Что является лучшим способом присоединиться к нескольким значениям элемента в новую строку в xslt 2.0

Эти части

video?.exportFilterVideo(videoComposition: avVideoComposition , completion: { (url) in 
   Merge(config: .standard).overlayVideo(video: filterVideoAsset, overlayImage: videoImage, completion: { (finalVideoUrl) in

делают основной поток очень занятым. поэтому вам нужна фоновая очередь, например

 func handleAddToStory() {
    print("Attempting to add to story")

    // hide the color slider so we can return the image of the tapView that contains the text field if they added one
    colorSlider.isHidden = true
    let videoImage = self.imageWithView(inView: self.tapView)
    colorSlider.isHidden = false
    self.dismiss(animated:false, completion: nil)
    self.videoPlayer?.replaceCurrentItem(with: nil)


    DispatchQueue.main.asyncAfter(deadline: .now() + 0.7 ) {

            video?.exportFilterVideo(videoComposition: avVideoComposition , completion: { (url) in

                if let videoImage = videoImage {

                    let filterVideoAsset = AVAsset(url: url! as URL)

                    // Now merge the filtered video with tapView image which will contain the textfield if the user added one
                    Merge(config: .standard).overlayVideo(video: filterVideoAsset, overlayImage: videoImage, completion: { (finalVideoUrl) in

                        upload(finalVideoUrl! as URL)  

                    }, progressHandler: { _ in })


                } else {

                   upload(url! as URL)

                }

            })

    }


}

func upload(_ url:URL) {

 DispatchQueue.global().async {

    let dateFormatter = ISO8601DateFormatter()
    let timeStamp = dateFormatter.string(from: Date())
    let uid = User.current.uid
    let storageRef = Storage.storage().reference().child("event_stories").child(self.eventKey).child(uid).child(timeStamp + ".mp4")
    StorageService.uploadVideo(url, at: storageRef) { (downloadUrl) in
        guard let downloadUrl = downloadUrl else {
            return
        }

        let videoUrlString = downloadUrl.absoluteString
        print(videoUrlString)
        PostService.create(for: self.eventKey, for: videoUrlString)
    }

 }

}