Example demonstrating how to use versioning for Codable structs

CodableVersioning.swift

// This gist demonstrates how you can implement versioning for a Codable struct to support loading
// old serialized data after changing the structure. Notable features of this solution:
//
// * No need to make new properties optional, or to perform post-processing on the struct after
//   loading in ordeer to populate missing values
// * No need to change the call site - from the outside this struct behaves just the same
//   as if we had implemented codable directly in the normal way.
// * Versioning can be applied individually to parents or leaves in a larger tree of
//   structs without affecting the other elements
// * This approach will work even if the original struct was not designed with versioning in mind
//   and was directly serialized without using an intermediate struct.
//
// Note: In this trivial case you could (and probably would) solve the problem in a simpler way
// without having multiple versions of the serialization struct by just making the `id` property
// optional, but the intention is to demonstrate how to handle an arbitrarily complex change.

import Foundation

struct User {
  var name: String
  var id: String
}

extension User: Codable {
  // V1 used email instead of opaque id
  private struct UserV1: Decodable {
    let name: String
    let email: String
  }

  // V2 matches current implementation
  private struct UserV2: Decodable {
    let name: String
    let id: String
  }

  init(from decoder: Decoder) throws {
    do {
      let userV2 = try UserV2(from: decoder)
      self.init(name: userV2.name, id: userV2.id)
    } catch {
      let userV1 = try UserV1(from: decoder)
      self.init(name: userV1.name, id: userV1.email)
    }
  }
}

let oldJSON = """
{
  "name": "Foo Bar",
  "email": "[email protected]"
}
"""

let data = oldJSON.data(using: .utf8)!
let user = try! JSONDecoder().decode(User.self, from: data)
print(user) // User(name: "Foo Bar", id: "[email protected]")