Decode JSON nested heterogeneous item with Codable

Before all, calm down and think thoroughly

If you're trying to decode JSON nested heterogeneous item on Codable, you may probably want to do it with manual decoding instead.
Even though we can decode it with Codable without any problem but write your own manual decoding rules could be more intuitive and have better performance.

Ref: Benchmarking Codable JSONDecoder vs. JSONSerialization... FIGHT!

Despite the above, Codable is fun so let's continue anyway.

Restriction

In the context of heterogeneous decoding, it's natural to design the polymorphic structure for decoding type purposes. Here comes the restriction: Swift Codable implementations do not write out subclass type information, the decode() method only static concrete type in the compile-time which means we would lose subclass type information during decode().

Ref: SwiftSR-5331 Swift 4 Decodable Loses Subclass Type Information

The restriction limits our encapsulation approaches.

Solution

Let's say we're trying to decode this JSON data.

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

HeterogeneousClassFamily is designed for expandability.
The business logic of heterogeneous decoding strategy all encapsulate in HeterogeneousClassFamily.