Return expression of type '()' does not conform to 'QLPreviewItem' QuickLook Swift

Question

My network manager returns an URL. Inside this URL I have a pdf document and I want to show it in a quick look. But I am getting an error. Plus I could not figure out how to configure this quicklook in a proper way. I have tried integrating quicklook as below but I was unsuccessful.

class PersonalInfoDetailViewController: BaseViewController, QLPreviewControllerDataSource {
func numberOfPreviewItems(in controller: QLPreviewController) -> Int {
    return 1
}

func previewController(_ controller: QLPreviewController, previewItemAt index: Int) -> QLPreviewItem {
    RtukNetworkManager.shared.getPayroll(year: selectedPayment.year, month: selectedPayment.month, salaryType: selectedPayment.paymentType) { [weak self] => // Return expression of type '()' does not conform to 'QLPreviewItem'
filePath in
            DispatchQueue.main.async {
                return filePath! as QLPreviewItem // => Cannot convert value of type 'QLPreviewItem' to closure result type 'Void'
            }
        }
    }

for clarification this is my getPayroll

func getPayroll(year: Int, month: Int, salaryType: Int, completion: @escaping ((URL?) -> ())) {
    let documentsUrl =  FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
    if let url = URL(string: "https://rip.rtuk.gov.tr/workplacebackend/api/Employee/GetPayroll?yil=\(year)&ay=\(month)&maasturu=\(salaryType)") {
        let session = URLSession(configuration: .default)
        let destinationUrl = documentsUrl.appendingPathComponent(url.lastPathComponent)
        if FileManager().fileExists(atPath: destinationUrl.path) {
            print("File already exists [\(destinationUrl.path)]")
            completion(destinationUrl)
        }
        var request = URLRequest(url: url)
        request.httpMethod = "GET"
        request.addValue(authToken, forHTTPHeaderField: "Authorization")
        request.addValue(workingToken, forHTTPHeaderField: "token")
    
        let task = session.dataTask(with: request as URLRequest) { (data, response, error) in
            if error == nil {
                if let response = response as? HTTPURLResponse  {
                    if response.statusCode == 200 {
                        if let data = data  {
                            if let _ = try? data.write(to: destinationUrl, options: Data.WritingOptions.atomic) {
                                completion(destinationUrl)
                            }
                            else {
                                completion(destinationUrl)
                            }
                        }
                    }
                }
            } else {
                print(error?.localizedDescription as Any)
                completion(nil)
            }
        }
        task.resume()
    }
}

and finally that is my didSelectRowAt

 func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
    switch cellType(rawValue: selected.title) {
    case .salary:
        selectedPayment = payment![indexPath.row]
        let previewQL = QLPreviewController()
        previewQL.dataSource = self
        previewQL.currentPreviewItemIndex = indexPath.row
        show(previewQL, sender: nil)
    case .leave: break
    case .personnel: break
    case .none: break
    }

Answer 1

previewItemAt needs to return synchronously. Here you are returning your file from the block you dispatch_asynced, not from the delegate method. If you need to Quick Look a file that is not available yet, you should put up your own UI while it is downloading, before presenting the QLPreviewController. (I also remember that if you give a non-existent URL to Quick Look it will actually show a loading screen, and that you can call reloadData when the file is actually available, but I may be wrong).