SomeLoader().selectedIndex = 1

class SomeLoader {

// MARK: - Public

  var selectedIndex: Int? {
    get {
      return dataStorage.selectedIndex
    }
    set {
      dataStorage.selectedIndex = newValue
    }
  }
}

SomeLoader().selectedIndex = 1

class SomeLoader {

// MARK: - Public

  var selectedIndex: Int? {
    get {
      return dataStorage.selectedIndex
    }
    set {
      dataStorage.updateSelected(index: newValue)
    }
  }
}

struct DataStorage<T: Hashable> {
  enum Keys: String {
    case selectedIndex
  }
  private func get<U>(forKey key: String) -> U? {
    guard let objectData = getData(forKey: key) else {
      return nil
    }
    let object: U? = get(forData: objectData)
    return object
  }
  private func get<U>(forData objectData: Data) -> U? {
    return NSKeyedUnarchiver.unarchiveObject(with: objectData) as? U
  }
  private func save<U>(forKey key: String, object: U) {
    let encodedData = NSKeyedArchiver.archivedData(withRootObject: object)
    UserDefaults.standard.set(encodedData, forKey: key)
  }
}
extension DataStorage {
  func updateSelected(index: Int?) {
    guard let index = index else {
      remove(forKey: Keys.selectedIndex.rawValue)
      return
    }
    saveSelected(index: index)
  }
}