Swift 4 JSONEncoder returns empty JSON array

Question

I have multiple Codeable classes that I want to convert to JSON strings.

class MyCodable: NSObject, Codable {

    override init() {

    }

    func encode(to encoder: Encoder) throws {

    }

}

class Category: MyCodable {

   required init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self);
        //... assign values and call super.init
    }

    override func encode(to encoder: Encoder) throws {

    }

    var categoyId: Int64;
    var categoryName: String;

    enum CodingKeys: String, CodingKey {
        case categoryId
        case categoryName
    }

}

class Product: MyCodable {

    required init(from decoder: Decoder) throws {
        //..assign values and call super.init
    }

    override func encode(to encoder: Encoder) throws {

    }

    var productId: Int64;
    var categoryId: Int64;
    var productName: String;

    enum CodingKeys: String, CodingKey {
        case productId
        case categoryId
        case productName
    }

}

I have a utility class that I am using for the conversion.

class JSONUtil: NSObject {

    public static func encode(objects: [MyCodable]) -> String {
        var json: String = "";
        do {
            let encoder = JSONEncoder();
            encoder.outputFormatting = .prettyPrinted;
            let jsonData = try encoder.encode(objects);
            json = String(data: jsonData, encoding: String.Encoding.utf8)!
        } catch let convertError { json = "[{error: '" + convertError.localizedDescription + "'}]"; }
        return json;
    }

    public static func toJson(object: MyCodable) -> String {
        var objects = [MyCodable]();
        objects.append(object);
        return encode(objects: objects);
    }

}

The return value is always "[{}]" no matter what is inside the array of MyCodable objects. While debugging, I can see the values of the Category object are populated.

Since MyCodable has no properties, is that why the JSONEncoder does not print out the Category properties?.. Or why doesn't it print the Category properties?

I am having the same issue with the Product object.

The goal is to avoid having to customize the JSON conversion for every class I want to print to JSON.

Answer 1

Your mistake is that you're trying to use inheritance instead of generics. Your JSONUtil class should be look like this.

class JSONUtil: NSObject {

    public static func encode<T: Encodable>(objects: [T]) -> String {
        var json: String = "";
        do {
            let encoder = JSONEncoder();
            encoder.outputFormatting = .prettyPrinted;
            let jsonData = try encoder.encode(objects);
            json = String(data: jsonData, encoding: String.Encoding.utf8)!
        } catch let convertError { json = "[{error: '" + convertError.localizedDescription + "'}]"; }
        return json;
    }

    public static func toJson<T: Encodable>(object: T) -> String {
        return encode(objects: [object]);
    }

}

I'm not sure why you're creating classes inheriting from NSObject. You may be stuck in the old Objective-C ways Prefer to use struct over class, unless you always need the object to be copied by reference.

Answer 2

Your reasoning is correct, MyCodable has no properties, so there is nothing to read/write. You have actually stumbled across a bug of the swift JSON Encoder and Decoder. They do not handle inheritance well. So your Product and Category class become anonymous MyCodable class that have nothing to be encoded or decoded.

Looking at your code, I see no reason for the MyCodable class. Simply have Product and Category adhere to the Codable protocol. If you need the polymorphism of being able to refer to Product and Category in the same way, define a protoocl that adheres to Codable and have Product and Category adhere to the protocol you defined. In my eyes this is redundant however there could be use cases where it is a plausible solution.