存储app状态 in https://www.raywenderlich.com/160653/design-patterns-ios-using-swift-part-22

memento.swift

// 1. 在 Storyboard 中设置RestorationID
// 2. AppDelegate.swift中告诉App我们想要save和restore
func application(_ application: UIApplication, shouldSaveApplicationState coder: NSCoder) -> Bool {
  return true
}

func application(_ application: UIApplication, shouldRestoreApplicationState coder: NSCoder) -> Bool {
  return true
}

// 3. ViewController.swift
static let IndexRestorationKey = "currentAlbumIndex"
override func encodeRestorableState(with coder: NSCoder) {
  coder.encode(currentAlbumIndex, forKey: Constants.IndexRestorationKey)
  super.encodeRestorableState(with: coder)
}

override func decodeRestorableState(with coder: NSCoder) {
  super.decodeRestorableState(with: coder)
  currentAlbumIndex = coder.decodeInteger(forKey: Constants.IndexRestorationKey)
  showDataForAlbum(at: currentAlbumIndex)
  horizontalScrollerView.reload()
}

// 4. 把 horizontalScrollerView 移动到合适的地方
override func viewDidAppear(_ animated: Bool) {
  super.viewDidAppear(animated)
  horizontalScrollerView.scrollToView(at: currentAlbumIndex, animated: false)
}