Processes & Concurrency

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Discover how the operating system manages multiple applications and processes simultaneously, ensuring smooth multitasking performance.

Concurrency Documentation

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Background Download Not Working in release builds- flutter_downloader
Issue: Background downloads using the flutter_downloader package work perfectly in debug mode and release mode when run directly from Xcode (plugged in). However, when I create an archive build and install the app separately (via TestFlight or direct IPA install), the background download stops working as soon as the app is minimized. ✅ What I’ve already done Info.plist <key>UIBackgroundModes</key> <array> <string>remote-notification</string> <string>fetch</string> <string>processing</string> <string>audio</string> <string>push-to-talk</string> </array> AppDelegate.swift import UIKit import Flutter import Firebase import flutter_downloader import BackgroundTasks @main @objc class AppDelegate: FlutterAppDelegate { static let backgroundChannel = "com.example.app/background_service" private var backgroundCompletionHandler: (() -> Void)? override func application( _ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]? ) -> Bool { FirebaseApp.configure() GeneratedPluginRegistrant.register(with: self) FlutterDownloaderPlugin.setPluginRegistrantCallback(registerPlugins) if #available(iOS 10.0, *) { UNUserNotificationCenter.current().delegate = self } if #available(iOS 13.0, *) { registerBackgroundTask() } return super.application(application, didFinishLaunchingWithOptions: launchOptions) } @available(iOS 13.0, *) private func registerBackgroundTask() { BGTaskScheduler.shared.register( forTaskWithIdentifier: "com.example.app.process_download_queue", using: nil ) { [weak self] task in guard let self = self else { return } self.handleDownloadQueueTask(task: task as! BGProcessingTask) } } @available(iOS 13.0, *) private func handleDownloadQueueTask(task: BGProcessingTask) { scheduleNextDownloadTask() let headlessEngine = FlutterEngine(name: "BackgroundTaskEngine", project: nil, allowHeadlessExecution: true) headlessEngine.run() let channel = FlutterMethodChannel( name: AppDelegate.backgroundChannel, binaryMessenger: headlessEngine.binaryMessenger ) task.expirationHandler = { channel.invokeMethod("backgroundTaskExpired", arguments: nil) } channel.invokeMethod("processNextInBackground", arguments: nil) { result in task.setTaskCompleted(success: (result as? Bool) ?? false) } } override func application( _ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void ) { self.backgroundCompletionHandler = completionHandler super.application(application, handleEventsForBackgroundURLSession: identifier, completionHandler: completionHandler) } override func applicationDidEnterBackground(_ application: UIApplication) { if #available(iOS 13.0, *) { scheduleNextDownloadTask() } } @available(iOS 10.0, *) override func userNotificationCenter( _ center: UNUserNotificationCenter, willPresent notification: UNNotification, withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions) -> Void ) { if #available(iOS 14.0, *) { completionHandler([.list, .banner, .badge, .sound]) } else { completionHandler([.alert, .badge, .sound]) } } @available(iOS 10.0, *) override func userNotificationCenter( _ center: UNUserNotificationCenter, didReceive response: UNNotificationResponse, withCompletionHandler completionHandler: @escaping () -> Void ) { completionHandler() } } // MARK: - Helper @available(iOS 13.0, *) func scheduleNextDownloadTask() { let request = BGProcessingTaskRequest(identifier: "com.example.app.process_download_queue") request.requiresNetworkConnectivity = true request.requiresExternalPower = false request.earliestBeginDate = Date(timeIntervalSinceNow: 60) do { try BGTaskScheduler.shared.submit(request) print("BGTask: Download queue processing task scheduled successfully.") } catch { print("BGTask: Could not schedule download queue task: \(error)") } } private func registerPlugins(registry: FlutterPluginRegistry) { if !registry.hasPlugin("FlutterDownloaderPlugin") { FlutterDownloaderPlugin.register(with: registry.registrar(forPlugin: "FlutterDownloaderPlugin")!) } } 🧩 Observations Background download works correctly when: The app is plugged in and run via Xcode (release/debug) It stops working when: The app is installed from an archived build (IPA/TestFlight) and minimized All entitlements and background modes are properly added. Provisioning profile includes required background modes. ❓Question Is there any known limitation or signing difference between Xcode run and archived release builds that could cause URLSession background tasks not to trigger? Has anyone faced a similar issue when using flutter_downloader on iOS 13+ with BGTaskScheduler or URLSession background configuration? Any help or working setup example for production/TestFlight would be appreciated.
