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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BGContinuedProcessingTask compatibility with background URLSession
My app does really large uploads. Like several GB. We use the AWS SDK to upload to S3. It seemed like using BGContinuedProcessingTask to complete a set of uploads for a particular item may improve UX as well as performance and reliability. When I tried to get BGContinuedProcessingTask working with the AWS SDK I found that the task would fail after maybe 30 seconds. It looked like this was because the app stopped receiving updates from the AWS upload and the task wants consistent updates. The AWS SDK always uses a background URLSession and this is not configurable. I understand the background URLSession runs in a separate process from the app and maybe that is why progress updates did not continue when the app was in the background. Is it expected that BGContinuedProcessingTask and background URLSession are not really compatible? It would not be shocking since they are 2 separate background APIs. Would the Apple recommendation be to use a normal URLSession for this, in which case AWS would need to change their SDK? Or does Apple think that BGContinuedProcessingTask should just not be used with uploads? In other words use an upload specific API. Thanks!
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153
Aug ’25
How to safely maximize concurrent UI rendering
I'm using Swift 6 and tasks to concurrently process multiple PDF files for rendering, and it's working well. But currently I'm manually limiting the number of simultaneous tasks to 2 out of fear that the system might run many tasks concurrently without having enough RAM to do the PDF processing. Testing on a variety of devices, I've tried increasing the task limit and haven't seen any crashes, but I'm quite concerned about the possibility. Any given device might be using a lot of RAM at any moment, and any given PDF might strain resources more than the average PDF. Is there a recommended technique for handling this kind of scenario? Should I not worry about it and just go ahead and start a high number of tasks, trusting that the system won't run too many concurrently and therefore won't run out of RAM?
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273
Mar ’25
Electron app with Express + Python child processes not running in macOS production build
Hi all, I’ve built an Electron application that uses two child processes: An Express.js server A Python executable (packaged .exe/binary) During the development phase, everything works fine — the Electron app launches, both child processes start, and the app functions as expected. But when I create a production build for macOS, the child processes don’t run. Here’s a simplified snippet from my electron.mjs: import { app, BrowserWindow } from "electron"; import { spawn } from "child_process"; import path from "path"; let mainWindow; const createWindow = () => { mainWindow = new BrowserWindow({ width: 1200, height: 800, webPreferences: { nodeIntegration: true, }, }); mainWindow.loadFile("index.html"); // Start Express server const serverPath = path.join(process.resourcesPath, "app.asar.unpacked", "server", "index.js"); const serverProcess = spawn(process.execPath, [serverPath], { stdio: "inherit", }); // Start Python process const pythonPath = path.join(process.resourcesPath, "app.asar.unpacked", "python", "myapp"); const pythonProcess = spawn(pythonPath, [], { stdio: "inherit", }); serverProcess.on("error", (err) => console.error("Server process error:", err)); pythonProcess.on("error", (err) => console.error("Python process error:", err)); }; app.whenReady().then(createWindow); I’ve already done the following: Configured package.json with the right build settings Set up extraResources / asarUnpack to include the server and Python files Verified both child processes work standalone Questions: What’s the correct way to package and spawn these child processes for macOS production builds? Do I need to move them into a specific location (like Contents/Resources/app.asar.unpacked) and reference them differently? Is there a more reliable pattern for handling Express + Python child processes inside an Electron app bundle? Any insights or working examples would be really appreciated!
