POC for serializing heterogeneous array of classes using Codable in Swift

AnyFoo.swift

import Foundation

enum FooType: String, Codable {
  case bar, baz
}

protocol Foo: AnyObject, Codable {
  var type: FooType { get }
}

extension Foo {
  func encode(to container: inout UnkeyedEncodingContainer) throws {
    try container.encode(self)
  }
}

struct AnyFoo: Codable {
  let value: Foo

  init(_ value: Foo) {
    self.value = value
  }

  init(from decoder: Decoder) throws {
    var container = try decoder.unkeyedContainer()
    switch try container.decode(FooType.self) {
    case .bar:
      self.init(try container.decode(Bar.self))
    case .baz:
      self.init(try container.decode(Baz.self))
    }
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.unkeyedContainer()
    try container.encode(value.type)
    try value.encode(to: &container)
  }
}

class Bar: Foo {
  let type = FooType.bar
  var bar: Int

  init(bar: Int) {
    self.bar = bar
  }

  private enum CodingKeys: CodingKey {
    case bar
  }

  required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    bar = try container.decode(Int.self, forKey: .bar)
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(bar, forKey: .bar)
  }
}

class Baz: Foo {
  let type = FooType.baz
  var baz: String

  init(baz: String) {
    self.baz = baz
  }

  private enum CodingKeys: CodingKey {
    case baz
  }

  required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    baz = try container.decode(String.self, forKey: .baz)
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(baz, forKey: .baz)
  }
}

let coder = JSONEncoder()
let decoder = JSONDecoder()

let values: [Foo] = [
  Bar(bar: 5),
  Baz(baz: "Hello")
]

let typeErasedValues = values.map(AnyFoo.init)
let data = coder.encode(typeErasedValues)

let decodedErasedValues = decoder.decode([AnyFoo].self, from: data)
let decodedValues = decodedErasedValues.map { $0.value }

print((decodedValues[0] as! Bar).bar) // 5
print((decodedValues[1] as! Baz).baz) // Hello

For comparison, here is the struct version: https://gist.github.com/nicklockwood/833fabacbc4b2d11ae7c7d4752b8fd18