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88
Nov ’25
What happens after BGContinuedProcessingTask "expires"?
If I create a BGContinuedProcessingTaskRequest, register it, and then "do work" within it appropriately reporting progress, and before my task has finished doing all the work it had to do, its expirationHandler triggers... does the task later try again? Or does it lose the execution opportunity until the app is next re-launched to the foreground? In my testing, I never saw my task execute again once expired (which suggests the latter?). I was able to easily force this expiry by starting my task, backgrounding my app, then launching the iOS Camera App. My example is just using test code inspired from https://developer.apple.com/documentation/backgroundtasks/performing-long-running-tasks-on-ios-and-ipados let request = BGContinuedProcessingTaskRequest(identifier: taskIdentifier, title: "Video Upload", subtitle: "Starting Upload") request.strategy = .queue BGTaskScheduler.shared.register(forTaskWithIdentifier: taskIdentifier, using: nil) { task in guard let task = task as? BGContinuedProcessingTask else { return } print("i am a good task") var wasExpired = false task.expirationHandler = { wasExpired = true } let progress = task.progress progress.totalUnitCount = 100 while !progress.isFinished && !wasExpired { progress.completedUnitCount += 1 let formattedProgress = String(format: "%.2f", progress.fractionCompleted * 100) task.updateTitle(task.title, subtitle: "Completed \(formattedProgress)%") sleep(1) } if progress.isFinished { print ("i was a good task") task.setTaskCompleted(success: true) } else { print("i was not a good task") task.setTaskCompleted(success: false) } } try? BGTaskScheduler.shared.submit(request) Apologies if this is clearly stated somewhere and I'm missing it.
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89
Nov ’25
Questions about `dispatch_sync` vs `dispatch_async_and_wait` and DispatchWorkloops
In the header for workloop.h there is this note: A dispatch workloop is a "subclass" of dispatch_queue_t which can be passed to all APIs accepting a dispatch queue, except for functions from the dispatch_sync() family. dispatch_async_and_wait() must be used for workloop objects. Functions from the dispatch_sync() family on queues targeting a workloop are still permitted but discouraged for performance reasons. I have a couple questions related to this. First, I'd like to better understand what the alluded-to 'performance reasons' are that cause this pattern to be discouraged in the 'queues targeting a workloop' scenario. From further interrogation of the headers, I've found these explicit callouts regarding differences in the dispatch_sync and dispatch_async_and_wait API: dispatch_sync: Work items submitted to a queue with dispatch_sync() do not observe certain queue attributes of that queue when invoked (such as autorelease frequency and QOS class). dispatch_async_and_wait: Work items submitted to a queue with dispatch_async_and_wait() observe all queue attributes of that queue when invoked (inluding [sic] autorelease frequency or QOS class). Additionally, dispatch_async_and_wait has a section of the headers devoted to 'Differences with dispatch_sync()', though I can't say I entirely follow the distinctions it attempts to draw. Based on that, my best guess is that the 'performance reasons' are something about either QoS not being properly respected/observed or some thread context switching differences that can degrade performance, but I would appreciate insight from someone with more domain knowledge. My second question is a bit more general – taking a step back, why exactly do these two API exist? It's not clear to me from the existing documentation I've found why I would/should prefer dispatch_sync over dispatch_async_and_wait (other than the aforementioned callout noting the former is unsupported on workloops). What is the motivation for preserving both these API vs deprecating dispatch_sync in favor of dispatch_async_and_wait (or functionally subsuming one with the other)? Credit to Luna for originally posing/inspiring these questions.
