Downloading in the background in iOS (Swift)

Hello, I'm using file download from Dropbox in my app. It works fine when app is active, but if my app goes into the background the download stops. How can I solve it? I use this code to download:

private let client = DropboxClientsManager.authorizedClient

func downloadFile(_ file: TrackModelProtocol?, callback: @escaping () -> ()) {

        guard let path = file?.path, let pathComponent = file?.fileName else { return }

        if let client = client {

            let fileManager = FileManager.default

            let directoryURL = fileManager.urls(for: .documentDirectory, in: .userDomainMask)[0]

            let destURL = directoryURL.appendingPathComponent(pathComponent)

            let destination: (URL, HTTPURLResponse) -> URL = { temporaryURL, response in

                return destURL

            }

            client.files.download(path: path, overwrite: true, destination: destination)

                .response { _ , error in

                    if let error = error{

                        print("Downloading from Dropbox fails: \(error)")

                        return

                    }

                    //Call some action after the download is complete

                    callback()

                }

        }

    }