Test `decodeIfPresent(_:forKey:)`

DecodeIfPresent.swift

import Foundation

let bothValues = """
{
  "key": "1",
  "value": "111"
}
""".data(using: .utf8)!

let valueNull = """
{
  "key": "1",
  "value": null
}
""".data(using: .utf8)!

let noValue = """
{
  "key": "1"
}
""".data(using: .utf8)!

struct Model: Decodable {
  fileprivate enum CodkingKeys : String, CodingKey {
    case key
    case value
  }
  
  let key: String
  let value: String?
  
  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodkingKeys.self)
    
    key = try container.decode(String.self, forKey: .key)
    value = try container.decodeIfPresent(String.self, forKey: .value)
  }
}

let decoder = JSONDecoder()

do {
  let model = try decoder.decode(Model.self, from: bothValues)
  print("1: \(model)")
} catch {
  print("1: \(error)")
}

do {
  let model = try decoder.decode(Model.self, from: noValue)
  print("2: \(model)")
} catch {
  print("2: \(error)")
}

do {
  let model = try decoder.decode(Model.self, from: valueNull)
  print("3: \(model)")
} catch {
  print("3: \(error)")
}