The BackgroundTasks framework

Since iOS 13, apps schedule deferred work through the BackgroundTasks framework. It offers two request types: BGAppRefreshTask for short, frequent updates (typically tens of seconds), and BGProcessingTask for longer, heavier work such as cleanup, indexing, or database maintenance, which can optionally require external power or network.

You register a launch handler for each task identifier in application(_:didFinishLaunchingWithOptions:) using BGTaskScheduler.shared.register(...), list those identifiers under BGTaskSchedulerPermittedIdentifiers in `Info.plist`, and enable the Background processing / Background fetch capabilities. The system, not the app, decides when each submitted BGTaskRequest actually runs.

Constraints and the expiration handler

iOS budgets background execution heavily. Run frequency adapts to how often you open the app, the device's battery level, Low Power Mode, and whether it is charging, so there is no guaranteed schedule. BGProcessingTask jobs are more likely to run while the device is charging and idle overnight.

Every task must set an expirationHandler and call setTaskCompleted(success:) when done. If you overrun the granted time, iOS invokes the expiration handler so you can save state and exit cleanly; ignoring it risks the system terminating the app and throttling future scheduling.

Why a cleaner uses it

Scanning the photo library for duplicates and similar shots through PhotoKit is exactly the kind of CPU-and-IO-heavy job that suits a BGProcessingTask. Cleanor can quietly pre-compute hashes and refresh its duplicate index overnight while charging, so that when you open the app the results are ready immediately instead of forcing you to wait for a fresh scan.