",
        "errorcode": 14
    }
}

For a given Error struct:

struct APIError: Decodable {
    let code: Int?
    let message: String?

    enum CodingKeys: String, CodingKey {
        case code = "errorcode"
        case message = "errormessage"
    }
}

Hit the web service, get the JSON, and initialize the struct:

let urlRequest = URLRequest(url: url)
let session = URLSession.shared
let task = session.dataTask(with: urlRequest)
{ (data, response, error) in
    // Doesn't work because the portion of the JSON we want is in the "data" key
    let e = try? JSONDecoder().decode(APIError.self, from: data)
}
task.resume()

Is there some easy way to do something like data["data"] ? What's the correct model to follow?

Solution A - Convert the data to a JSON object, get the object we want, then convert it to a Data object and decode.

let jsonFull = (try? JSONSerialization.jsonObject(with: data, options: [])) as? [String: Any]
let json = jsonFull["data"]
let data_error = try? JSONSerialization.data(withJSONObject: json, options: [])
let e = try? JSONDecoder().decode(APIError.self, from: data_error)

Solution B - Wrap the target item in another struct

struct temp : Decodable {
    let status: String?
    let data: APIError?
}

let e = try? JSONDecoder().decode(temp.self, from: data).data

Solution C - Set the nested structure in decode (what if it is several objects deep?)

let e = try? JSONDecoder().decode([Any, APIError.self], from: data)

What patterns am I missing? What's the most elegant way to do this?

Answer 1

You can use the following approach:

struct APIError: Decodable {
    let code: Int
    let message: String

    enum CodingKeys: String, CodingKey {
        case data
    }

    enum ErrorCodingKeys: String, CodingKey {
        case code = "errorcode"
        case message = "errormessage"
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let nestedContainer = try container.nestedContainer(keyedBy: ErrorCodingKeys.self, forKey: .data)

        code = try nestedContainer.decode(Int.self, forKey: .code)
        message = try nestedContainer.decode(String.self, forKey: .message)
    }
}

let data = try! JSONSerialization.data(withJSONObject: ["status": "error", "data": ["errorcode": 14, "errormessage": "Could not get user with ID: -1."]], options: [])
let error = try! JSONDecoder().decode(APIError.self, from: data)