/// Represents a single frame in a GIF.
struct AnimatedFrame {

  /// The image to display for this frame. Its value is nil when the frame is removed from the buffer.
  let image: UIImage?
  
  /// The duration that this frame should remain active.
  let duration: TimeInterval

  /// A placeholder frame with no image assigned.
  /// Used to replace frames that are no longer needed in the animation.
  var placeholderFrame: AnimatedFrame {
    return AnimatedFrame(image: nil, duration: duration)
  }

  /// Whether this frame instance contains an image or not.
  var isPlaceholder: Bool {
    return image == .none
  }

  /// Returns a new instance from an ptional image.
  ///
  /// - parameter image: An optional `UIImage` instance to be assigned to the new frame.
  /// - returns: An `AnimatedFrame` instance.
  func animatedFrame(with newImage: UIImage?) -> AnimatedFrame {
    return AnimatedFrame(image: newImage, duration: duration)
  }
}