Camera Capture Diagnostics

Systematic troubleshooting for AVFoundation camera issues: frozen preview, wrong rotation, slow capture, session interruptions, and permission problems.

Overview

Core Principle: When camera doesn't work, the problem is usually:

1. Threading (session work on main thread) - 35%
2. Session lifecycle (not started, interrupted, not configured) - 25%
3. Rotation (deprecated APIs, missing coordinator) - 20%
4. Permissions (denied, not requested) - 15%
5. Configuration (wrong preset, missing input/output) - 5%

Always check threading and session state BEFORE debugging capture logic.

Red Flags

Symptoms that indicate camera-specific issues:

Symptom	Likely Cause
Preview shows black screen	Session not started, permission denied, no camera input
UI freezes when opening camera	startRunning() called on main thread
Camera freezes on phone call	No interruption handling
Preview rotated 90° wrong	Not using RotationCoordinator (iOS 17+)
Captured photo rotated wrong	Rotation angle not applied to output connection
Front camera photo not mirrored	This is correct! (preview mirrors, photo does not)
"Camera in use by another app"	Another app has exclusive access
Capture takes 2+ seconds	photoQualityPrioritization set to .quality
Session won't start on iPad	Split View - camera unavailable
Crash on older iOS	Using iOS 17+ APIs without availability check

Mandatory First Steps

Before investigating code, run these diagnostics:

Step 1: Check Session State

print("📷 Session state:")
print("  isRunning: \(session.isRunning)")
print("  inputs: \(session.inputs.count)")
print("  outputs: \(session.outputs.count)")

for input in session.inputs {
    if let deviceInput = input as? AVCaptureDeviceInput {
        print("  Input: \(deviceInput.device.localizedName)")
    }
}

for output in session.outputs {
    print("  Output: \(type(of: output))")
}

Expected output:

• ✅ isRunning: true, inputs ≥ 1, outputs ≥ 1 → Session working
• ⚠️ isRunning: false → Session not started or interrupted
• ❌ inputs: 0 → Camera not added (permission? configuration?)

Step 2: Check Threading

print("🧵 Thread check:")

// When setting up session
sessionQueue.async {
    print("  Setup thread: \(Thread.isMainThread ? "❌ MAIN" : "✅ Background")")
}

// When starting session
sessionQueue.async {
    print("  Start thread: \(Thread.isMainThread ? "❌ MAIN" : "✅ Background")")
}

Expected output:

• ✅ All background → Correct
• ❌ Any main thread → UI will freeze

Step 3: Check Permissions

let status = AVCaptureDevice.authorizationStatus(for: .video)
print("🔐 Camera permission: \(status.rawValue)")

switch status {
case .authorized: print("  ✅ Authorized")
case .notDetermined: print("  ⚠️ Not yet requested")
case .denied: print("  ❌ Denied by user")
case .restricted: print("  ❌ Restricted (parental controls?)")
@unknown default: print("  ❓ Unknown")
}

Step 4: Check for Interruptions

// Add temporary observer to see interruptions
NotificationCenter.default.addObserver(
    forName: .AVCaptureSessionWasInterrupted,
    object: session,
    queue: .main
) { notification in
    if let reason = notification.userInfo?[AVCaptureSessionInterruptionReasonKey] as? Int {
        print("🚨 Interrupted: reason \(reason)")
    }
}