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85
Sep ’25
Processes & Concurrency Resources
General: DevForums subtopic: App & System Services > Processes & Concurrency Processes & concurrency covers a number of different technologies: Background Tasks Resources Concurrency Resources — This includes Swift concurrency. Service Management Resources XPC Resources Share and Enjoy — Quinn “The Eskimo!” @ Developer Technical Support @ Apple let myEmail = "eskimo" + "1" + "@" + "apple.com"
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195
Jul ’25
BGContinuedProcessingTask expiring unpredictably
I've adopted the new BGContinuedProcessingTask in iOS 26, and it has mostly been working well in internal testing. However, in production I'm getting reports of the tasks failing when the app is put into the background. A bit of info on what I'm doing: I need to download a large amount of data (around 250 files) and process these files as they come down. The size of the files can vary: for some tasks each file might be around 10MB. For other tasks, the files might be 40MB. The processing is relatively lightweight, but the volume of data means the task can potentially take over an hour on slower internet connections (up to 10GB of data). I set the totalUnitCount based on the number of files to be downloaded, and I increment completedUnitCount each time a file is completed. After some experimentation, I've found that smaller tasks (e.g. 3GB, 10MB per file) seem to be okay, but larger tasks (e.g. 10GB, 40MB per file) seem to fail, usually just a few seconds after the task is backgrounded (and without even opening any other apps). I think I've even observed a case where the task expired while the app was foregrounded! I'm trying to understand what the rules are with BGContinuedProcessingTask and I can see at least four possibilities that might be relevant: Is it necessary to provide progress updates at some minimum rate? For my larger tasks, where each file is ~40MB, there might be 20 or 30 seconds between progress updates. Does this make it more likely that the task will be expired? For larger tasks, the total time to complete can be 60–90 mins on slower internet connections. Is there some maximum amount of time the task can run for? Does the system attempt some kind of estimate of the overall time to complete and expire the task on that basis? The processing on each file is relatively lightweight, so most of the time the async stream is awaiting the next file to come down. Does the OS monitor the intensity of workload and suspend the task if it appears to be idle? I've noticed that the task UI sometimes displays a message, something along the lines of "Do you want to continue this task?" with a "Continue" and "Stop" option. What happens if the user simply ignores or doesn't see this message? Even if I tap "Continue" the task still seems to fail sometimes. I've read the docs and watched the WWDC video, but there's not a whole lot of information on the specific issues I mention above. It would be great to get some clarity on this, and I'd also appreciate any advice on alternative ways I could approach my specific use case.
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311
3w
Proper initialization - views, dependencies, laoder and viewcontroller
So i am pretty new to Xcode, but i have been using Python and other language for some while. But I am quite new to the game of view and view control. So it may be that i have over complicated this a bit - and it may be that I have some wrong understanding of the dependencies and appcontroller (that i thought would be a good idea). So here we have a main file we call it app.swift, we have a startupmanager.swift, a appcoordinator and a dependeciescontainer. But it may be that this is either a overkill - or that I am doing it wrong. So my thought was that i had a dependeciecontainer, a appcoordinator for the views and a startupmanager that controll the initialized fetching. I have controlled the memory when i run it - checking if it is higher, lower eg - but it was first when i did my 2 days profile i saw a lot of new errors, like this: Fikser(7291,0x204e516c0) malloc: xzm: failed to initialize deferred reclamation buffer (46). and i also get macro errors, probably from the @Query in my feedview. So my thought was that a depencecie manager and a startupmanager was a good idea together with a app coordinator. But maybe I am wrong - maybe this is not a good idea? Or maybe I am doing some things twice? I have added a lot of prints and debugs for checking. But it seems that it starts off to heavy? import SwiftUI import Combine @MainActor class AppCoordinator: ObservableObject { @Published var isLoggedIn: Bool = false private var authManager: AuthenticationManager = .shared private var cancellables = Set<AnyCancellable>() private let startupManager: StartupManager private let container: DependencyContainer @Published var path = NavigationPath() enum Screen: Hashable, Identifiable { case profile case activeJobs case offers case message var id: Self { self } } init(container: DependencyContainer) { self.container = container self.startupManager = container.makeStartupManager() setupObserving() startupManager.start() print("AppCoordinator initialized!") } private func setupObserving() { authManager.