background-processing

dpearson2699/swift-ios-skillsDeveloper ToolsReview advisedSnapshot 4a92d44db2e5

Summary

Schedule and execute background work on iOS using BGTaskScheduler. Use when registering BGAppRefreshTask for short background fetches, BGProcessingTask for long-running maintenance, BGContinuedProcessingTask (iOS 26+) for foreground-started work that continues in background, background URLSession downloads, or background push notifications. Covers Info.plist configuration, expiration handling, task completion, and debugging with simulated launches.

SKILL.md

Background Processing

Register, schedule, and execute background work on iOS using the BackgroundTasks framework, background URLSession, and background push notifications.

Info.plist Configuration
BGTaskScheduler Registration
BGAppRefreshTask Patterns
BGProcessingTask Patterns
BGContinuedProcessingTask (iOS 26+)
Background URLSession Downloads
Background Push Triggers
Common Mistakes
Review Checklist
References

Info.plist Configuration

Every task identifier must be declared in Info.plist under BGTaskSchedulerPermittedIdentifiers, or submit(_:) throws BGTaskScheduler.Error.Code.notPermitted.

<key>BGTaskSchedulerPermittedIdentifiers</key>
<array>
    <string>com.example.app.refresh</string>
    <string>com.example.app.db-cleanup</string>
    <string>com.example.app.export.*</string>
</array>

Also enable the required UIBackgroundModes:

<key>UIBackgroundModes</key>
<array>
    <string>fetch</string>       <!-- Required for BGAppRefreshTask -->
    <string>processing</string>  <!-- Required for BGProcessingTask -->
</array>

import BackgroundTasks

@main
class AppDelegate: UIResponder, UIApplicationDelegate {
    func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
    ) -> Bool {
        BGTaskScheduler.shared.register(
            forTaskWithIdentifier: "com.example.app.refresh",
            using: nil  // nil = default background queue
        ) { task in
            self.handleAppRefresh(task: task as! BGAppRefreshTask)
        }

        BGTaskScheduler.shared.register(
            forTaskWithIdentifier: "com.example.app.db-cleanup",
            using: nil
        ) { task in
            self.handleDatabaseCleanup(task: task as! BGProcessingTask)
        }

        return true
    }
}

import SwiftUI
import BackgroundTasks

@main
struct MyApp: App {
    init() {
        BGTaskScheduler.shared.register(
            forTaskWithIdentifier: "com.example.app.refresh",
            using: nil
        ) { task in
            BackgroundTaskManager.shared.handleAppRefresh(
                task: task as! BGAppRefreshTask
            )
        }
    }

    var body: some Scene {
        WindowGroup { ContentView() }
    }
}

func scheduleAppRefresh() {
    let request = BGAppRefreshTaskRequest(
        identifier: "com.example.app.refresh"
    )
    request.earliestBeginDate = Date(timeIntervalSinceNow: 15 * 60)
    do {
        try BGTaskScheduler.shared.submit(request)
    } catch {
        print("Could not schedule app refresh: \(error)")
    }
}

func handleAppRefresh(task: BGAppRefreshTask) {
    // Schedule the next refresh before doing work
    scheduleAppRefresh()

    let fetchTask = Task {
        do {
            let data = try await APIClient.shared.fetchLatestFeed()
            await FeedStore.shared.update(with: data)
            task.setTaskCompleted(success: true)
        } catch {
            task.setTaskCompleted(success: false)
        }
    }

    // CRITICAL: Handle expiration -- system can revoke time at any moment
    task.expirationHandler = {
        fetchTask.cancel()
        task.setTaskCompleted(success: false)
    }
}

func scheduleProcessingTask() {
    let request = BGProcessingTaskRequest(
        identifier: "com.example.app.db-cleanup"
    )
    request.requiresNetworkConnectivity = false
    request.requiresExternalPower = true
    request.earliestBeginDate = Date(timeIntervalSinceNow: 60 * 60)
    do {
        try BGTaskScheduler.shared.submit(request)
    } catch {
        print("Could not schedule processing task: \(error)")
    }
}

func handleDatabaseCleanup(task: BGProcessingTask) {
    scheduleProcessingTask()

    let cleanupTask = Task {
        do {
            try await DatabaseManager.shared.purgeExpiredRecords()
            try await DatabaseManager.shared.rebuildIndexes()
            task.setTaskCompleted(success: true)
        } catch {
            task.setTaskCompleted(success: false)
        }
    }

    task.expirationHandler = {
        cleanupTask.cancel()
        task.setTaskCompleted(success: false)
    }
}

import BackgroundTasks

func startExport() {
    // Register the task handler at app launch, not here.
    // BGTaskScheduler requires registration before app launch completes.
    let jobID = UUID().uuidString
    let request = BGContinuedProcessingTaskRequest(
        identifier: "com.example.app.export.\(jobID)",
        title: "Exporting Photos",
        subtitle: "Processing 247 items"
    )
    // Use a permitted base wildcard identifier: com.example.app.export.*
    // earliestBeginDate is ignored for continued processing requests.
    // .queue: begin as soon as possible if can't run immediately
    // .fail: fail submission if can't run immediately
    request.strategy = .queue