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210
Nov ’25
iOS BGProcessingTask + Background Upload Not Executing Reliably on TestFlight (Works in Debug)
iOS BGProcessingTask + Background Upload Not Executing Reliably on TestFlight (Works in Debug) Description: We are facing an issue with BGTaskScheduler and BGProcessingTask when trying to perform a background audio-upload flow on iOS. The behavior is inconsistent between Debug builds and TestFlight (Release) builds. Summary of the Problem Our application records long audio files (up to 1 hour) and triggers a background upload using: BGTaskScheduler BGProcessingTaskRequest Background URLSession (background with identifier) URLSession background upload task + AppDelegate.handleEventsForBackgroundURLSession In Debug mode (Xcode → Run on device), everything works as expected: BGProcessingTask executes handleEventsForBackgroundURLSession fires Background URLSession continues uploads reliably Long audio files successfully upload even when the app is in background or terminated However, in TestFlight / Release mode, the system does not reliably launch the BGProcessingTask or Background URLSession events. Technical Details We explicitly register BGTaskScheduler: BGTaskScheduler.shared.register( forTaskWithIdentifier: "example.background.process", using: nil ) { task in self.handleBackgroundProcessing(task: task as! BGProcessingTask) } We schedule it using: let request = BGProcessingTaskRequest(identifier: "example.background.process") request.requiresNetworkConnectivity = true request.requiresExternalPower = false try BGTaskScheduler.shared.submit(request) We also use Background URLSession: let config = URLSessionConfiguration.background(withIdentifier: sessionId) config.sessionSendsLaunchEvents = true config.isDiscretionary = false AppDelegate.handleEventsForBackgroundURLSession is implemented correctly and works in Debug. Issue Observed (TestFlight Only) In TestFlight builds: BGProcessingTask rarely triggers, or the system marks it as NO LONGER RUNNING. Background upload tasks sometimes never start or complete. No logs appear from our BGProcessingTask handler. system logs show messages like: NO LONGER RUNNING bgProcessing-example.background.process Tasks running in group [com.apple.dasd.defaultNetwork] are 1! This occurs most frequently for large audio uploads (30–60 minutes), while small files behave normally. What We Have Verified Proper Info.plist values: Permitted background modes: processing, audio, fetch BGTaskSchedulerPermittedIdentifiers contains our identifier BGProcessingTask is being submitted successfully (no errors) App has microphone permission + background audio works Device plugged/unplugged doesn’t change outcome Key Question for Apple We need clarification on: Why BGProcessingTask behave differently between Debug and TestFlight builds? Are there additional restrictions or heuristics (related to file size, CPU usage, runtime, network load, or power constraints) that cause BGProcessingTask to be throttled or skipped in Release/TestFlight? How can we guarantee a background upload continues reliably for large files (100MB–500MB) on TestFlight and App Store builds? Is there an Apple-recommended pattern to combine BGProcessingTask + Background URLSession for long-running uploads? Expected Result Background uploads should continue reliably for long audio files (>30 minutes) when the app goes to background or is terminated, in the same way they currently function in Debug builds.
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156
Nov ’25
Some questions about how to use the Background Assets capability on iOS
Regarding the Background Assets capability on iOS: In the install scenario, resources defined as the "install" type are incorporated into the App Store download progress. Do resources of the "update" type in the update scenario also get incorporated into the App Store download progress in the same way? If an exception occurs during the download of install-type resources and the download cannot proceed further, will the system no longer actively block users from launching the app and instead enable the launch button? Currently, if a user has enabled automatic updates on their device, after the app is updated and released on the App Store, will the Background Assets download start immediately once the automatic update completes? Or does Background Assets have its own built-in scheduling logic that prevents it from running concurrently with the automatic update?
