Codableでクラス継承対応

import UIKit

var data = """
{
    "A": "A",
    "B": "B",
    "C": "C",
}
"""

class A : Codable {
    
    var A:String = "A"
    
    enum CodingKeys: String, CodingKey {
        case A = "A"
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(A, forKey: .A)
    }
}

class B: A {
    
    var B:String = "B"

    enum CodingKeys: String, CodingKey {
        case B = "B"
        case A = "A"
    }
    
    override func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(B, forKey: .B)
        try container.encode(A, forKey: .A)
    }
}

var b = B()
b.A = "やべい"
b.B = "すごい"

var enc = try! JSONEncoder().encode(b)
print(String(data: enc, encoding: .utf8)!)


var a = A()
a.A = "どすこい"

var enc1 = try! JSONEncoder().encode(a)
print(String(data: enc1, encoding: .utf8)!)