Axiom Background Processing Diag

Time-Cost Analysis

LLDB Quick Test

Verify handler is correctly registered:

e -l objc -- (void)[[BGTaskScheduler sharedScheduler] _simulateLaunchForTaskWithIdentifier:@"com.yourapp.refresh"]

If breakpoint hits → Registration correct, issue is scheduling/system factors. If nothing happens → Registration broken.

Symptom 2: Task Terminates Unexpectedly

Handler called but work doesn't complete before termination.

Quick Diagnosis (5 minutes)

Task terminates early?
│
├─ Step 1: Check expiration handler (1 min)
│  ├─ Expiration handler set FIRST in handler?
│  │  └─ NO → Move to very first line
│  └─ Expiration handler actually cancels work?
│     └─ NO → Add cancellation logic
│
├─ Step 2: Check setTaskCompleted (2 min)
│  ├─ Called in success path?
│  ├─ Called in failure path?
│  ├─ Called after expiration?
│  └─ ANY path missing → Task never signals completion
│
├─ Step 3: Check work duration (2 min)
│  ├─ BGAppRefreshTask work > 30 seconds?
│  │  └─ YES → Chunk work or use BGProcessingTask
│  └─ BGProcessingTask work > system limit?
│     └─ YES → Save progress, resume on next launch

Common Causes

Test Expiration Handling

// First simulate launch
e -l objc -- (void)[[BGTaskScheduler sharedScheduler] _simulateLaunchForTaskWithIdentifier:@"com.yourapp.refresh"]

// Then force expiration
e -l objc -- (void)[[BGTaskScheduler sharedScheduler] _simulateExpirationForTaskWithIdentifier:@"com.yourapp.refresh"]

Verify expiration handler runs and work stops gracefully.

Symptom 3: Background URLSession Delegate Not Called

Download completes but didFinishDownloadingTo never fires.

Quick Diagnosis (5 minutes)

URLSession delegate not called?
│
├─ Step 1: Check session configuration (2 min)
│  ├─ Using URLSessionConfiguration.background()?
│  │  └─ NO → Must use background config
│  ├─ Session identifier unique?
│  │  └─ NO → Use unique bundle-prefixed ID
│  └─ sessionSendsLaunchEvents = true?
│     └─ NO → Set for app relaunch on completion
│
├─ Step 2: Check AppDelegate handler (2 min)
│  ├─ handleEventsForBackgroundURLSession implemented?
│  │  └─ NO → Required for session events
│  └─ Completion handler stored and called later?
│     └─ NO → Store handler, call after events processed
│
└─ Step 3: Check delegate assignment (1 min)
   ├─ Session created with delegate?
   └─ Delegate not nil when task completes?

Required AppDelegate Code

// Store completion handler
var backgroundSessionCompletionHandler: (() -> Void)?

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

// Call after all events processed
func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
    DispatchQueue.main.async {
        self.backgroundSessionCompletionHandler?()
        self.backgroundSessionCompletionHandler = nil
    }
}

Symptom 4: Works in Development, Not Production

Task runs with debugger but fails in release builds or for users.

Quick Diagnosis (10 minutes)

Works in dev, not prod?
│
├─ Step 1: Check system constraints (3 min)
│  ├─ Low Power Mode enabled?
│  │  └─ Check ProcessInfo.isLowPowerModeEnabled
│  ├─ Background App Refresh disabled?
│  │  └─ Check UIApplication.backgroundRefreshStatus
│  └─ Battery < 20%?
│     └─ System pauses discretionary work
│
├─ Step 2: Check app state (2 min)
│  ├─ App force-quit from App Switcher?
│  │  └─ YES → No background until foreground launch
│  └─ App recently used?
│     └─ Rarely used apps get lower priority
│
├─ Step 3: Check build differences (3 min)
│  ├─ Debug vs Release optimization differences?
│  ├─ #if DEBUG code excluding production?
│  └─ Different bundle identifier in release?
│
└─ Step 4: Add production logging (2 min)
   └─ Log task schedule/launch/complete to analytics

The 7 Scheduling Factors

All affect task execution in production:

Production Debugging

Add logging to track what's happening:

func scheduleRefresh() {
    let request = BGAppRefreshTaskRequest(identifier: "com.app.refresh")
    do {
        try BGTaskScheduler.shared.submit(request)
        Analytics.log("background_task_scheduled")
    } catch {
        Analytics.log("background_task_schedule_failed", error: error)
    }
}

func handleRefresh(task: BGAppRefreshTask) {
    Analytics.log("background_task_started")
    // ... work ...
    Analytics.log("background_task_completed")
    task.setTaskCompleted(success: true)
}

Symptom 5: Inconsistent Task Scheduling

Task runs sometimes but not predictably.

