iOS Swift how to get an image capture on hls(http live stream)?

Question

How to get image capture on hls(http live stream) with Swift on iOS?

return nil at "capturedImage" on below code.

func onCaptureButtonClick(sender : UIButton){

    let generator = AVAssetImageGenerator(asset: avAsset)
    generator.maximumSize = self.videolayer.frame.size

    let   time = CMTimeMake(1, 1);

    let capturedImage : CGImageRef! = generator.copyCGImageAtTime(time, actualTime: nil, error: nil)

    let imageView = UIImageView(frame: CGRectMake(0,self.view.bounds.height/2, self.view.bounds.width, self.view.bounds.height / 2))
    imageView.image = UIImage(CGImage: capturedImage)

    self.view.addSubview(imageView)

}