ObjectMapperSucks.swift

import Foundation
import Cocoa

/*
    SETUP
 */

let payload = """
{
    "things": [
        null,
        {
            "number": 3
        }
    ]
}
""".data(using: .utf8)!

struct Number: Decodable {
    let number: Double
    
    init?(dictionary: [String: Any]) {
        guard let number = dictionary["number"] as? Double else {
            return nil
        }
        self.number = number
    }
}
    
struct Payload: Decodable {
    let things: [Number?]
    
    init?(dictionary: [String: Any]) {
        guard let things = dictionary["things"] as? [[String: Any]?] else {
            return nil
        }
        
        /*
         Following code does not distinguish between failure of
         dictionary casting and failure of actual deserialization into Number
         structure. That's why I use a separate `success` flag
         */
        var success = true
        let numbers = things
            .map { dictionary -> Number? in
                guard let dictionary = dictionary else {
                    return nil
                }
                guard let number = Number(dictionary: dictionary) else {
                    success = false // means, actual parsing failed
                    return nil
                }
                return number
            }
        guard success else {
            return nil
        }
        self.things = numbers
    }
}


/*
    ATTEMPTS
 */


let jsonObject = try JSONSerialization.jsonObject(with: payload) as! [String: Any] // this one is guaranteed cast

let parsedObject = Payload(dictionary: jsonObject)
let stuff = parsedObject?.things
// Here, the playgound will be showing proper values, that nils inside array have indeed been parsed successfully

let decoder = JSONDecoder()

let parsedDecodable = try? decoder.decode(Payload.self, from: payload)

let things = parsedDecodable?.things
// Same thing.

// This means that it's the ObjectMapper that is incapable of properly handling nils inside an array and not the Foundation JSONSerialization API.