    do {
        try BGTaskScheduler.shared.submit(request)
    } catch {
        print("Could not submit continued processing task: \(error)")
    }
}

func performExport(task: BGContinuedProcessingTask) async {
    let items = await PhotoLibrary.shared.itemsToExport()
    let progress = task.progress
    progress.totalUnitCount = Int64(items.count)

    for (index, item) in items.enumerated() {
        if Task.isCancelled { break }

        await PhotoExporter.shared.export(item)
        progress.completedUnitCount = Int64(index + 1)

        // Update the user-facing title/subtitle
        task.updateTitle(
            "Exporting Photos",
            subtitle: "\(index + 1) of \(items.count) complete"
        )
    }

    task.setTaskCompleted(success: !Task.isCancelled)
}

let supported = BGTaskScheduler.supportedResources
if supported.contains(.gpu) {
    request.requiredResources = .gpu
}

class DownloadManager: NSObject, URLSessionDownloadDelegate {
    static let shared = DownloadManager()

    private lazy var session: URLSession = {
        let config = URLSessionConfiguration.background(
            withIdentifier: "com.example.app.background-download"
        )
        config.isDiscretionary = true
        config.sessionSendsLaunchEvents = true
        return URLSession(configuration: config, delegate: self, delegateQueue: nil)
    }()

    func startDownload(from url: URL) {
        let task = session.downloadTask(with: url)
        task.earliestBeginDate = Date(timeIntervalSinceNow: 60)
        task.resume()
    }

    func urlSession(
        _ session: URLSession,
        downloadTask: URLSessionDownloadTask,
        didFinishDownloadingTo location: URL
    ) {
        // Move file from tmp before this method returns
        let dest = FileManager.default.urls(
            for: .documentDirectory, in: .userDomainMask
        )[0].appendingPathComponent("download.dat")
        try? FileManager.default.moveItem(at: location, to: dest)
    }

    func urlSession(
        _ session: URLSession,
        task: URLSessionTask,
        didCompleteWithError error: (any Error)?
    ) {
        if let error { print("Download failed: \(error)") }
    }
}

// In AppDelegate:
func application(
    _ application: UIApplication,
    handleEventsForBackgroundURLSession identifier: String,
    completionHandler: @escaping () -> Void
) {
    backgroundSessionCompletionHandler = completionHandler
}

// In URLSessionDelegate — call stored handler when events finish:
func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
    Task { @MainActor in
        self.backgroundSessionCompletionHandler?()
        self.backgroundSessionCompletionHandler = nil
    }
}

{ "aps": { "content-available": 1 }, "custom-data": "new-messages" }

func application(
    _ application: UIApplication,
    didReceiveRemoteNotification userInfo: [AnyHashable: Any],
    fetchCompletionHandler completionHandler:
        @escaping (UIBackgroundFetchResult) -> Void
) {
    Task {
        do {
            let hasNew = try await MessageStore.shared.fetchNewMessages()
            completionHandler(hasNew ? .newData : .noData)
        } catch {
            completionHandler(.failed)
        }
    }
}

UIApplication.shared.registerForRemoteNotifications()

// DON'T: Submit a task whose identifier isn't in BGTaskSchedulerPermittedIdentifiers
let request = BGAppRefreshTaskRequest(identifier: "com.example.app.refresh")
try BGTaskScheduler.shared.submit(request)  // Throws .notPermitted

// DO: Add every identifier to Info.plist BGTaskSchedulerPermittedIdentifiers
// <string>com.example.app.refresh</string>

// DON'T: Start a 10-minute operation assuming it will finish
func handleRefresh(task: BGAppRefreshTask) {
    Task { await tenMinuteSync() }
}

// DO: Design work to be incremental and cancellable
func handleRefresh(task: BGAppRefreshTask) {
    let work = Task {
        for batch in batches {
            try Task.checkCancellation()
            await processBatch(batch)
            await saveBatchProgress(batch)
        }
        task.setTaskCompleted(success: true)
    }
    task.expirationHandler = {
        work.cancel()
        task.setTaskCompleted(success: false)
    }
}

background-processing

Summary

SKILL.md

Background Processing

Contents

Info.plist Configuration

BGTaskScheduler Registration

UIKit Registration

SwiftUI Registration

BGAppRefreshTask Patterns

BGProcessingTask Patterns

BGContinuedProcessingTask (iOS 26+)

Background URLSession Downloads

Background Push Triggers

Common Mistakes

1. Missing Info.plist identifiers

2. Not calling setTaskCompleted(success:)

3. Ignoring the expiration handler

4. Scheduling too frequently

5. Over-relying on background time

Review Checklist

References

Related skills