Decision Tree

Camera not working as expected?
│
├─ Black/frozen preview?
│  ├─ Check Step 1 (session state)
│  │  ├─ isRunning = false → See Pattern 1 (session not started)
│  │  ├─ inputs = 0 → See Pattern 2 (no camera input)
│  │  └─ isRunning = true, inputs > 0 → See Pattern 3 (preview layer)
│
├─ UI freezes when opening camera?
│  └─ Check Step 2 (threading)
│     └─ Main thread → See Pattern 4 (move to session queue)
│
├─ Camera freezes during use?
│  ├─ After phone call → See Pattern 5 (interruption handling)
│  ├─ In Split View (iPad) → See Pattern 6 (multitasking)
│  └─ Random freezes → See Pattern 7 (thermal pressure)
│
├─ Preview/photo rotated wrong?
│  ├─ Preview rotated → See Pattern 8 (RotationCoordinator preview)
│  ├─ Captured photo rotated → See Pattern 9 (capture rotation)
│  └─ Front camera "wrong" → See Pattern 10 (mirroring expected)
│
├─ Capture too slow?
│  ├─ 2+ seconds delay → See Pattern 11 (quality prioritization)
│  └─ Slight delay → See Pattern 12 (deferred processing)
│
├─ Permission issues?
│  ├─ Status: notDetermined → See Pattern 13 (request permission)
│  └─ Status: denied → See Pattern 14 (settings prompt)
│
└─ Crash on some devices?
   └─ See Pattern 15 (API availability)

Diagnostic Patterns

Pattern 1: Session Not Started

Symptom: Black preview, isRunning = false

Common causes:

1. startRunning() never called
2. startRunning() called but session has no inputs
3. Session stopped and never restarted

Diagnostic:

// Check if startRunning was called
print("isRunning before start: \(session.isRunning)")
session.startRunning()
print("isRunning after start: \(session.isRunning)")

Fix:

// Ensure session is started on session queue
func startSession() {
    sessionQueue.async { [self] in
        guard !session.isRunning else { return }

        // Verify we have inputs before starting
        guard !session.inputs.isEmpty else {
            print("❌ Cannot start - no inputs configured")
            return
        }

        session.startRunning()
    }
}

Time to fix: 10 min

Pattern 2: No Camera Input

Symptom: session.inputs.count = 0

Common causes:

1. Camera permission denied
2. AVCaptureDeviceInput creation failed
3. canAddInput() returned false
4. Configuration not committed

Diagnostic:

// Step through input setup
guard let camera = AVCaptureDevice.default(for: .video) else {
    print("❌ No camera device found")
    return
}
print("✅ Camera: \(camera.localizedName)")

do {
    let input = try AVCaptureDeviceInput(device: camera)
    print("✅ Input created")

    if session.canAddInput(input) {
        print("✅ Can add input")
    } else {
        print("❌ Cannot add input - check session preset compatibility")
    }
} catch {
    print("❌ Input creation failed: \(error)")
}

Fix: Ensure permission is granted BEFORE creating input, and wrap in configuration block:

session.beginConfiguration()
// Add input here
session.commitConfiguration()

Time to fix: 15 min

Pattern 3: Preview Layer Not Connected

Symptom: isRunning = true, inputs configured, but preview is black

Common causes:

1. Preview layer session not set
2. Preview layer not in view hierarchy
3. Preview layer frame is zero

Diagnostic:

print("Preview layer session: \(previewLayer.session != nil)")
print("Preview layer superlayer: \(previewLayer.superlayer != nil)")
print("Preview layer frame: \(previewLayer.frame)")
print("Preview layer connection: \(previewLayer.connection != nil)")

Fix:

// Ensure preview layer is properly configured
previewLayer.session = session
previewLayer.videoGravity = .resizeAspectFill

// Ensure frame is set (common in SwiftUI)
previewLayer.frame = view.bounds

Time to fix: 10 min

Pattern 4: Main Thread Blocking

Symptom: UI freezes for 1-3 seconds when camera opens

Root cause: startRunning() is a blocking call executed on main thread

Diagnostic:

// If this prints on main thread, that's the problem
print("startRunning on thread: \(Thread.current)")
session.startRunning()

Fix:

// Create dedicated serial queue
private let sessionQueue = DispatchQueue(label: "camera.session")

func startSession() {
    sessionQueue.async { [self] in
        session.startRunning()
    }
}

Time to fix: 15 min

Pattern 5: Phone Call Interruption

Symptom: Camera works, then freezes when phone call comes in

Root cause: Session interrupted but no handling/UI feedback

Diagnostic:

// Check if session is still running after returning from call
print("Session running: \(session.isRunning)")
// Will be false during active call, true after call ends

Fix: Add interruption observers (see camera-capture skill Pattern 5)

Key point: Session AUTOMATICALLY resumes after interruption ends. You don't need to call startRunning() again. Just update your UI.

