Background execution

How it works

• States: Not running · Inactive · Active · Background (running, off screen) · Suspended (in memory, threads frozen). A suspended app can be purged with no callback, so save state on entering the background.
• beginBackgroundTask: time to finish a save or upload, ~30 s (backgroundTimeRemaining is an estimate). Every begin needs an endBackgroundTask, also in the expiration handler.
• BGTaskScheduler (iOS 13): BGAppRefreshTask = short (~30 s), keeps content fresh; BGProcessingTask = minutes, deferrable maintenance (DB clean-up, ML), usually idle/overnight, can set requiresExternalPower / requiresNetworkConnectivity. Needs the identifiers in Info.plist BGTaskSchedulerPermittedIdentifiers, background mode fetch and/or processing, and register for each before launch finishes. Tasks are one-shot, so re-submit. The system decides when, from app usage, battery, Low Power Mode and the Background App Refresh switch.
• Debug (the debugger stops tasks running): pause, then in LLDB
  e -l objc -- (void)[[BGTaskScheduler sharedScheduler]
_simulateLaunchForTaskWithIdentifier:@"com.you.refresh"]
  (_simulateExpiration… tests the expiration handler).
• Background URLSession: .background(withIdentifier:) config, delegate-based (no completion handlers), uploads from a file. The daemon transfers, then relaunches the app in handleEventsForBackgroundURLSession: store its handler, recreate the session with the same id, call the handler on main after urlSessionDidFinishEvents. isDiscretionary lets iOS wait for Wi-Fi and power.
• Background modes (UIBackgroundModes): audio, location, voip (PushKit + CallKit), bluetooth-central/-peripheral, remote-notification, external-accessory, fetch, processing. The continuous ones hold only while doing that work. Silent push (content-available:1, priority 5) = a throttled server-side wake for ~30 s.

What gets you killed

• Jetsam: memory pressure; background/suspended and big apps go first, with no callback. Keep the background footprint small.
• Watchdog (0x8badf00d): main thread blocked too long (slow launch, sync I/O), or a background task never ended.
• 0xdead10cc: suspended while holding a file lock (SQLite in an App Group). Wrap the write in a background task.
• Too much background CPU, or a faked mode (silent audio).

Example

let bg = BGTaskScheduler.shared, rid = "com.you.refresh"
// in didFinishLaunching, BEFORE it returns:
bg.register(forTaskWithIdentifier: rid, using: nil) {
  handle($0 as! BGAppRefreshTask) }
func schedule() {
  let r = BGAppRefreshTaskRequest(identifier: rid)
  r.earliestBeginDate = .now + 15 * 60  // not before; no promise
  try? bg.submit(r) }
func handle(_ task: BGAppRefreshTask) {
  schedule()                             // one-shot: re-arm first
  let t = Task { task.setTaskCompleted(success: await sync()) }
  task.expirationHandler = { t.cancel() } }
var id = UIBackgroundTaskIdentifier.invalid  // finish a save
id = app.beginBackgroundTask { app.endBackgroundTask(id) }
store.save { app.endBackgroundTask(id) }

Interview traps

• “My BG task never runs”: running under the debugger, never re-submitted, identifier missing from Info.plist, registered too late, user rarely opens the app, Low Power Mode, or Background App Refresh is off.
• earliestBeginDate is a not-before date, not a schedule.
• After the user force-quits (swipe up): no BG tasks, no silent push, and pending background-session transfers are cancelled. VoIP PushKit still works.
• On a background-session relaunch, recreate the session with the same identifier or its delegate events are never delivered.
• Skipping setTaskCompleted or the URLSession handler costs future background time.
• beginBackgroundTask is for finishing, not polling (was ~3 min, now ~30 s).

Remember

Grant · Request · Hand-off · Mode — none is a promise.

Likely questions

1. Refresh vs processing? — ~30 s fresh content vs minutes of deferrable work.
2. Download 1 GB? — background URLSession (discretionary).
3. Test a BG task? — pause, LLDB _simulateLaunch….
4. Suspended app vanished? — jetsam; no applicationWillTerminate.
5. Why register before launch ends? — iOS may launch you only to run the task.