# What is the correct way to call UIApplication.shared.beginBackgroundTask no on the main thread? **URL:** \ **Category:** Using Swift\ **Tags:** concurrency, uikit, mainactor, actor\ **Created:** [October 7, 2024, 5:19am UTC](https://forums.swift.org/t/what-is-the-correct-way-to-call-uiapplication-shared-beginbackgroundtask-no-on-the-main-thread/75196 "2024-10-07T05:19:54Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![nomnom](https://avatars.discourse-cdn.com/v4/letter/n/3d9bf3/32.png) [@nomnom](https://forums.swift.org/u/nomnom)\ **Post date:** [October 7, 2024, 5:19am UTC](https://forums.swift.org/t/what-is-the-correct-way-to-call-uiapplication-shared-beginbackgroundtask-no-on-the-main-thread/75196/1 "2024-10-07T05:19:54Z")
This is my newbie implementation for beginBackgroundTask ```swift @MainActor class BackgroundTaskManager { var backgroundTaskID: UIBackgroundTaskIdentifier = .invalid func startBackgroundTask() { backgroundTaskID = UIApplication.shared.beginBackgroundTask { print("Background task about to expire with \(UIApplication.shared.backgroundTimeRemaining)") UIApplication.shared.endBackgroundTask(self.backgroundTaskID) } print("Started background task with \(UIApplication.shared.backgroundTimeRemaining)") } func endBackgroundTask() { UIApplication.shared.endBackgroundTask(backgroundTaskID) } } ``` When I search online, I see that apple has commented on their forum that these methods in fact does not need to be called from the main thread. But if I were to remove @MainActor and just make this class an actor instead, I get these warnings: Main actor-isolated class property 'shared' can not be referenced on a nonisolated actor instance; this is an error in the Swift 6 language mode Actor-isolated property 'backgroundTaskID' can not be referenced from the main actor; this is an error in the Swift 6 language mode Ideally, I dont want to isolate to main, as that's an extra dependencies for my background jobs. ---
**Author:** ![vns](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/vns/32/37396_2.png) [@vns](https://forums.swift.org/u/vns)\ **Post date:** [October 9, 2024, 12:42pm UTC](https://forums.swift.org/t/what-is-the-correct-way-to-call-uiapplication-shared-beginbackgroundtask-no-on-the-main-thread/75196/3 "2024-10-09T12:42:34Z")
You can make both methods `async` and get instance of the `UIApplication`: ```swift func startBackgroundTask() async { let app = await UIApplication.shared backgroundTaskID = app.beginBackgroundTask { /* ... */ } } ``` Some of APIs in UIKit and related parts are questionable at the moment (another example is `UIDevice` which got a few topics here as well). And this is really a bit of a complication here to just call an API from the background. I think over time this will get an improvement, but right now you can use a workaround. > [@dima\_kozhinov](#): > > I'm not a structured concurrency expert, so please correct me if I'm wrong, but do we have Actor Helltm already? It's just a difficulties bridging old (largely ObjC still I guess) systems. I don't think it is anything to blame at all, just a transition case. ---
**Author:** ![tera](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/tera/32/38201_2.png) [@tera](https://forums.swift.org/u/tera)\ **Post date:** [October 10, 2024, 10:23pm UTC](https://forums.swift.org/t/what-is-the-correct-way-to-call-uiapplication-shared-beginbackgroundtask-no-on-the-main-thread/75196/4 "2024-10-10T22:23:20Z")
> [@dima\_kozhinov](#): > > Actor Helll™ What's that? ---
**Author:** ![dima\_kozhinov](https://avatars.discourse-cdn.com/v4/letter/d/258eb7/32.png) [@dima\_kozhinov](https://forums.swift.org/u/dima_kozhinov)\ **Post date:** [October 11, 2024, 9:57am UTC](https://forums.swift.org/t/what-is-the-correct-way-to-call-uiapplication-shared-beginbackgroundtask-no-on-the-main-thread/75196/5 "2024-10-11T09:57:30Z")
A kind of a mess created by good intentions. Not sure if the term already exists, or I just coined it. the tm mark was intended as a joke. ---
**Author:** ![eskimo](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/eskimo/32/13558_2.png) [@eskimo](https://forums.swift.org/u/eskimo)\ **Post date:** [October 11, 2024, 3:31pm UTC](https://forums.swift.org/t/what-is-the-correct-way-to-call-uiapplication-shared-beginbackgroundtask-no-on-the-main-thread/75196/6 "2024-10-11T15:31:21Z")
nomnom wrote: > I see that apple has commented on their forum that these > methods in fact does not need to be called from the main > thread. It’s more official than that. In the Objective-C header these methods are decorated with `NS_SWIFT_NONISOLATED`, which is why you’re able to call them from non-main-actor contexts. vns wrote: > You can make both methods `async` and get instance of the > `UIApplication`: That’ll work, but it requires bouncing to the main thread, which undermines the purpose of making this API thread safe. I kinda like this approach: ```start actor BackgroundTaskManager { @MainActor init() { self.app = UIApplication.shared } let app: UIApplication func runJob(name: String) { let t = self.app.beginBackgroundTask(withName: name) { … handle expiration … } defer { if t != .invalid { self.app.endBackgroundTask(t) } } … do something … } } ``` Of course the expiry handler is main actor isolated, courtesy of the `NS_SWIFT_UI_ACTOR` decoration in the header, which is another tricky complication )-: Honestly, I’m not a fan of these APIs [1] and I encourage you to file an [enhancement request](https://developer.apple.com/forums/thread/712889) for a better option. Please post your bug number, just for the record. Share and Enjoy Quinn “The Eskimo!” @ DTS @ Apple [1] Although I’m even less of a fan of the `ProcessInfo` API that app extensions have to use. I talk about this in [UIApplication Background Task Notes](https://developer.apple.com/forums/thread/85066). ---
**Author:** ![vns](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/vns/32/37396_2.png) [@vns](https://forums.swift.org/u/vns)\ **Post date:** [October 11, 2024, 3:52pm UTC](https://forums.swift.org/t/what-is-the-correct-way-to-call-uiapplication-shared-beginbackgroundtask-no-on-the-main-thread/75196/7 "2024-10-11T15:52:51Z")
> [@eskimo](#): > > ```swift > @MainActor > init() { > self.app = UIApplication.shared > } > > ``` That’s a clever way to handle this, thanks for sharing! Might be useful in other cases as well.