nullable.swift

import Foundation

enum Nullable<Value: Codable>: Codable {
    case some(_ value: Value)
    case null
    
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        if container.decodeNil() {
            self = .null
        } else {
            self = .some(try container.decode(Value.self))
        }
    }
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        switch self {
        case .some(let value):
            try container.encode(value)
        case .null:
            try container.encodeNil()
        }
    }
}

struct SomePatchRequest: Codable {
    var nullableIntProperty: Nullable<Int>?
}

let json = #"{"nullableIntProperty": null}"#
//let json = #"{}"#

let data = json.data(using: .utf8)!
// The `Nullable` decode function won't be called since null is treated the same as if the value was missing entirely
// and therefore does not need to be decoded.
let patchRequest = try JSONDecoder().decode(SomePatchRequest.self, from: data)
print(patchRequest)

You can achieve this using a custom decoder for the parent object like this

struct SomeCustomDecoderPatchRequest: Decodable {
    var nullableIntProperty: Nullable<Int>?
    
    enum DecodingKeys: CodingKey { case nullableIntProperty }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: DecodingKeys.self)
        do {
            nullableIntProperty = try container.decode(Nullable<Int>.self, forKey: .nullableIntProperty)
        } catch DecodingError.keyNotFound {
            nullableIntProperty = nil
        }
    }
}

Is there a better way?