title: UIImageWriteToSavedPhotosAlbum freezes my view
body: <p>I have a problem using <code>UIImageWriteToSavedPhotosAlbum</code> because I wanted to know at what point it started to download, because I have to download an image of 1.4M size and it takes a long time to download it, and I wanted to put a loader that indicate at user that is actually download the image. Any ideas?</p>
  
  <p>I have to download my Image from a URL, </p>
  
  <pre><code>LoadingOverlay.shared.showOverlay(view: self.view) //NEVER EXECUTED, FREEZES BUTTON
  
  let imagestring = Constants.baseURL + "photos/download/" + imagePhoto
  
  if let url = URL(string: imagestring),
      let data = try? Data(contentsOf: url),
      let image = UIImage(data: data) {
      UIImageWriteToSavedPhotosAlbum(image, self, #selector(image(_:didFinishSavingWithError:contextInfo:)), nil)
  }
  
  @objc func image(_ image: UIImage, didFinishSavingWithError error: Error?, contextInfo: UnsafeRawPointer) {
      LoadingOverlay.shared.hideOverlayView(view: self.view)
      if let error = error {
          // we got back an error!
          let ac = UIAlertController(title: NSLocalizedString("Error", comment: ""), message: error.localizedDescription, preferredStyle: .alert)
          ac.addAction(UIAlertAction(title: "OK", style: .default))
          present(ac, animated: true)
      } else {
          let ac = UIAlertController(title: "Imagen guardada", message: "Tu imágen se guardó correctamente", preferredStyle: .alert)
          ac.addAction(UIAlertAction(title: "OK", style: .default))
          present(ac, animated: true)
      }
  }
  </code></pre>
  
  <p>I wanted to know how can I know where starts downloading or if there's another way to save photos to library.</p>