Time to fix: 30 min

Pattern 6: Split View Camera Unavailable

Symptom: Camera stops working when iPad enters Split View

Root cause: Camera not available with multiple foreground apps

Diagnostic:

// Check interruption reason
// InterruptionReason.videoDeviceNotAvailableWithMultipleForegroundApps

Fix: Show appropriate UI message and resume when user exits Split View:

case .videoDeviceNotAvailableWithMultipleForegroundApps:
    showMessage("Camera unavailable in Split View. Use full screen.")

Time to fix: 15 min

Pattern 7: Thermal Pressure

Symptom: Camera stops randomly, especially after prolonged use

Root cause: Device getting hot, system reducing resources

Diagnostic:

// Check thermal state
print("Thermal state: \(ProcessInfo.processInfo.thermalState.rawValue)")
// 0 = nominal, 1 = fair, 2 = serious, 3 = critical

Fix: Reduce quality or show cooling message:

case .videoDeviceNotAvailableDueToSystemPressure:
    // Reduce quality
    session.sessionPreset = .medium
    showMessage("Camera quality reduced due to device temperature")

Time to fix: 20 min

Pattern 8: Preview Rotation Wrong

Symptom: Preview is rotated 90° from expected

Root cause: Not using RotationCoordinator (iOS 17+) or not observing updates

Diagnostic:

print("Preview connection rotation: \(previewLayer.connection?.videoRotationAngle ?? -1)")

Fix:

// Create and observe RotationCoordinator
let coordinator = AVCaptureDevice.RotationCoordinator(device: camera, previewLayer: previewLayer)

// Set initial rotation
previewLayer.connection?.videoRotationAngle = coordinator.videoRotationAngleForHorizonLevelPreview

// Observe changes
observation = coordinator.observe(\.videoRotationAngleForHorizonLevelPreview) { [weak previewLayer] coord, _ in
    DispatchQueue.main.async {
        previewLayer?.connection?.videoRotationAngle = coord.videoRotationAngleForHorizonLevelPreview
    }
}

Time to fix: 30 min

Pattern 9: Captured Photo Rotation Wrong

Symptom: Preview looks correct, but captured photo is rotated

Root cause: Rotation angle not applied to photo output connection

Diagnostic:

if let connection = photoOutput.connection(with: .video) {
    print("Photo connection rotation: \(connection.videoRotationAngle)")
}

Fix:

func capturePhoto() {
    // Apply current rotation to capture
    if let connection = photoOutput.connection(with: .video) {
        connection.videoRotationAngle = rotationCoordinator.videoRotationAngleForHorizonLevelCapture
    }

    photoOutput.capturePhoto(with: settings, delegate: self)
}

Time to fix: 15 min

Pattern 10: Front Camera Mirroring

Symptom: Designer says "front camera photo doesn't match preview"

Reality: This is CORRECT behavior, not a bug.

Explanation:

• Preview is mirrored (like looking in a mirror - user expectation)
• Captured photo is NOT mirrored (text reads correctly when shared)
• This matches the system Camera app behavior

If business requires mirrored photos (selfie apps):

func mirrorImage(_ image: UIImage) -> UIImage? {
    guard let cgImage = image.cgImage else { return nil }
    return UIImage(cgImage: cgImage, scale: image.scale, orientation: .upMirrored)
}

Time to fix: 5 min (explanation) or 15 min (if mirroring required)

Pattern 11: Slow Capture (Quality Priority)

Symptom: Photo capture takes 2+ seconds

Root cause: photoQualityPrioritization = .quality (default for some devices)

Diagnostic:

print("Max quality prioritization: \(photoOutput.maxPhotoQualityPrioritization.rawValue)")
// Check what you're requesting in AVCapturePhotoSettings

Fix:

var settings = AVCapturePhotoSettings()

// For fast capture (social/sharing)
settings.photoQualityPrioritization = .speed

// For balanced (general use)
settings.photoQualityPrioritization = .balanced

// Only use .quality when image quality is critical

Time to fix: 5 min

Pattern 12: Deferred Processing

Symptom: Want maximum responsiveness (zero-shutter-lag)

Solution: Enable deferred processing (iOS 17+)

photoOutput.isAutoDeferredPhotoDeliveryEnabled = true

// Then handle proxy in delegate:
// - didFinishProcessingPhoto gives proxy for immediate display
// - didFinishCapturingDeferredPhotoProxy gives final image later

Time to fix: 30 min

Pattern 13: Permission Not Requested

Symptom: authorizationStatus = .notDetermined

Fix:

// Must request before setting up session
Task {
    let granted = await AVCaptureDevice.requestAccess(for: .video)
    if granted {
        setupSession()
    }
}

Time to fix: 10 min

Pattern 14: Permission Denied

Symptom: authorizationStatus = .denied

Fix: Show settings prompt

func showSettingsPrompt() {
    let alert = UIAlertController(
        title: "Camera Access Required",
        message: "Please enable camera access in Settings to use this feature.",
        preferredStyle: .alert
    )
    alert.addAction(UIAlertAction(title: "Settings", style: .default) { _ in
        if let url = URL(string: UIApplication.openSettingsURLString) {
            UIApplication.shared.open(url)
        }
    })
    alert.addAction(UIAlertAction(title: "Cancel", style: .cancel))
    present(alert, animated: true)
}

Time to fix: 15 min

Pattern 15: API Availability Crash

Symptom: Crash on iOS 16 or earlier

Root cause: Using iOS 17+ APIs without availability check

Fix:

if #available(iOS 17.0, *) {
    // Use RotationCoordinator
    let coordinator = AVCaptureDevice.RotationCoordinator(device: camera, previewLayer: preview)
} else {
    // Fallback to deprecated videoOrientation
    if let connection = previewLayer.connection {
        connection.videoOrientation = .portrait
    }
}

Time to fix: 20 min

Quick Reference Table

Symptom	Check First	Likely Pattern
Black preview	Step 1 (session state)	1, 2, or 3
UI freezes	Step 2 (threading)	4
Freezes on call	Step 4 (interruptions)	5
Wrong rotation	Print rotation angle	8 or 9
Slow capture	Print quality setting	11
Denied access	Step 3 (permissions)	14
Crash on old iOS	Check @available	15

Checklist

Before escalating camera issues:

Basics:

• ☑ Session has at least one input
• ☑ Session has at least one output
• ☑ Session isRunning = true
• ☑ Preview layer connected to session
• ☑ Preview layer has non-zero frame

Threading:

• ☑ All session work on sessionQueue
• ☑ startRunning() on background thread
• ☑ UI updates on main thread

Permissions:

• ☑ Authorization status checked
• ☑ Permission requested if notDetermined
• ☑ Graceful UI for denied state

Rotation:

• ☑ RotationCoordinator created with device AND previewLayer
• ☑ Observation set up for preview angle changes
• ☑ Capture angle applied when taking photos

Interruptions:

• ☑ Interruption observer registered
• ☑ UI feedback for interrupted state
• ☑ Tested with incoming phone call

Resources

WWDC: 2021-10247, 2023-10105

Docs: /avfoundation/avcapturesession, /avfoundation/avcapturesessionwasinterruptednotification

Skills: axiom-camera-capture, axiom-camera-capture-ref