Make class conforming to Codable by default in Swift

Question

Such feature of Swift as Codable (Decodable&Encodable) protocol is very useful. But I found such issue: Let we have class Parent conforming to Codable:

class Parent: Codable {
    var name: String
    var email: String?
    var password: String?
}

Ok, that class is conforming to Codable protocol "from the box", you don't need write any initializers, it's ready to be initialized from JSON like that:

{ "name": "John", "email": "johndoe@yahoo.com", "password": <null>}

But let's say we need other class, Child inherits from Parent and be conforming to Codable:

class Child: Parent {
   var token: String
   var date: Date?
}

so class Child must be conforming to Codable by conformance to Parent, BUT properties of class Child won't be initialized from JSON properly. Decision I found is write all Codable stuff for class Child by myself, like:

class Child: Parent {
    var token: String
    var date : Date?

    enum ChildKeys: CodingKey {
        case token, date
    }

    required init(from decoder: Decoder) throws {
        try super.init(from: decoder)
        let container = try decoder.container(keyedBy: ChildKeys.self)
        self.token = try container.decode(String.self, forKey: .token)
        self.date = try container.decodeIfPresent(Date.self, forKey: .date)
    }

    override func encode(to encoder: Encoder) throws {
        try super.encode(to: encoder)
        var container = encoder.container(keyedBy: ChildKeys.self)
        try container.encode(self.token, forKey: .token)
        try container.encodeIfPresent(self.date, forKey: .date)
    }
}

But I feel it can't be right, did I missed something? How to make class Child conforming to Codable properly without writing all that stuff?

Possible duplicate of [Using Decodable in Swift 4 with Inheritance](https://stackoverflow.com/questions/44553934/using-decodable-in-swift-4-with-inheritance) — J. Doe, Aug 31 '18 at 13:16
Same problem everywhere you look. In .Net, they use explicit uninherited attributes to describe serialization on a type...so that you're not lulled into thinking there's any way to inherit serializability. Swift's approach strikes me as an anti-pattern...leaving a consumer to expect that a subtype of a conforming type to conform at birth. Sure it technically does...but as you have identified, it breaks. — Clay, Aug 31 '18 at 13:39

score 12 · Accepted Answer · answered Aug 31 '18 at 13:25

Here's a good blog post which includes an answer to your question: source

Scroll down to inheritance and you'll see the following:

Assuming we have the following classes:

class Person : Codable {
    var name: String?
}

class Employee : Person {
    var employeeID: String?
}

We get the Codable conformance by inheriting from the Person class, but what happens if we try to encode an instance of Employee?

let employee = Employee()
employee.employeeID = "emp123"
employee.name = "Joe"

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let data = try! encoder.encode(employee)
print(String(data: data, encoding: .utf8)!)

{
  "name" : "Joe"
}

This is not the expected result, so we have to add a custom implementation like this:

class Person : Codable {
    var name: String?

    private enum CodingKeys : String, CodingKey {
        case name
    }

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

Same story for the subclass:

class Employee : Person {
    var employeeID: String?

    private enum CodingKeys : String, CodingKey {
        case employeeID = "emp_id"
    }

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

The result would be:

{
  "emp_id" : "emp123"
}

Which again is not the expected result, so here we are using inheritance by calling super.

// Employee.swift
    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)
    }

Which finally gives us what we really wanted from the beginning:

{
    "name": "Joe",
    "emp_id": "emp123"
}

If you're not happy with the flattened result, there's a tip on how to avoid that too.

All the credits to the guy who wrote the blog post and my thanks. Hope it helps you as well, cheers!

Make class conforming to Codable by default in Swift

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