ymd.swift

import Foundation

@propertyWrapper
struct YMD {
  var wrappedValue: Date?
}

extension YMD: Codable {
  func encode(to encoder: Encoder) throws {
    if let date = self.wrappedValue {
      let dateFormatter = DateFormatter()
      dateFormatter.dateFormat = "YYYY/MM/dd"
      let string = dateFormatter.string(from: date)

      var container = try encoder.singleValueContainer()
      try container.encode(string)
    }
  }

  init(from decoder: Decoder) throws {
    var container = try decoder.singleValueContainer()
    let string = try container.decode(String.self)
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "YYYY/MM/dd"
    self.wrappedValue = dateFormatter.date(from: string)
  }
}

struct Movie: Codable {
  @YMD var released: Date?
}

let data = """
{
  "released": "2019/11/15"
}
""".data(using: .utf8)!

let decoder = JSONDecoder()
let encoder = JSONEncoder()
do {
  let movie = try decoder.decode(Movie.self, from: data)
  print(movie)

  let data = try encoder.encode(movie)
  print(String(data: data, encoding: .utf8))
} catch {
  print(error)
}