Convenient way to decode nested JSON object in Swift? - json

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)