Make class conforming to Codable by default in Swift - json

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?

Answer 1

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!