background-processing

What this skill does

# Background Processing

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

## Contents

- [Info.plist Configuration](#infoplist-configuration)
- [BGTaskScheduler Registration](#bgtaskscheduler-registration)
- [BGAppRefreshTask Patterns](#bgapprefreshtask-patterns)
- [BGProcessingTask Patterns](#bgprocessingtask-patterns)
- [BGContinuedProcessingTask (iOS 26+)](#bgcontinuedprocessingtask-ios-26)
- [Background URLSession Downloads](#background-urlsession-downloads)
- [Background Push Triggers](#background-push-triggers)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#references)

## Info.plist Configuration

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

```xml
<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`:

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

In Xcode: target > Signing & Capabilities > Background Modes > enable "Background fetch" and "Background processing".

## BGTaskScheduler Registration

Register handlers **before** app launch completes. In UIKit, register in
`application(_:didFinishLaunchingWithOptions:)`; in SwiftUI, register in `App.init()`.

### UIKit Registration

```swift
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
    }
}
```

### SwiftUI Registration

```swift
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() }
    }
}
```

## BGAppRefreshTask Patterns

Short-lived tasks (~30 seconds) for fetching small data updates. The system
decides when to launch based on usage patterns. Review notes should say
`earliestBeginDate` is a lower-bound hint and the system may run the task later.

```swift
func scheduleAppRefresh() {
    let request = BGAppRefreshTaskRequest(
        identifier: "com.example.app.refresh"
    )
    request.earliestBeginDate = Date(timeIntervalSinceNow: 15 * 60)
    // earliestBeginDate is a lower-bound hint; the system may delay launch.
    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)
    }
}
```

## BGProcessingTask Patterns

Long-running tasks (minutes) for maintenance, data processing, or cleanup.
Runs only when device is idle and (optionally) charging. Review notes should say
`earliestBeginDate` is a lower-bound hint and the system may run the task later.

```swift
func scheduleProcessingTask() {
    let request = BGProcessingTaskRequest(
        identifier: "com.example.app.db-cleanup"
    )
    request.requiresNetworkConnectivity = false
    request.requiresExternalPower = true
    request.earliestBeginDate = Date(timeIntervalSinceNow: 60 * 60)
    // earliestBeginDate is a lower-bound hint; the system may delay launch.
    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)
    }
}
```

## BGContinuedProcessingTask (iOS 26+)

A task initiated in the foreground by a user action that continues running in the
background. The system displays progress via a Live Activity. Conforms to
`ProgressReporting`.

**Availability:** iOS 26.0+, iPadOS 26.0+

Unlike `BGAppRefreshTask` and `BGProcessingTask`, this task starts immediately
from the foreground. The system can terminate it under resource pressure,
prioritizing tasks that report minimal progress first. Set `expirationHandler` for user or system cancellation, cancel in-flight work, and clean up partial output before reporting completion.

```swift
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)
}
```

For GPU work, check support and enable Background GPU Access (`com.apple.developer.background-tasks.continued-processing.gpu`):

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

## Background URLSession Downloads

Use `URLSessionC