$isAuthenticated .receive(on: RunLoop.main) .sink { [weak self] isAuthenticated in self?.isLoggedIn = isAuthenticated } .store(in: &cancellables) } func userDidLogout() { authManager.logout() path.removeLast(path.count) } func showProfile() { path.append(Screen.profile) } func showActiveJobs() { path.append(Screen.activeJobs) } func showOffers() { path.append(Screen.offers) } func showMessage() { path.append(Screen.message) } @ViewBuilder func viewForDestination(_ destination: Screen) -> some View { switch destination { case .profile: ProfileView() case .activeJobs: ActiveJobsView() case .offers: OffersView() case .message: ChatView() } } @ViewBuilder func viewForJob(_ job: Job) -> some View { PostDetailView( job: job, jobUserDetailsRepository: container.makeJobUserDetailsRepository() ) } @ViewBuilder func viewForProfileSubview(_ destination: ProfileView.ProfileSubviews) -> some View { switch destination{ case .personalSettings: PersonalSettingView() case .historicData: HistoricDataView() case .transactions: TransactionView() case .helpCenter: HelpcenterView() case .helpContract: HelpContractView() } } enum HomeBarDestinations: Hashable, Identifiable { case postJob case jobPosting var id: Self { self } } @ViewBuilder func viewForHomeBar(_ destination: HomeBarView.HomeBarDestinations) -> some View { switch destination { case .postJob: PostJobView() } } } import Apollo import FikserAPI import SwiftData class DependencyContainer { static var shared: DependencyContainer! private let modelContainer: ModelContainer static func initialize(with modelContainer: ModelContainer) { shared = DependencyContainer(modelContainer: modelContainer) } private init(modelContainer: ModelContainer) { self.modelContainer = modelContainer print("DependencyContainer being initialized at ") } @MainActor private lazy var userData: UserData = { return UserData(apollo: Network.shared.apollo) }() @MainActor private lazy var userDetailsRepository: UserDetailsRepository = { return UserDetailsRepository(userData: makeUserData()) }() @MainActor private lazy var jobData: JobData = { return JobData(apollo: Network.shared.apollo) }() @MainActor private lazy var jobRepository: JobRepository = { return JobRepository(jobData: makeJobData(), modelContainer: modelContainer) }() @MainActor func makeUserData() -> UserData { return userData } @MainActor func makeUserDetailsRepository() -> UserDetailsRepository { return userDetailsRepository } @MainActor func makeStartupManager() -> StartupManager { return StartupManager( userDetailsRepository: makeUserDetailsRepository(), jobRepository: makeJobRepository(), authManager: AuthenticationManager.shared, lastUpdateRepository: makeLastUpdateRepository() ) } @MainActor func makeJobData() -> JobData { return jobData } @MainActor func makeJobRepository() -> any JobRepositoryProtocol { return jobRepository } @MainActor private lazy var jobUserData: JobUserData = { return JobUserData(apollo: Network.shared.apollo) }() @MainActor private lazy var jobUserDetailsRepository: JobUserDetailsRepository = { return JobUserDetailsRepository(jobUserData: makeJobUserData()) }() @MainActor func makeJobUserData() -> JobUserData { return jobUserData } @MainActor func makeJobUserDetailsRepository() -> JobUserDetailsRepository { return jobUserDetailsRepository } @MainActor private lazy var lastUpdateData: LastUpdateData = { return LastUpdateData(apollo: Network.shared.apollo) }() @MainActor private lazy var lastUpdateRepository: LastUpdateRepository = { return LastUpdateRepository(lastUpdateData: makeLastUpdateData()) }() @MainActor func makeLastUpdateData() -> LastUpdateData { return lastUpdateData } @MainActor func makeLastUpdateRepository() -> LastUpdateRepository { return lastUpdateRepository } }```
1
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346
Feb ’25
Background Refresh Stalls After Charging on watchOS 26
Hello everyone, I’m a new developer still learning as I go. I’m building a simple watchOS app that tracks Apple Watch battery consumption, records hourly usage data, and uses that information to predict battery life in hours. I’ve run into an issue where background refresh completely stalls after charging and never recovers, regardless of what I do. The only way to restore normal behavior is to restart the watch. Background refresh can work fine for days, but if the watch is charging and a scheduled background refresh tries to run during that period, it appears to be deferred—and then remains in that deferred state indefinitely. Even reopening the app or scheduling new refreshes doesn’t recover it. Has anyone else encountered this behavior? Is there a reliable workaround? I’ve seen a few reports suggesting that there may be a regression in scheduleBackgroundRefresh() on watchOS 26, where tasks are never delivered after certain states. Any insights or confirmations would be greatly appreciated. Thank you!
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184
Oct ’25
Mac: Best way to distinguish native app process and script process spawned from executable (e.g. python node) through process_id
I'm working on a Mac app that receives a process ID via NSXPCConnection, and I'm trying to figure out the best way to determine whether that process is a native macOS app like Safari—with bundles and all—or just a script launched by something like Node or Python. The executable is signed with a Team ID using codesign. I was thinking about getting the executable's path as one way to handle it, but I’m wondering if there’s a more reliable method than relying on the folder structure.
