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I'm trying the sample app from here: https://developer.apple.com/documentation/vision/detecting_moving_objects_in_a_video I made a tweak to read the video from library instead of document picker: var recordedVideoURL: AVAsset? @IBAction func uploadVideoForAnalysis(_ sender: Any) { var configuration = PHPickerConfiguration() configuration.filter = .videos configuration.selectionLimit = 1 let picker = PHPickerViewController(configuration: configuration) picker.delegate = self present(picker, animated: true, completion: nil) } The delegation method: extension HomeViewController: PHPickerViewControllerDelegate { func picker(_ picker: PHPickerViewController, didFinishPicking results: [PHPickerResult]) { guard let selectedResult = results.first else { print("assetIdentifier: null") dismiss(animated: true, completion: nil) return } selectedResult.itemProvider.loadFileRepresentation(forTypeIdentifier: UTType.movie.identifier) { [weak self] url, error in guard error == nil, let url = url else { print(error?.localizedDescription ?? "Failed to load image") return } let asset = AVAsset(url: url) self?.recordedVideoURL = asset DispatchQueue.main.async { [weak self] in self?.dismiss(animated: true) { // dismiss the picker self?.performSegue(withIdentifier: ContentAnalysisViewController.segueDestinationId, sender: self!) self?.recordedVideoURL = nil } } } } } everything else is pretty much the same. Then, in the camera controller, it raised an error: "The requested URL was not found on this server." I put a debug break point and it show the error was from the line init AssetReader: let reader = AVAssetReader(asset: asset) func startReadingAsset(_ asset: AVAsset, reader: AVAssetReader? = nil) { videoRenderView = VideoRenderView(frame: view.bounds) setupVideoOutputView(videoRenderView) videoFileReadingQueue.async { [weak self] in do { guard let track = asset.tracks(withMediaType: .video).first else { throw AppError.videoReadingError(reason: "No video tracks found in the asset.") } let reader = AVAssetReader(asset: asset) let settings = [kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_420YpCbCr8BiPlanarFullRange] let output = AVAssetReaderTrackOutput(track: track, outputSettings: settings) if reader.canAdd(output) { reader.add(output) } else { throw AppError.videoReadingError(reason: "Couldn't add a track to the asset reader.") } if !reader.startReading() { throw AppError.videoReadingError(reason: "Couldn't start the asset reader.") } ... I tried to create a reader directly in the asset creation block, it worked: selectedResult.itemProvider.loadFileRepresentation(forTypeIdentifier: UTType.movie.identifier) { [weak self] url, error in guard error == nil, let url = url else { print(error?.localizedDescription ?? "Failed to load image") return } let asset = AVAsset(url: url) do { let reader = try AVAssetReader(asset: asset) self?.assetReader = reader print("reader: \(reader)") } catch let e { print("No reader: \(e)") } ... but if I just move it a little bit to the Dispatch.main.async block, it printed out "No reader: The requested URL was not found on this server. Therefore, I have to keep an instance of the reader and pass it to the cameraVC. Can someone please explain why is this happening? What's the logic behind this?