Axiom Memory Debugging

Memory Debugging

Overview

Memory issues manifest as crashes after prolonged use. Core principle 90% of memory leaks follow 3 patterns (retain cycles, timer/observer leaks, collection growth). Diagnose systematically with Instruments, never guess.

Example Prompts

• "My app crashes after 10-15 minutes with no error messages"
• "Memory jumps from 50MB to 200MB+ on a specific action — leak or cache?"
• "View controllers don't deallocate after dismiss"
• "Timers/observers causing memory leaks — how to verify?"
• "App uses 200MB and I don't know if that's normal"

Red Flags — Memory Leak Likely

• Progressive memory growth: 50MB → 100MB → 200MB (not plateauing)
• App crashes after 10-15 minutes with no error in Xcode console
• Memory warnings appear repeatedly in device logs
• View controllers don't deallocate after dismiss (visible in Memory Graph Debugger)
• Same operation run multiple times causes linear memory growth

Leak vs normal: Normal = stays at 100MB. Leak = 50MB → 100MB → 150MB → 200MB → CRASH.

Mandatory First Steps

ALWAYS diagnose FIRST (before reading code):

1. Check device logs for "Memory pressure critical", "Jetsam killed", "Low Memory"
2. Use Memory Graph Debugger (below) — shows object count growth
3. Xcode → Product → Profile → Memory. Perform action 5 times, note if memory keeps growing

What this tells you: Flat = not a leak. Linear growth = classic leak. Spike then flat = normal cache. Spikes stacking = compound leak.

Why diagnostics first: Finding leak with Instruments: 5-15 min. Guessing: 45+ min.

Detecting Leaks — Step by Step

Step 1: Memory Graph Debugger (Fastest)

1. Open app in simulator
2. Debug → Memory Graph Debugger (or toolbar icon)
3. Look for PURPLE/RED circles with "⚠" badge
4. Click them → Xcode shows retain cycle chain

Step 2: Instruments (Detailed Analysis)

1. Product → Profile (Cmd+I) → "Memory" template
2. Perform action 5-10 times
3. Memory line goes UP for each action? = Leak confirmed

Key instruments: Heap Allocations (object count), Leaked Objects (direct detection), VM Tracker (by type).

Step 3: Deallocation Check

// Add deinit logging to suspect classes
class MyViewController: UIViewController {
    deinit { print("✅ MyViewController deallocated") }
}

@MainActor
class ViewModel: ObservableObject {
    deinit { print("✅ ViewModel deallocated") }
}

Navigate to view, navigate away. See "✅ deallocated"? Yes = no leak. No = retained somewhere.

Jetsam (Memory Pressure Termination)

Jetsam is not a bug — iOS terminates background apps to free memory. Not a crash (no crash log), but frequent kills hurt UX.

Reducing Jetsam Rate

Clear caches on backgrounding:

// SwiftUI
.onChange(of: scenePhase) { _, newPhase in
    if newPhase == .background {
        imageCache.clearAll()
        URLCache.shared.removeAllCachedResponses()
    }
}

State Restoration

Users shouldn't notice jetsam. Use @SceneStorage (SwiftUI) or stateRestorationActivity (UIKit) to restore navigation position, drafts, and scroll position.

Monitoring with MetricKit

class JetsamMonitor: NSObject, MXMetricManagerSubscriber {
    func didReceive(_ payloads: [MXMetricPayload]) {
        for payload in payloads {
            guard let exitData = payload.applicationExitMetrics else { continue }
            let bgData = exitData.backgroundExitData
            if bgData.cumulativeMemoryPressureExitCount > 0 {
                // Send to analytics
            }
        }
    }
}

App memory grows while in USE? → Memory leak (fix retention)
App killed in BACKGROUND? → Jetsam (reduce bg memory)

Common Memory Leak Patterns (With Fixes)

Pattern 1: Timer Leaks (Most Common — 50% of leaks)

Why [weak self] alone doesn't fix timer leaks: The RunLoop retains scheduled timers. [weak self] only prevents the closure from retaining self — the Timer object itself continues to exist and fire. You must explicitly invalidate() to break the RunLoop's retention.

❌ Leak — Timer never invalidated

progressTimer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { [weak self] _ in
    self?.updateProgress()
}
// Timer never stopped → RunLoop keeps it alive and firing forever

✅ Best fix: Combine (auto-cleanup)

cancellable = Timer.publish(every: 1.0, tolerance: 0.1, on: .main, in: .default)
    .autoconnect()
    .sink { [weak self] _ in self?.updateProgress() }
// No deinit needed — cancellable auto-cleans when released

Alternative: Call timer?.invalidate(); timer = nil in both the appropriate teardown method (viewWillDisappear, stop method, etc.) AND deinit.

Pattern 2: Observer/Notification Leaks (25% of leaks)

❌ Leak — No removeObserver

NotificationCenter.default.addObserver(self, selector: #selector(handle),
    name: AVAudioSession.routeChangeNotification, object: nil)
// No matching removeObserver → accumulates listeners

✅ Best fix: Combine publisher

NotificationCenter.default.publisher(for: AVAudioSession.routeChangeNotification)
    .sink { [weak self] _ in self?.handleChange() }
    .store(in: &cancellables)  // Auto-cleanup with viewModel

Alternative: NotificationCenter.default.removeObserver(self) in deinit.

