Updating Feeds in the Background

Episode #416 | 10 minutes | published on November 7, 2019 | Uses Xcode-11.1, Swift-5.1

Subscribers Only

In this episode we update all of the episodes in the background when the application is launched. We leverage Operations to do this work and set the qualityOfService to prefer foreground work that the user is actively requesting.

This episode is part of a series: Making a Podcast App From Scratch.

1. Intro 1 min

2. New Project Setup & Theming 14 min

3. Storyboard References 5 min

4. Reusable Views 10 min

5. Styling Custom Cells 15 min

6. Working with Remote Images 16 min

7. Creating an API Client 18 min

8. Parsing JSON into Models 15 min

9. Displaying Search Results and Recommendations 25 min

10. Parsing RSS and Atom Feeds 29 min

11. Podcast Detail Screen 27 min

12. Custom Gradient Background 13 min

13. Padded Label and Separator View 11 min

14. Making a Custom Subscribe Button 16 min

15. Refactoring to a Data Manager 26 min

16. Adding Episode Cells 27 min

17. Polish and Cleanup 23 min

18. Setting up Core Data to Save Subscriptions 24 min

19. Player Screen 27 min

20. Audio Playback 24 min

21. Player Bar Part 1 21 min

22. Player Bar Part 2 15 min

23. My Podcasts Screen 28 min

24. Setting up Core Data for Episodes 5 min

25. Refactor Core Data Context Handling 11 min

26. Importing Episodes from a Feed 22 min

27. Updating Feeds in the Background 10 min

28. Playlist Screen 18 min

29. Tracking Playback Progress 33 min

Background Update of Subscribed Feeds

As the user launches the application, we'll start by importing the new episodes of the subscribed feeds. We'll use a qualityOfService of .background.

private var priorityQueue: OperationQueue = {
    let q = OperationQueue()
    q.maxConcurrentOperationCount = 2
    q.qualityOfService = .userInitiated
    return q
}()

private var backgroundQueue: OperationQueue = {
    let q = OperationQueue()
    q.maxConcurrentOperationCount = 2
    q.qualityOfService = .background
    return q
}()

Updating the Podcast

As we fetch the feeds and import new episodes, we'll update the podcast by adding these episodes. We'll make sure this operation runs in the background and simultaneously logs the details of the subscribed podcast.

func updatePodcasts() {
    backgroundQueue.addOperation {
        let context = PersistenceManager.shared.newBackgroundContext()
        let subscriptionStore = SubscriptionStore(context: context)
        do {
            let subs = try subscriptionStore.fetchSubscriptions()
            for sub in subs {
                guard let podcast = sub.podcast else { continue }
                guard let id = podcast.id else { continue }
                print("Queueing operation to update subscribed podcast: \(podcast.title ?? "?")")
                let updateOperation = ImportEpisodesOperation(podcastId: id)
                self.backgroundQueue.addOperation(updateOperation)
            }
        } catch {
            print("Error fetching subscriptions for background update. \(error)")
        }
    }
}

As the application launches, we'll update the podcast on a background queue.

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    window = UIWindow()

    Theme.apply(to: window!)

    window?.rootViewController = UIViewController()
    window?.makeKeyAndVisible()

    PersistenceManager.shared.initializeModel(then: {
        FeedImporter.shared.startListening()
        FeedImporter.shared.updatePodcasts()

        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        self.window?.rootViewController = storyboard.instantiateInitialViewController()            
    })

    return true
}

Note that we've also bumped the minimum version of the app to iOS 13.

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