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116
Dec ’25
Running processing task for data upload together with state restoration
Hi All, In continuation of this thread https://developer.apple.com/forums/thread/804439 I want to perform data upload after getting it from the BLE device. As state restoration wake should not deal with data upload i though of using a processing task to perform the data upload. So the flow will be something like: Connect to device -> listen to notification -> go to background -> wake from notification -> handle data download from ble device -> register processing task for data upload -> hopefully get the data uploaded From reading about processing task i understand that the task execution is completely handled by the OS and depends on user behaviour and app usage. I even saw that if the user is not using the app for a while, the OS might not even perfoirm the task. So my quesiton is: does state restoration wakeup and perfroming data dowloads in the backgound considered app usage that will increase the likeluhood the task will get execution time? Can we rely on this for a scenario that the user opens the app for the first time, register, onboard for ble, connect to devie and then put it in the background for days or weeks and only relying on state restoration and processing tasks to do their thing? Sorry for the long read and appreciate your support! Shimon
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99
Dec ’25
Can an app launch automatically after watchOS restarts?
Regarding App Update Synchronization During Workout Mode: My watchOS app has workout mode enabled. When I update the app from the App Store on my iPhone while a workout session is active on my Apple Watch, the update does not sync to the watch. Why does this happen, and when can I expect the watch app to be updated? Regarding Automatic App Launch After a Prolonged Shutdown: I would like my watchOS app to launch automatically on my Apple Watch after it has been powered off for an extended period and then turned back on. Is this functionality possible to implement? If not, please provide a definitive answer regarding this capability.
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100
2w
nonisolated Execution Differences Before and After Xcode 26.2
I have an older project that was created before Xcode 26.2. In Xcode versions prior to 26.2, there was no Swift Compiler – Concurrency build setting. With those older versions, the following behavior occurs: a nonisolated function executes off the main thread. class ViewController: UIViewController { override func viewDidLoad() { super.viewDidLoad() run() } private func run() { Task { await runInMainThread() } } func runInMainThread() async { print(">>>> IN runInMainThread(), Thread.isMainThread \(Thread.isMainThread)") await runInBackgroundThread() } private nonisolated func runInBackgroundThread() async { print(">>>> IN runInBackgroundThread(), Thread.isMainThread \(Thread.isMainThread)") } } Output: >>>> IN runInMainThread(), Thread.isMainThread true >>>> IN runInBackgroundThread(), Thread.isMainThread false However, starting with Xcode 26.2, Apple introduced the Swift Compiler – Concurrency settings. When running the same code with the default configuration: Approachable Concurrency = Yes Default Actor Isolation = MainActor This is the output Output: >>>> IN runInMainThread(), Thread.isMainThread true >>>> IN runInBackgroundThread(), Thread.isMainThread true the nonisolated function now executes on the main thread. This raises the following questions: What is the correct Swift Compiler – Concurrency configuration if I want a nonisolated function to run off the main thread? Is nonisolated still an appropriate way to ensure code runs on a background thread?
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2w
BGProcessingTask Not Triggering at Scheduled Time After Updating to Xcode 26.1.1
I’m reaching out regarding an issue we’ve been experiencing with BGProcessingTask since upgrading to Xcode 26.1.1. Issue Summary Our daily background processing task—scheduled shortly after end‑of‑day—has stopped triggering reliably at night. This behavior started occurring only after updating to Xcode 26.1.1. Prior to this update, the task consistently ran around midnight, executed for ~10–15 seconds, and successfully rescheduled itself for the next day. Expected Behavior BGProcessingTask should run at/near the scheduled earliestBeginDate, which we set to roughly 2 hours after end-of-day. The task should execute, complete, and then reschedule itself. Actual Behavior On devices running builds compiled with Xcode 26.1.1, the task does not trigger at all during the night. The same code worked reliably before the Xcode update. No system logs indicate rejection, expiration, or background task denial. Technical Details This is the identifier we use: private enum DayEndProcessorConst {    static let taskIdentifier = "com.company.sdkmanagement.daysummary.manager" } The task is registered as follows: When app launched BGTaskScheduler.shared.register(    forTaskWithIdentifier: DayEndProcessorConst.taskIdentifier,    using: nil ) { [weak self] task in    self?.handleDayEndTask(task) } And scheduled like this: let date = Calendar.current.endOfDay(for: Date()).addingTimeInterval(60 * 60 * 2) let request = BGProcessingTaskRequest(identifier: DayEndProcessorConst.taskIdentifier) request.requiresNetworkConnectivity = true request.requiresExternalPower = false request.earliestBeginDate = date try BGTaskScheduler.shared.submit(request) As per our logs, tasks scheduled successfully The handler wraps the work in an operation queue, begins a UI background task, and marks completion appropriately: task.setTaskCompleted(success: true) Could you please advise whether: There are known issues with BGProcessingTask scheduling or midnight execution in Xcode 26.1.1 or iOS versions associated with it? Any new entitlement, configuration, or scheduler behavior has changed in recent releases? Additional logging or diagnostics can help pinpoint why the scheduler never fires the task?
