Pfthb

I believe in the case of inheritance you must implement Coding yourself. That is, you must specify CodingKeys and implement init(from:) and encode(to:) in both superclass and subclass. Per the WWDC video (around 49:28, pictured below), you must call super with the super encoder/decoder.

required init(from decoder: Decoder) throws 

 // Get our container for this subclass' coding keys
 let container = try decoder.container(keyedBy: CodingKeys.self)
 myVar = try container.decode(MyType.self, forKey: .myVar)
 // otherVar = ...

 // Get superDecoder for superclass and call super.init(from:) with it
 let superDecoder = try container.superDecoder()
 try super.init(from: superDecoder)

The video seems to stop short of showing the encoding side (but it's container.superEncoder() for the encode(to:) side) but it works in much the same way in your encode(to:) implementation. I can confirm this works in this simple case (see playground code below).

I'm still struggling with some odd behavior myself with a much more complex model I'm converting from NSCoding, which has lots of newly-nested types (including struct and enum) that's exhibiting this unexpected nil behavior and "shouldn't be". Just be aware there may be edge cases that involve nested types.

Edit: Nested types seem to work fine in my test playground; I now suspect something wrong with self-referencing classes (think children of tree nodes) with a collection of itself that also contains instances of that class' various subclasses. A test of a simple self-referencing class decodes fine (that is, no subclasses) so I'm now focusing my efforts on why the subclasses case fails.

Update June 25 '17: I ended up filing a bug with Apple about this. rdar://32911973 - Unfortunately an encode/decode cycle of an array of Superclass that contains Subclass: Superclass elements will result in all elements in the array being decoded as Superclass (the subclass' init(from:) is never called, resulting in data loss or worse).

//: Fully-Implemented Inheritance

class FullSuper: Codable 

 var id: UUID?

 init() 

 private enum CodingKeys: String, CodingKey case id 

 required init(from decoder: Decoder) throws 

 let container = try decoder.container(keyedBy: CodingKeys.self)
 id = try container.decode(UUID.self, forKey: .id)

 

 func encode(to encoder: Encoder) throws 

 var container = encoder.container(keyedBy: CodingKeys.self)
 try container.encode(id, forKey: .id)

 



class FullSub: FullSuper 

 var string: String?
 private enum CodingKeys: String, CodingKey case string 

 override init() super.init() 

 required init(from decoder: Decoder) throws 

 let container = try decoder.container(keyedBy: CodingKeys.self)
 let superdecoder = try container.superDecoder()
 try super.init(from: superdecoder)

 string = try container.decode(String.self, forKey: .string)

 

 override func encode(to encoder: Encoder) throws 

 var container = encoder.container(keyedBy: CodingKeys.self)
 try container.encode(string, forKey: .string)

 let superencoder = container.superEncoder()
 try super.encode(to: superencoder)

 


let fullSub = FullSub()
fullSub.id = UUID()
fullSub.string = "FullSub"

let fullEncoder = PropertyListEncoder()
let fullData = try fullEncoder.encode(fullSub)

let fullDecoder = PropertyListDecoder()
let fullSubDecoded: FullSub = try fullDecoder.decode(FullSub.self, from: fullData)

Both the super- and subclass properties are restored in fullSubDecoded.

Found This Link - Go down to inheritance section

override func encode(to encoder: Encoder) throws 
 try super.encode(to: encoder)
 var container = encoder.container(keyedBy: CodingKeys.self)
 try container.encode(employeeID, forKey: .employeeID)

For Decoding I did this:

 required init(from decoder: Decoder) throws 

 try super.init(from: decoder)

 let values = try decoder.container(keyedBy: CodingKeys.self)
 total = try values.decode(Int.self, forKey: .total)
 

private enum CodingKeys: String, CodingKey

 case total

I was able to make it work by making my base class and subclasses conform to Decodable instead of Codable. If I used Codable it would crash in odd ways, such as getting a EXC_BAD_ACCESS when accessing a field of the subclass, yet the debugger could display all the subclass values with no problem.

Additionally, passing the superDecoder to the base class in super.init() didn't work. I just passed the decoder from the subclass to the base class.

How about using the following way?

protocol Parent: Codable 
 var inheritedProp: Int? get set


struct Child: Parent 
 var inheritedProp: Int?
 var title: String?

 enum CodingKeys: String, CodingKey 
 case inheritedProp = "inherited_prop"
 case title = "short_title"
 

Additional info on composition: http://mikebuss.com/2016/01/10/interfaces-vs-inheritance/

Here is a library TypePreservingCodingAdapter to do just that (can be installed with Cocoapods or SwiftPackageManager).

The code below compiles and works just fine with Swift 4.2. Unfortunately for every subclass you'll need to implement encoding and decoding of properties on your own.

import TypePreservingCodingAdapter
import Foundation

// redeclared your types with initializers
class Server: Codable 
 var id: Int?

 init(id: Int?) 
 self.id = id
 


class Development: Server 
 var name: String?
 var userId: Int?

 private enum CodingKeys: String, CodingKey 
 case name
 case userId
 

 init(id: Int?, name: String?, userId: Int?) 
 self.name = name
 self.userId = userId
 super.init(id: id)
 

 required init(from decoder: Decoder) throws 
 try super.init(from: decoder)
 let container = try decoder.container(keyedBy: CodingKeys.self)

 name = try container.decodeIfPresent(String.self, forKey: .name)
 userId = try container.decodeIfPresent(Int.self, forKey: .userId)
 

 override func encode(to encoder: Encoder) throws 
 try super.encode(to: encoder)
 var container = encoder.container(keyedBy: CodingKeys.self)

 try container.encode(name, forKey: .name)
 try container.encode(userId, forKey: .userId)
 



// create and adapter
let adapter = TypePreservingCodingAdapter()
let encoder = JSONEncoder()
let decoder = JSONDecoder()

// inject it into encoder and decoder
encoder.userInfo[.typePreservingAdapter] = adapter
decoder.userInfo[.typePreservingAdapter] = adapter

// register your types with adapter
adapter.register(type: Server.self).register(type: Development.self)


let server = Server(id: 1)
let development = Development(id: 2, name: "dev", userId: 42)

let servers: [Server] = [server, development]

// wrap specific object with Wrap helper object
let data = try! encoder.encode(servers.map Wrap(wrapped: $0) )

// decode object back and unwrap them force casting to a common ancestor type
let decodedServers = try! decoder.decode([Wrap].self, from: data).map $0.wrapped as! Server 

// check that decoded object are of correct types
print(decodedServers.first is Server) // prints true
print(decodedServers.last is Development) // prints true