Decode websocket message (JSON string) to Swift object depending on JSON field value?

Question

in common case you know what type do you need to decode. For example:

func decode<T: Decodable>(_ result: Result<Response, MoyaError>) -> Result<T, Error> {
  let nResult = Result {
    try JSONDecoder().decode(T.self, from: data)
  }
  return nResult
}

Before calling of decode you know which type you need.

But what if T depends on json contents? For example, json contains field "type". It is ok to use switch-case to choose the appropriate class but how to get this "type" value correctly? Should I decode json twice - to get type and to convert to the object I need?

Answer 1

First you need to create model with the field value:

enum FieldValues: String, Decodable {
    case one, two, three
}

struct ResponseTypeModel: Decodable {
   let fieldName: FieldValues
}

Then you can decode it depending on this field:

let type = try JSONDecoder().decode(ResponseTypeModel.self, from: data)
switch type.fieldName {
case .one:
   let result = try JSONDecoder().decode(FirstModel.self, from: data)
case two:
   let result = try JSONDecoder().decode(SecondModel.self, from: data)
case three:
   let result = try JSONDecoder().decode(ThirdModel.self, from: data)
}