Used for http://blog.gitter.im/2016/02/10/how-to-cache-view-controllers-in-ios/

DetailViewController.swift

class DetailViewController: UIViewController, NSDiscardableContent {
  override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
  }

  func beginContentAccess() -> Bool {
    return true
  }
  func endContentAccess() {}
  func discardContentIfPossible() {}
  func isContentDiscarded() -> Bool {
    return false
  }

  ...

}