Advanced Networking with URLSession

Sep 15 2022 · Swift 5.6, iOS 15, Xcode 13.4.1

Part 1: Upload Data, Background Downloads & WebSockets

04. Background Downloads

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Notes: 04. Background Downloads

URLSession - Apple Developer

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You’ve seen that you can download and upload files, and these transfers are done in a background thread. This means your app can respond to user interface events.

private var session: URLSession
override init() {
  let identifier = "com.razeTunes.mutableSongDownloader"
  let configuration = URLSessionConfiguration.background(withIdentifier: identifier)
  session = URLSession(configuration: configuration, delegate: self, delegateQueue: nil)
private var session: URLSession!
import UIKit

class AppDelegate: NSObject, UIApplicationDelegate {

@UIApplicationDelegateAdaptor(AppDelegate.self) var appDelegate
class AppDelegate: NSObject, UIApplicationDelegate {
  var backgroundCompletionHandler: (() -> Void)?
func application(
  _ application: UIApplication,
  handleEventsForBackgroundURLSession identifier: String,
  completionHandler: @escaping () -> Void
  ) {
  print("URLSession identifier: \(identifier)")
  backgroundCompletionHandler = completionHandler
static let BackgroundSongDownloadDidFinish = 
NSNotification.Name(rawValue: "BackgroundSongDownloadDidFinish")
func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
  Task { @MainActor in
    print("urlSessionDidFinishEvents called.")
  Self.BackgroundSongDownloadDidFinish, object: nil)
func application(
  _ application: UIApplication,
  didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil
  ) -> Bool {
                                         selector: #selector(backgroundSongDidDownload),
                                         name: MutableSongDownloader.BackgroundSongDownloadDidFinish,
                                         object: nil)
  return true
@objc private func backgroundSongDidDownload() {
  if let backgroundCompletionHandler = backgroundCompletionHandler {
guard let documentsPath = fileManager.urls(for: .documentDirectory,
                                           in: .userDomainMask).first
else {
  Task {
    await {
      state = .failed
let lastPathComponent = downloadURL?.lastPathComponent ?? "Song.m4a"

Testing Background Downloads

And with that, you’re done and have implemented the ability to perform background downloads. Build and run your app. Tap the download button and background your app.