Notifications Usage Example from https://www.raywenderlich.com/160653/design-patterns-ios-using-swift-part-22

Notifications.swift

// 📢名字：放在NSNotificationExtension.swift中，作为广播名字
extension NSNotification.Name {
  static let BLDownloadImage = Notification.Name("BLDownloadImageNotification")
} 

// 📢发送：放在 AlbumView.swift中

  init(frame: CGRect, coverUrl: String) {
    super.init(frame: frame)
    commonInit()
    
    // 发送出广播
    NotificationCenter.default.post(name: .BLDownloadImage, object: self, userInfo: ["imageView" : coverImageView, "coverUrl": coverUrl])
  }

// 📢接收：放在LibraryAPI.swift中
// 一旦开始init，便接收广播,接收到广播之后调用的方法是downloadImage(with:)
  private init() {
    NotificationCenter.default.addObserver(self, selector: #selector(downloadImage(with:)), name: .BLDownloadImage, object: nil)
  }
  
  // 之前post广播的时候就把一些消息放进了userInfo这个字典里
  @objc func downloadImage(with notification: Notification) {
  guard let userInfo = notification.userInfo,
    let imageView = userInfo["imageView"] as? UIImageView,
    let coverUrl = userInfo["coverUrl"] as? String,
    let filename = URL(string: coverUrl)?.lastPathComponent else {
      return
  }
  
  // 调用persistencyManager的 static method
  // 如果我们已经在cache中有了这张image，我们就直接调用缓存的
  if let savedImage = persistencyManager.getImage(with: filename) {
    imageView.image = savedImage
    return
  }
  
  // 否则我们用gcd先下载这张照片，然后让他显示到imageView中，再缓存它
  DispatchQueue.global().async {
    let downloadedImage = self.httpClient.downloadImage(coverUrl) ?? UIImage()
    DispatchQueue.main.async {
      imageView.image = downloadedImage
      self.persistencyManager.saveImage(downloadedImage, filename: filename)
    }
  }
}

// 辅助方法
// PersistencyManager.swift中
// 找到Cache directory
private var cache: URL {
  return FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask)[0]
}

// 存
func saveImage(_ image: UIImage, filename: String) {
  let url = cache.appendingPathComponent(filename)
  guard let data = UIImagePNGRepresentation(image) else {
    return
  }
  try? data.write(to: url)
}

// 取
func getImage(with filename: String) -> UIImage? {
  let url = cache.appendingPathComponent(filename)
  guard let data = try? Data(contentsOf: url) else {
    return nil
  }
  return UIImage(data: data)
}