如何使用 Codable 处理 JSON 响应的失败案例？

Question

I have some JSON response, that I take from a server. In success case, it might be like:

{
  "success": true, 
  "data": [
    {
       /// something here 
    }
  ]
}

If all server responses would be successful, it would be really easy to parse that JSON. But we have also failure cases like:

{
  "success": false, 
  "msg": "Your session expired",
  "end_session": true 
}

That means we need to handle two cases. As you noticed, attributes like success, msg may occur in any response. In order to handle that, I created following struct:

struct RegularResponse<T: Codable>: Codable {
let success: Bool
let msg: String?
let endSession: Bool?
let data: T?

enum CodingKeys: String, CodingKey {
    case success, msg, data
    case endSession = "end_session"
}

}

It may contain some data if response is successfull or otherwise, it is possible to identify why the error occurred(using success attribute or msg ). Parsing process would go like following:

let model = try JSONDecoder().decode(RegularResponse<MyModel>.self, from: data)

            if model.success {
                // do something with data 
            } else {
               // handle error 
            }

Everything works fine, but what if following JSON comes as following:

{
  "success": true, 
  "name": "Jon Snow", 
  "living_place": "Nights Watch",
  //some other fields
}

Here, I don't have data attribute. It means, my RegularResponse cannot be parsed. So, the question is how to handle these kind of situations? My idea for solution is simple: always put data in success cases into data field on my API. By doing so, my RegularResponse will always work, no matter what is inside data . But, it requires changes on a server side. Can this be fixed in a client side, not changing a server side? In other words, how to handle above situation in Swift using Codable?

Answer 1

I'm not sure if this is the best solution but if you know that your error response is in that shape, ie:

{
  "success": false, 
  "msg": "Some error", 
  "end_session": "true",
}

then you could make another Codable struct/class that follows this response.

struct ErrorResponse: Codable {
    let success: Bool
    let msg: String
    let end_session: String
}

and then when you are responding to your JSON you could adjust your code to:

if let successResponse = try? JSONDecoder().decode(RegularResponse<MyModel>.self, from: data) {
    //handle success
} else if let responseError = try? JSONDecoder().decode(ErrorResponse.self, from data) {
    //handle your error
}