Pattern 3: Closure Capture Leaks (15% of leaks)

❌ Leak — Closure in array captures self

updateCallbacks.append { [self] track in
    self.refreshUI(with: track)  // Strong capture → cycle
}

✅ Fix: Use [weak self]

updateCallbacks.append { [weak self] track in
    self?.refreshUI(with: track)
}

Clear callback arrays in deinit. Use [unowned self] only when certain self outlives the closure.

Pattern 4: Strong Reference Cycles

❌ Leak — Mutual strong references

player?.onPlaybackEnd = { [self] in self.playNextTrack() }
// self → player → closure → self (cycle)

✅ Fix: [weak self] in closure

player?.onPlaybackEnd = { [weak self] in self?.playNextTrack() }

Pattern 5: View/Layout Callback Leaks

Use the delegation pattern with AnyObject protocol (enables weak references) instead of closures that capture view controllers.

Pattern 6: PhotoKit Image Request Leaks

PHImageManager.requestImage() returns a PHImageRequestID that must be cancelled. Without cancellation, pending requests queue up and hold memory when scrolling.

class PhotoCell: UICollectionViewCell {
    private var imageRequestID: PHImageRequestID = PHInvalidImageRequestID

    func configure(with asset: PHAsset, imageManager: PHImageManager) {
        if imageRequestID != PHInvalidImageRequestID {
            imageManager.cancelImageRequest(imageRequestID)
        }
        imageRequestID = imageManager.requestImage(for: asset, targetSize: PHImageManagerMaximumSize,
            contentMode: .aspectFill, options: nil) { [weak self] image, _ in
            self?.imageView.image = image
        }
    }

    override func prepareForReuse() {
        super.prepareForReuse()
        if imageRequestID != PHInvalidImageRequestID {
            PHImageManager.default().cancelImageRequest(imageRequestID)
            imageRequestID = PHInvalidImageRequestID
        }
        imageView.image = nil
    }
}

Similar patterns: AVAssetImageGenerator → cancelAllCGImageGeneration(), URLSession.dataTask() → cancel().

Systematic Debugging Workflow

Phase 1: Confirm Leak (5 min)

Profile with Memory template, repeat action 10 times. Flat = not a leak (stop). Steady climb = leak (continue).

Phase 2: Locate Leak (10-15 min)

Memory Graph Debugger → purple/red circles → click → read retain cycle chain.

Common locations: Timers (50%), Notifications/KVO (25%), Closures in collections (15%), Delegate cycles (10%).

Phase 3: Fix and Verify (5 min)

Apply fix from patterns above. Add deinit { print("✅ deallocated") }. Run Instruments again — memory should stay flat.

Compound Leaks

Real apps often have 2-3 leaks stacking. Fix the largest first, re-run Instruments, repeat until flat.

Non-Reproducible / Intermittent Leaks

When Instruments prevents reproduction (Heisenbug) or leaks only happen with specific user data:

Lightweight diagnostics (when Instruments can't be attached):

1. deinit logging as primary diagnostic — Add deinit { print("✅ ClassName deallocated") } to all suspect classes. Run 20+ sessions. When the leak occurs (e.g., 1 in 5 runs), missing deinit messages reveal which objects are retained.
2. Isolate the trigger — Test each navigation path independently. Rapidly toggle background/foreground if timing-dependent. Narrow to the specific path that leaks.
3. MetricKit for field diagnostics — Monitor peak memory in production via MXMetricPayload.memoryMetrics.peakMemoryUsage. Alert when exceeding threshold (e.g., 400MB). This catches leaks that only manifest with real user data volumes.

Common cause of intermittent leaks: Notification observers added on lifecycle events (viewWillAppear, applicationDidBecomeActive) without removing duplicates first. Each re-registration accumulates a listener — timing determines whether the duplicate fires.

TestFlight verification: Ship diagnostic build to affected users. Add os_log memory milestones. Monitor MetricKit for 24-48 hours after fix deployment.

Common Mistakes

• [weak self] without invalidate() — Timer keeps running, consuming CPU. ALWAYS call invalidate() or cancel()
• Invalidate without nil — timer?.invalidate() stops firing but reference remains. Always follow with timer = nil
• Local AnyCancellable — Goes out of scope immediately, subscription dies. Store in Set<AnyCancellable> property
• deinit with only logging — Add actual cleanup (invalidate timers, remove observers), not just print statements
• Wrong Instruments template — Memory shows usage. Leaks detects actual leaks. Use both

Instruments Quick Reference

Command Line Tools

xcrun xctrace record --template "Memory" --output memory.trace
xcrun xctrace dump memory.trace
leaks -atExit -excludeNoise YourApp

Real-World Impact

Before: 50+ PlayerViewModel instances with uncleared timers → 50MB → 200MB → Crash (13min) After: Timer properly invalidated → 50MB stable for hours

Key insight 90% of leaks come from forgetting to stop timers, observers, or subscriptions. Always clean up in deinit or use reactive patterns that auto-cleanup.

Resources

WWDC: 2021-10180, 2020-10078, 2018-416

Docs: /xcode/gathering-information-about-memory-use, /metrickit/mxbackgroundexitdata

Skills: axiom-performance-profiling, axiom-objc-block-retain-cycles, axiom-metrickit-ref