a.swift

class DataProducer<T: Codable> {
  let object: T?
  let image: Image?

  init(object: T) {
    self.object = object
    self.image = nil
  }

  init(image: Image) {
    self.image = image
    self.object = nil
  }

  func toData() -> Data {
    if let object = object {
      let encoder = JSONEncoder()
      return try! encoder.encode(object)
    } else if let image = image {
      return image.cache_toData()!
    } else {
      return Data()
    }
  }
}

class MyStorage {
  func save<T>(dataProducer: DataProducer<T>, forKey key: String) {
    save(data: dataProducer.toData(), forKey: key)
  }

  func save(data: Data, forKey key: String) {
    diskStorage.save(data, forKey: String)
  }
}


let storage = MyStorage()
storage.save(dataProducer: DataProducer<String>(object: "hello world"), forKey: "string")