DBImageDecoderFrameSize.swift

func frameSize(at index: Int, downsamplingLevel: DownSamplingLevel = .default) -> CGSize? {
        guard let properties = CGImageSourceCopyPropertiesAtIndex(imageSource, index, imageSourceOptions(with: downsamplingLevel)) as? [CFString: Any] else {
            return nil
        }

        guard let width = properties[kCGImagePropertyPixelWidth] as? Int, let height = properties[kCGImagePropertyPixelHeight] as? Int else {
            return nil
        }

        return CGSize(width: width, height: height)
}