axiom-background-processing-diag

Background Processing Diagnostics

Symptom-based troubleshooting for background task issues.

Related skills: axiom-background-processing (patterns, checklists), axiom-background-processing-ref (API reference)

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Symptom 1: Task Never Runs

Handler never called despite successful submit().

Quick Diagnosis (5 minutes)

Task never runs?
│
├─ Step 1: Check Info.plist (2 min)
│  ├─ BGTaskSchedulerPermittedIdentifiers contains EXACT identifier?
│  │  └─ NO → Add identifier, rebuild
│  ├─ UIBackgroundModes includes "fetch" or "processing"?
│  │  └─ NO → Add required mode
│  └─ Identifiers case-sensitive match code?
│     └─ NO → Fix typo, rebuild
│
├─ Step 2: Check registration timing (2 min)
│  ├─ Registered in didFinishLaunchingWithOptions?
│  │  └─ NO → Move registration before return true
│  └─ Registration before first submit()?
│     └─ NO → Ensure register() precedes submit()
│
└─ Step 3: Check app state (1 min)
   ├─ App swiped away from App Switcher?
   │  └─ YES → No background until user opens app
   └─ Background App Refresh disabled in Settings?
      └─ YES → Enable or inform user

Time-Cost Analysis

Approach	Time	Success Rate
Check Info.plist + registration	5 min	70% (catches most issues)
Add console logging	15 min	90%
LLDB simulate launch	5 min	95% (confirms handler works)
Random code changes	2+ hours	Low

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.

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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

Cause	Fix
Missing expiration handler	Set handler as first line
setTaskCompleted not called	Add to ALL code paths
Work takes too long	Chunk and checkpoint
Network timeout > task time	Use background URLSession
Async callback after expiration	Check shouldContinue flag

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.

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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
    }
}

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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:

Factor	Check
Critically Low Battery	Battery < 20%?
Low Power Mode	ProcessInfo.isLowPowerModeEnabled
App Usage	User opens app frequently?
App Switcher	App NOT swiped away?
Background App Refresh	Settings enabled?
System Budgets	Many recent background launches?
Rate Limiting	Requests too frequent?

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)
}

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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

Task Type	Scheduling Behavior
BGAppRefreshTask	Runs before predicted app usage times
BGProcessingTask	Runs when charging + idle (typically overnight)
Silent Push	Rate-limited; 14 pushes may = 7 launches

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

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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)
}

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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 (

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