I am using QlPreviewController in my applications, I am download the data one by one using observer, The observer detects currentIndexItem in the preview controller, The image is only download when the currentIndexItem will changed ,After downloading i will refresh the currentPreviewItem , My Problem is , while downloading an files in first item and i am moving to the next items , which also downloading an files,And both screens are freezing , after click the menu button, in the sections show images is loaded , but not reflected. While i am calling download data, The thread is changed to Background thread DispatchQueue.global().async { interactor.processData(kbPath: categoriesPath) } }else{ interactor.processData(kbPath: categoriesPath) } This is the downloading file which will happens in Background thread, The above code will change let url = fileData.contentUrl let fileName = fileData.name let contentUrl = fileData.contentUrl let documentsURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0] let destinationURL = documentsURL.appendingPathComponent(fileName) // Check if the file already exists at the destination URL if FileManager.default.fileExists(atPath: destinationURL.path){ return completion(.success(File(data: destinationURL, name: fileName, contentUrl: contentUrl, hasData: true))) } // If the file doesn't exist, proceed with downloading let request = URLRequest(url: URL(string: url)!) let task = URLSession.shared.downloadTask(with: request) {(tempFileURL, response, error) in if let error = error { print("Error downloading file: \(error)") completion(.failure(.FailedToDownload)) return } guard let httpResponse = response as? HTTPURLResponse , httpResponse.statusCode == 200,let tempFileURL = tempFileURL else { print("Failed to download file. Invalid response.") completion(.failure(.FailedToDownload)) return } do { try FileManager.default.moveItem(at: tempFileURL, to: destinationURL) completion(.success(File(data: destinationURL, name: fileName, contentUrl: url, hasData: true))) } catch { print("Error moving file: \(error)") completion(.failure(.FailedToDownload)) } } task.resume() } After Downloading the data , it will inform to the viewcontroller to check the thread and call DispatchQueue.main.async { handler() } }else{ handler() } This is my Observer if context == &previewControllerContext{ if keyPath == "currentPreviewItemIndex" { let currentIndex = preview.currentPreviewItemIndex //api calls for the specific files if currentIndex < previewData.count && !previewData[currentIndex].hasData { self.loadingView.startAnimating() let builder = Builder() self.previewData[currentIndex].hasData = true builder.build(instance: self, categoriesPath: .KBDownloadFile(fileData: self.previewData[currentIndex])) } } }else { super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context) } After downloaded image the result will notify and i will refresh the current previewItem self.preview.refreshCurrentPreviewItem() The Issue video link -> https://drive.google.com/file/d/18pBZawbvcUU3SA5vS53X9R9q-AGMG5Sv/view Any one facing this issues with this ,my thought this issues is thread based , but i will handle the thread