DecodableDate.swift

import Foundation

extension String {
    var stringToDateCustomMethod: Date {
        return Date()
    }
}

extension KeyedDecodingContainer {
    subscript<T: Decodable>(key: KeyedDecodingContainer.Key) -> T? {
        return try? decode(T.self, forKey: key)
    }
}

struct Post: Decodable {
    var id: Int?
    var userId: Int?
    var date: Date?
    var title: String?
    var subtitle: String?
    
    enum CustomJsonKeys: String, CodingKey {
        case id
        case userId
        case date
        case title = "__title"
        case subtitle = "__subtitle"
    }
    
    init(from decoder: Decoder) throws {
        let decoderContainer = try decoder.container(keyedBy: CustomJsonKeys.self)
        
        self.id = decoderContainer[.id]
        self.userId = decoderContainer[.userId]
        self.date = (decoderContainer[.date] as String?)?.stringToDateCustomMethod
        self.title = decoderContainer[.title]
        self.subtitle = decoderContainer[.subtitle]
    }
}

let postInfoJson = """
{
"userId": 12212,
"id": 112371239,
"date": "22/11/1222",
"__title": "Swift 4 Codable: tips and tricks",
"__subtitle": "Many of you may agree with me when I say the Codable protocol is the best Swift 4 feature, here are some of my impressions about it."
}
"""

let decoder = JSONDecoder()

if let jsonData = postInfoJson.data(using: .utf8) {
    do {
        let post = try decoder.decode(Post.self, from: jsonData)
        print(post)
    } catch let error as NSError {
        print(error)
    }
}