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2w
Capturing screen buffer at macOS Login Window with ScreenCaptureKit and PrivilegedHelper
I am developing a remote support tool for macOS. While we have successfully implemented a Privileged Helper Tool and LaunchDaemon architecture that works within an active Aqua session, we have observed a total failure to capture the screen buffer or receive input at the macOS Login Window. Our observation of competitor software (AnyDesk, TeamViewer) shows they maintain graphical continuity through logout/restart. We are seeking the official architectural path to replicate this system-level access. Current Technical Implementation Architecture: A root-level LaunchDaemon manages the persistent network connection. A PrivilegedHelperTool (installed in /Library/PrivilegedHelperTools/) is used for elevated tasks. Environment: Tested on macOS 14.x (Sonoma) and macOS 15.x (Sequoia) on Apple Silicon. Capture Methods: We have implemented ScreenCaptureKit (SCK) as the primary engine and CGDisplayCreateImage as a fallback. Binary Status: All components are signed with a Developer ID and have been successfully Notarized. Observed Behavior & Blockers The "Aqua" Success: Within a logged-in user session, our CGI correctly identifies Display IDs and initializes the capture stream. Remote control is fully functional. The "Pre-Login" Failure: When the Mac is at the Login Window (no user logged in), the following occurs: The Daemon remains active, but the screen capture buffer returns NULL or an empty frame. ScreenCaptureKit fails to initialize, citing a lack of graphical context. No TCC (Transparency, Consent, and Control) prompt can appear because no user session exists. The "Bootstrap" Observation: We have identified that the loginwindow process exists in a restricted Mach bootstrap namespace that our Daemon (running in the System domain) cannot natively bridge. Comparative Analysis (Competitor Benchmarking) We have analyzed established remote desktop solutions like AnyDesk and Jump Desktop to understand their success at the login screen. Our findings suggest: Dual-Context Execution: They appear to use a Global LaunchAgent with LimitLoadToSessionType = ["LoginWindow"]. This allows a child process to run as root inside the login window’s graphical domain. Specialized Entitlements: These apps have migrated to the com.apple.developer.persistent-content-capture entitlement. This restricted capability allows them to bypass the weekly/monthly TCC re-authorization prompts and function in unattended scenarios where a user cannot click "Allow." Questions Entitlement Requirement: Is the persistent-content-capture entitlement the only supported way for a third-party app to capture the LoginWindow buffer without manual user intervention? LaunchAgent Strategy: To gain a graphical context at the login screen, is it recommended to load a specialized agent into the loginwindow domain via launchctl bootstrap loginwindow ...? ScreenCaptureKit vs. Legacy: Does ScreenCaptureKit officially support the LoginWindow session, or does it require an active Aqua session to initialize? MDM Bypass: For Enterprise environments, can a Privacy Preferences Policy Control (PPPC) payload grant "Screen Recording" to a non-entitled Daemon specifically for the login window context?
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2d
Where did I screw up trying concurrency?
I tried making a concurrency-safe data queue. It was going well, until memory check tests crashed. It's part of an unadvertised git project. Its location is: https://github.com/CTMacUser/SynchronizedQueue/commit/84a476e8f719506cbd4cc6ef513313e4e489cae3 It's the blocked-off method "`memorySafetyReferenceTypes'" in "SynchronizedQueueTests.swift." Note that the file and its tests were originally AI slop.