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208
Sep ’25
Why is xpc_connection_set_peer_code_signing_requirement() closing the connection instead of returning XPC_ERROR_PEER_CODE_SIGNING_REQUIREMENT?
I'm using libxpc in a C server and Swift client. I set up a code-signing requirement in the server using xpc_connection_set_peer_code_signing_requirement(). However, when the client doesn't meet the requirement, the server just closes the connection, and I get XPC_ERROR_CONNECTION_INTERRUPTED on the client side instead of XPC_ERROR_PEER_CODE_SIGNING_REQUIREMENT, making debugging harder. What I want: To receive XPC_ERROR_PEER_CODE_SIGNING_REQUIREMENT on the client when code-signing fails, for better debugging. What I’ve tried: Using xpc_connection_set_peer_code_signing_requirement(), but it causes the connection to be dropped immediately. Questions: Why does the server close the connection without sending the expected error? How can I receive the correct error on the client side? Are there any other methods for debugging code-signing failures with libxpc? Thanks for any insights!
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482
Feb ’25
How to Handle Asynchronous Operations in BGContinuedProcessingTask
I would like to know whether BGContinuedProcessingTaskRequest supports executing asynchronous tasks internally, or if it can only execute synchronous tasks within BGContinuedProcessingTaskRequest? Our project is very complex, and we now need to use BGContinuedProcessingTaskRequest to perform some long-running operations when the app enters the background (such as video encoding/decoding & export). However, our export interface is an asynchronous function, for example video.export(callback: FinishCallback). This export call returns immediately, and when the export completes internally, it calls back through the passed-in callback. So when I call BGTaskScheduler.shared.register to register a BGContinuedProcessingTask, what should be the correct approach? Should I directly call video.export(nil) without any waiting, or should I wait for the export function to complete in the callback? For example: BGTaskScheduler.shared.register(forTaskWithIdentifier: "com.xxx.xxx.xxx.xxx", using: nil) { task in guard let continuedTask = task as? BGContinuedProcessingTask else { task.setTaskCompleted(success: false) return } let scanner = SmartAssetsManager.shared let semaphore = DispatchSemaphore(value: 0) continuedTask.expirationHandler = { logError(items: "xwxdebug finished.") semaphore.signal() } logInfo(items: "xwxdebug start!") video.export { _ in semaphore.signal() } semaphore.wait() logError(items: "xwxdebug finished!") }
3
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108
Nov ’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.
2
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395
Feb ’25
iOS26 background lock screen Blood glucose monitoring Bluetooth low energy disconnect sleep
First, our app communicates with our blood glucose monitor (CGM) using Bluetooth Low Energy (BLE). On an iPhone 14 Pro with iOS 26.0.1, Bluetooth communication works properly even when the app is in the background and locked. Even if the phone and CGM are disconnected, the app continues to scan in the background and reconnects when the phone and CGM are back in close proximity. It won't be dormant in the background or when the screen is locked. This effectively ensures that diabetic users can monitor their blood glucose levels in real time. However, after using iOS 26.0.1 on the iPhone 17, we've received user feedback about frequent disconnections in the background. Our logs indicate that Bluetooth communication is easily disconnected when switching to the background, and then easily dormant by the system, especially when the user's screen is locked. This situation significantly impacts users' blood glucose monitoring, and users are unacceptable. What can be done?
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167
Oct ’25
SMAppService
Hello, https://developer.apple.com/forums/thread/802443 https://developer.apple.com/documentation/servicemanagement/updating-helper-executables-from-earlier-versions-of-macos https://developer.apple.com/documentation/ServiceManagement/updating-your-app-package-installer-to-use-the-new-service-management-api#Run-the-sample-launch-agent Read these. Earlier we had a setup with SMJobBless, now we have migrated to SMAppService. Everything is working fine, the new API seems easier to manage, but we are having issues with updating the daemon. I was wondering, what is the right process for updating a daemon from app side? What we are doing so far: App asks daemon for version If version is lower than expected: daemon.unregister(), wait a second and daemon.register() again. The why? We have noticed that unregistering/registering multiple times, of same daemon, can cause the daemon to stop working as expected. The daemon toggle in Mac Settings -> Login Items & Extensions can be on or off, but the app can still pickup daemon running, but no daemon running in Activity monitor. Registration/unregistration can start failing and nothing helps to resolve this, only reseting with sfltool resetbtm and a restart seems to does the job. This is usually noticeable for test users, testing same daemon version with different app builds. In production app, we also increase the bundle version of daemon in plist, in test apps we - don't. I haven't found any sources of how the update of pre-bundled app daemon should work. Initial idea is register/unregister, but from what I have observed, this seems to mess up after multiple registrations. I have a theory, that sending the daemon a command to kill itself after app update, would load the latest daemon. Also, I haven't observed for daemon, with different build versions to update automatically. What is the right way to update a daemon with SMAppService setup? Thank you in advance.
