codable_polymorphism.swift

import Foundation

let json = """
{
   "results":[
      {
         "blah":"blah",
         "nested_object":{
            "type":"a",
            "id":69,
            "aVar":"aaa"
         }
      },
      {
         "blah":"blah",
         "nested_object":{
            "type":"b",
            "id":42,
            "bVar":"bbb"
         }
      }
   ]
}
"""

// Types

protocol AbstractType {
    var id: Int { get set }
}

struct SubtypeA: AbstractType {
    var id: Int
    var aVar: String
}

struct SubtypeB: AbstractType {
    var id: Int
    var bVar: String
}

// Response Types

struct Response: Codable {
    var results: [Result]
}
struct Result: Codable {
    var blah: String
    var nested_object: NestedObject
}
struct NestedObject: Codable {
    var type: String
    var id: Int
    var aVar: String?
    var bVar: String?
    func getObject() -> AbstractType {
        switch type {
        case "a":
            return SubtypeA(id: id, aVar: aVar!)
        case "b":
            return SubtypeB(id: id, bVar: bVar!)
        default:
            fatalError() 
        }
    }
}

// Usage

let response = try JSONDecoder().decode(Response.self, from: json.data(using: .utf8)!)
let nestedObjects = response.results.map { $0.nested_object.getObject() }
let isA = nestedObjects[0] is SubtypeA
let isB = nestedObjects[1] is SubtypeB