Swift - JSONDecoder - decoding generic nested classes

Question

I am very new to Swift and I am trying to create a tree of classes to model my data. I want to use JSONEncoder and JSONDecoder to send and receive objects. I have an issue when decoding generic classes inside another object (nested) because in the init(from: decoder) method I do not have access to other properties that could help me.

In my code:

NestedSecondObject extends NestedObjects, which extends Codable - NestedObject can be extended by NesteThirtObject and so on...

Contact extends Object1, which extends Codable; Contact contains a NestedObject type (which can be any subclass of NestedObject at runtime)

Because JSONEncoder and JSONDecoder do not support inheritance by default, i override the methods "encode" and init(from: decoder) as described here: Using Decodable in Swift 4 with Inheritance

My code is:

class NestedSecondObject: NestedObject {
    var subfield2: Int?

    private enum CodingKeys : String, CodingKey {
        case subfield2
    }

    override init() { super.init() }

    override func encode(to encoder: Encoder) throws {
        try super.encode(to: encoder)
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(subfield2, forKey: .subfield2)
    }

    required init(from decoder: Decoder) throws
    {
        try super.init(from: decoder)
        let values = try decoder.container(keyedBy: CodingKeys.self)
        self.subfield2 = try values.decode(Int.self, forKey: .subfield2)

    }

}


class Contact:Object1 {
    var name: String = ""
    var age: Int = 0
    var address: String = ""
//    var className = "biz.ebas.platform.generic.shared.EContactModel"
    var nestedObject:NestedObject?

    private enum CodingKeys : String, CodingKey {
        case name,age,address,nestedObject
    }

    override init() { super.init() }

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

    required init(from decoder: Decoder) throws
    {
        try super.init(from: decoder)
        print(type(of: self))
        print(type(of: decoder))

        let values = try decoder.container(keyedBy: CodingKeys.self)
        print(type(of: values))


        self.name = try values.decode(String.self, forKey: .name)
        self.age = try values.decode(Int.self, forKey: .age)
        self.address = try values.decode(String.self, forKey: .address)
        self.nestedObject = try values.decodeIfPresent(???.self, forKey: .nestedObject)  // HERE i need to know what ???.self is
    }
}

Decoding is:

let jsonDec = JSONDecoder()
let jsonData = json?.data(using: .utf8)
let decodedContact: Contact = try jsonDec.decode(Contact.self, from: jsonData!)

So, basically, when I make a request to the server, I know what types I receive (let's say I request NestedSecondObject), but how do I pass it to the method "init(from decoder:Decoder)" ?

I tried to extend class JSONDecoder and add a simple property like this:

class EJSONDecoder: JSONDecoder {

    var classType:AnyObject.Type?

}

But inside the required init(from decoder:Decoder) method, the type of decoder is not EJSONDecoder, is _JSONDecoder, so I cannot access the decoder.classType property.

Can anyone help with a solution or some sort of workaround? Thanks!

Generally I'd say you have to inspect the "raw" JSON data of the nested object to figure out what type it is, then use this type for decoding. — dr_barto

Fabian Fabian · Accepted Answer · 2018-07-26T13:31:45

New answer

You can give the decoder a userInfo array where you can store things you want to use during decoding.

let decoder = JSONDecoder()
decoder.userInfo = [.type: type(of: NestedSecondObject.self)]
let decodedContact: Contact = try! decoder.decode(Contact.self, from: json)

extension CodingUserInfoKey {
    static let type = CodingUserInfoKey(rawValue: "Type")!
}

Then use it during decoding:

switch decoder.userInfo[.type]! {
case let second as NestedSecondObject.Type:
    self.nestedObject = try values.decode(second, forKey: .nestedObject)
case let first as NestedObject.Type:
    self.nestedObject = try values.decode(first, forKey: .nestedObject)
default:
    fatalError("didnt work")
}

I have sadly not found a way to skip the switch.

Old answer:

Decode as

NestedSecondObject.self

and if that fails decode inside the catch with

NestedObject.self

. Do not catch the NestedObject-decoding cause you want to fail if its not even decodable to the basic type.

Swift - JSONDecoder - decoding generic nested classes

1 Answers

New answer

Old answer: