urlsession-init.swift

private var urlSession: URLSession?

private func upload() {
    if urlSession == nil {
        let config = URLSessionConfiguration.background(withIdentifier: (Bundle.main.bundleIdentifier ?? "") + UUID().uuidString)
        urlSession = URLSession(configuration: config, delegate: self, delegateQueue: nil)
    }
}

extension ViewController: URLSessionDelegate, URLSessionTaskDelegate, URLSessionDataDelegate {
    func urlSession(_ session: URLSession, didBecomeInvalidWithError error: Error?) {}
    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {}
    func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {}
}