extension MainViewController: UIImagePickerControllerDelegate, 
                              UINavigationControllerDelegate {
  func imagePickerControllerDidCancel(_ picker: UIImagePickerController) {
    // 1
    picker.dismiss(animated: true)
  }

  func imagePickerController(
    _ picker: UIImagePickerController,
    didFinishPickingMediaWithInfo
    info: [UIImagePickerController.InfoKey : Any]
    ) {
    picker.dismiss(animated: true)

    // 2
    guard let image = info[.originalImage] as? UIImage else {
      return
    }

    // 3
    originalImage = image
    updateImages()
  } 
}

let picker = UIImagePickerController()
picker.delegate = self

present(picker, animated: true)

private func compressJPGImage(with quality: CGFloat) {
  jpgImageView.image = originalImage
}

private func compressHEICImage(with quality: CGFloat) {
  heicImageView.image = originalImage
}

private var previousQuality: Float = 0

@objc private func sliderEndedTouch() {
  updateImages()
}

@objc private func sliderDidChange() {
  let diff = abs(compressionSlider.value - previousQuality)

  guard diff > 0.1 else {
    return
  }

  previousQuality = compressionSlider.value

  updateImages()
}

compressionSlider.addTarget(
  self,
  action: #selector(sliderEndedTouch),
  for: [.touchUpInside, .touchUpOutside]
)

private let compressionQueue = OperationQueue()

compressionQueue.cancelAllOperations()

// 1
jpgImageView.image = nil
jpgActivityIndicator.startAnimating()

// 2
compressionQueue.addOperation {
  // 3
  guard let data = self.originalImage.jpegData(compressionQuality: quality) else {
    return
  }

  // 4
  DispatchQueue.main.async {
    self.jpgImageView.image = UIImage(data: data)
    // TODO: Add image size here...
    // TODO: Add compression time here...
    // TODO: Disable share button here...

    UIView.animate(withDuration: 0.3) {
      self.jpgActivityIndicator.stopAnimating()
    }
  }
}

heicImageView.image = nil
heicActivityIndicator.startAnimating()

compressionQueue.addOperation {
  do {
    let data = try self.originalImage.heicData(compressionQuality: quality)

    DispatchQueue.main.async {
      self.heicImageView.image = UIImage(data: data)
      // TODO: Add image size here...
      // TODO: Add compression time here...
      // TODO: Disable share button here...

      UIView.animate(withDuration: 0.3) {
        self.heicActivityIndicator.stopAnimating()
      }
    }
  } catch {
    print("Error creating HEIC data: \(error.localizedDescription)")
  }
}

enum HEICError: Error {
  case heicNotSupported
  case cgImageMissing
  case couldNotFinalize
}

// 1
let data = NSMutableData()
guard let imageDestination =
  CGImageDestinationCreateWithData(
    data, AVFileType.heic as CFString, 1, nil
  )
  else {
    throw HEICError.heicNotSupported
}

// 2
guard let cgImage = self.cgImage else {
  throw HEICError.cgImageMissing
}

// 3
let options: NSDictionary = [
  kCGImageDestinationLossyCompressionQuality: compressionQuality
]

// 4
CGImageDestinationAddImage(imageDestination, cgImage, options)
guard CGImageDestinationFinalize(imageDestination) else {
  throw HEICError.couldNotFinalize
}

return data as Data