Quick Diagnosis (5 minutes)

Inconsistent scheduling?
│
├─ Step 1: Understand earliestBeginDate (2 min)
│  ├─ This is MINIMUM delay, not scheduled time
│  │  └─ System runs when convenient AFTER this date
│  └─ Set too far in future (> 1 week)?
│     └─ System may skip task entirely
│
├─ Step 2: Check scheduling pattern (2 min)
│  ├─ Scheduling same task multiple times?
│  │  └─ Call getPendingTaskRequests to check
│  └─ Scheduling in handler for continuity?
│     └─ Required for continuous refresh
│
└─ Step 3: Understand system behavior (1 min)
   ├─ BGAppRefreshTask runs based on USER patterns
   │  └─ User rarely opens app = rare runs
   └─ BGProcessingTask runs when charging
      └─ User doesn't charge overnight = no runs

Expected Behavior

Key insight: You request a time window. System decides when (or if) to run.

Symptom 6: App Crashes on Background Launch

App crashes when launched by system for background task.

Quick Diagnosis (5 minutes)

Crash on background launch?
│
├─ Step 1: Check launch initialization (2 min)
│  ├─ UI setup before task handler?
│  │  └─ Background launch may not have UI context
│  ├─ Accessing files before first unlock?
│  │  └─ Use completeUntilFirstUserAuthentication protection
│  └─ Force unwrapping optionals that may be nil?
│     └─ Guard against nil in background context
│
├─ Step 2: Check handler safety (2 min)
│  ├─ Handler captures self strongly?
│  │  └─ Use [weak self] to prevent retain cycles
│  └─ Handler accesses UI on non-main thread?
│     └─ Dispatch UI work to main queue
│
└─ Step 3: Check data protection (1 min)
   └─ Files accessible when device locked?
      └─ Use .completeUnlessOpen or .completeUntilFirstUserAuthentication

File Protection for Background Tasks

// Set appropriate protection when creating files
try data.write(to: url, options: .completeFileProtectionUntilFirstUserAuthentication)

// Or configure in entitlements for entire app

Safe Handler Pattern

BGTaskScheduler.shared.register(
    forTaskWithIdentifier: "com.app.refresh",
    using: nil
) { [weak self] task in
    guard let self = self else {
        task.setTaskCompleted(success: false)
        return
    }

    // Don't access UI
    // Use background-safe APIs only
    self.performBackgroundWork(task: task)
}

Symptom 7: Task Runs Multiple Times

Same task appears to run repeatedly or in parallel.

Quick Diagnosis (5 minutes)

Task runs multiple times?
│
├─ Step 1: Check scheduling logic (2 min)
│  ├─ Scheduling on every app launch?
│  │  └─ Check getPendingTaskRequests first
│  ├─ Scheduling in handler AND elsewhere?
│  │  └─ Consolidate to single location
│  └─ Using same identifier for different purposes?
│     └─ Use unique identifiers per task type
│
├─ Step 2: Check for duplicate submissions (2 min)
│  └─ Multiple submit() calls queued?
│     └─ System may batch into single execution
│
└─ Step 3: Check handler execution (1 min)
   └─ setTaskCompleted called promptly?
      └─ Delay may cause system to think task hung

Prevent Duplicate Scheduling

func scheduleRefreshIfNeeded() {
    BGTaskScheduler.shared.getPendingTaskRequests { requests in
        let alreadyScheduled = requests.contains {
            $0.identifier == "com.app.refresh"
        }

        if !alreadyScheduled {
            self.scheduleRefresh()
        }
    }
}

Quick Diagnostic Checklist

30-Second Check

• Info.plist has identifier?
• Registration in didFinishLaunchingWithOptions?
• App not swiped away?

5-Minute Check

• Identifiers exactly match (case-sensitive)?
• Background mode enabled (fetch/processing)?
• setTaskCompleted called in all paths?
• Expiration handler set first?

15-Minute Investigation

• LLDB simulate launch works?
• LLDB simulate expiration handled?
• Console shows registration/scheduling logs?
• Real device (not just simulator)?
• Release build (not just debug)?
• Background App Refresh enabled in Settings?

Console Log Filters

// All background task events