5
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204
Nov ’25
Background refresh or processing app
I am writing an app which mainly is used to update data used by other apps on the device. After the user initializes some values in the app, they almost never have to return to it (occasionally to add a "friend"). The app needs to run a background task at least daily, however, without the user's intervention (or even awareness, once they've given permission). My understanding of background refresh tasks is that if the user doesn't activate the app in the foreground periodically, the scheduled background tasks may never run. If this is true, do I want to use a background processing task instead, or is there a better solution (or have I misunderstood entirely)?
1
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412
Jan ’25
Combine delay & switchToLatest publisher don't emit value sometimes
Hello, I recently implemented a conditional debounce publisher using Swift's Combine. If a string with a length less than 2 is passed, the event is sent downstream immediately without delay. If a string with a length of 2 or more is passed, the event is emitted downstream with a 0.2-second delay. While writing test logic related to this, I noticed a strange phenomenon: sometimes the publisher, which should emit events with a 0.2-second delay, does not emit an event. The test code below should have all indices from 1 to 100 in the array, but sometimes some indices are missing, causing the assertion to fail. Even after observing completion, cancel, and output events through handleEvents, I couldn't find any cause. Am I using Combine incorrectly, or is there a bug in Combine? I would appreciate it if you could let me know. import Foundation import Combine var cancellables: Set<AnyCancellable> = [] @MainActor func text(index: Int, completion: @escaping () -> Void) { let subject = PassthroughSubject<String, Never>() let textToSent = "textToSent" subject .map { text in if text.count >= 2 { return Just<String>(text) .delay(for: .seconds(0.2), scheduler: RunLoop.main) .eraseToAnyPublisher() } else { return Just<String>(text) .eraseToAnyPublisher() } } .switchToLatest() .sink { if $0.count >= 2 { completion() } }.store(in: &cancellables) for i in 0..<textToSent.count { let stringIndex = textToSent.index(textToSent.startIndex, offsetBy: i) let stringToSent = String(textToSent[textToSent.startIndex...stringIndex]) subject.send(stringToSent) } } var array = [Int]() for i in 1...100 { text(index: i) { array.append(i) } } DispatchQueue.main.asyncAfter(deadline: .now() + 5) { for i in 1...100 { assert(array.contains(i)) } } RunLoop.main.run(until: .now + 10)
0
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403
Feb ’25
SSMenuAgent consuming lots of CPU
My load average on a largely idle system is around 22, going up to 70 or so periodically; SSMenuAgent seems to be consuming lots of CPU (and, looking at spindump, it certainly seems busy), but... it's not happening on any other system whose screens I am observing. (Er, I know about load average limitations, the process is also consuming 70-98% CPU according to both top and Activity Monitor.) Since this machine (although idle) has our network extension, I'm trying to figure out if this is due to that, or of this is generally expected. Anyone?
2
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444
May ’25
Did GCD change in macOS 26
Some users of my Mac app are complaining of redrawing delays. Based on what I see in logs, my GCD timer event handlers are not being run in a timely manner although the runloop is still pumping events: sometimes 500ms pass before a 15ms timer runs. During this time, many keypresses are routed through -[NSApplication sendEvent:], which is how I know it's not locked up in synchronous code. This issue has not been reported in older versions of macOS. I start the timer like this: _gcdUpdateTimer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue()); dispatch_source_set_timer(_gcdUpdateTimer, dispatch_time(DISPATCH_TIME_NOW, period * NSEC_PER_SEC), period * NSEC_PER_SEC, 0.0005 * NSEC_PER_SEC); dispatch_source_set_event_handler(_gcdUpdateTimer, ^{ …redraw… });
1
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111
Sep ’25