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2d
macOS 14 XPC vs Foundation XPC
I'm looking into a newer XPC API available starting with macOS 14. Although it's declared as a low-level API I can't figure it how to specify code signing requirement using XPCListener and XPCSession. How do I connect it with xpc_listener_set_peer_code_signing_requirement and xpc_connection_set_peer_code_signing_requirement which require xpc_listener_t and xpc_connection_t respectively? Foundation XPC is declared as a high-level API and provides easy ways to specify code signing requirements on both ends of xpc. I'm confused with all these XPC APIs and their future: Newer really high-level XPCListener and XPCSession API (in low-level framework???) Low-level xpc_listener_t & xpc_connection_t -like API. Is it being replaced by newer XPCListener and XPCSession? How is it related to High-level Foundation XPC? Are NSXPCListener and NSXPCConnection going to be deprecated and replaced by XPCListener and XPCSession??
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779
Aug ’25
Handling XPC Communication to Multiple Clients: Is Storing Connections a Reliable Approach?
This is the functionality I am trying to achieve with libxpc: There's one xpc server and two xpc clients. When the xpc server receives a particular dictionary item from clientB, the server needs to send a response to both clientA and clientB. This is the approach I am currently using: First, clientA creates a dictionary item that indicates that this item is from clientA. Now, clientA sends this dictionary to server. When server receives this item, it stores the connection instance with clientA in a global variable. Next, when clientB sends a particular dictionary item, server uses this global variable where it perviously stored clientA's connection instance to send a response back to clientA, alongside clientB. Only one edge case I can see is that when clientA closes this connection instance, server will be trying to send a response to an invalidated connection. Question: Is this approach recommended? Any edge cases I should be aware of? Is there any better way to achieve this functionality?
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481
Feb ’25
Spawning electron app from deamon process
Hi all, Our company has an application that runs on several machines, this app is launched via a deamon that keeps it alive. One of the feature of this app, is to start a headless electron application to run some tests. When spawning this electron application with the new arm64 OS, we are getting this issue: Silent Test Agent Worker exited with code: 133 [ERROR] [75873:0205/135842.347044:ERROR:mach_port_rendezvous.cc(384)] bootstrap_look_up com.hivestreaming.silenttestagent.MachPortRendezvousServer.1: Permission denied (1100) [ERROR] [75873:0205/135842.347417:ERROR:shared_memory_switch.cc(237)] No rendezvous client, terminating process (parent died?) [ERROR] [75872:0205/135842.347634:ERROR:mach_port_rendezvous.cc(384)] bootstrap_look_up com.hivestreaming.silenttestagent.MachPortRendezvousServer.1: Permission denied (1100) [ERROR] [75872:0205/135842.347976:ERROR:shared_memory_switch.cc(237)] No rendezvous client, terminating process (parent died?) Both application (main app and electron one) are signed and notarized, but it seems that there is some other permission issue. If we run the electron application manually, all runs as expected. I added the crash report as attachment CrashReport.log
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553
Feb ’25
EXC_BREAKPOINT in BGAppRefreshTask
When I run my app with XCode on my iPhone, and then moved into the background, I'm getting a EXC_BREAKPOINT exception after a few minutes, seemingly when iOS attempts to call my app with a BGAppRefreshTask: Thread 23 Queue: com.apple.BGTaskScheduler (com.mycompany.MyApp.RefreshTask) (serial) 0 _dispatch_assert_queue_fail 12 _pthread_wqthread Enqueued from com.apple.duet.activityscheduler.client.xpcqueue (Thread 23) 0 dispatch_async 20 start_wqthread I can't quite understand the reason from this crash. In the background task, I'm attempting to update live activities. In the process, it might encounter code that calls MainActor and manipulate @Observable objects. Might that be the reason?
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486
Feb ’25
Possible to allow x code builds to run background processes for over 3 minutes
I have an app that I'm using for my own purposes and is not in the app store. I would like to run an http server in the background for more than the allotted 3 minutes to allow persistent communications with a connected Bluetooth device. The Bluetooth device would poll the service at intervals. Is this possible to do? This app does not need app store approval since it's only for personal use.
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421
Feb ’25