swift4 - Inheritance of Encodable Class

Question

I am writing a program using Swift 4 and Xcode 9.2. I have faced difficulties with writing encodable class (exactly class, not struct). When I am trying to inherit one class from another, JSONEncoder does not take all properties from sub class (child). Please look at this:

class BasicData: Encodable {

    let a: String
    let b: String

    init() {
        a = "a"
        b = "b"
    }
}

class AdditionalData: BasicData {

    let c: String

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

}

let encode = AdditionalData(c: "c")

do {
    let data = try JSONEncoder().encode(encode)
    let string = String(data: data, encoding: .utf8)
    if let string = string {
        print(string)
    }
} catch {
}

It will print this: {"a":"a","b":"b"}

But I need this: {"a":"a","b":"b","c":"c"}

It look like c property of class AdditionalData just lost somewhere and somehow.

So question is: if I have class signed with protocol Encodable how to make sub class (child of this class, inherit) class properly?

I will be thankful for any help or advice.

Answer 1

Encodable and Decodable involve some code synthesis where the compiler essentially writes the code for you. When you conform BasicData to Encodable, these methods are written to the BasicData class and hence they are not aware of any additional properties defined by subclasses. You have to override the encode(to:) method in your subclass:

class AdditionalData: BasicData {
    let c: String
    let d: Int

    init(c: String, d: Int) {
        self.c = c
        self.d = d
    }

    private enum CodingKeys: String, CodingKey {
        case c, d
    }

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

See this question for a similar problem with Decodable.