iOS pairing scanner: start camera only when foreground (fix black preview)

Root cause (from on-device logs): the session was started from
viewDidLoad while the pairing sheet was still presenting, so iOS
interrupted it with reason 1 (videoDeviceNotAvailableInBackground) and it
never ran (isRunning=false) — a black preview. The CMVideoFormatDescription
-12710 errors were unrelated noise.

Defer startRunning until the camera can actually run: configure the graph
up front, then start only once the view is on screen and the app is
foreground-active (viewDidAppear + a guarded startSessionIfReady). Recover
on AVCaptureSessionInterruptionEnded / didBecomeActive / runtimeError. All
start triggers funnel through the session queue and no-op if already
running, so it's idempotent.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

NucleicRemote/NucleicRemote/Views/PairingScannerView.swift

@@ -120,6 +120,7 @@ final class QRScannerController: UIViewController, AVCaptureMetadataOutputObject
     /// AVCaptureSession isn't thread-safe — all configuration + start/stop happen here; only the
     /// preview layer and delegate callbacks touch the main thread.
     private let sessionQueue = DispatchQueue(label: "nucleic.qrscanner.session")
+    private var configured = false
     private var didFind = false
 
     private var previewView: CameraPreviewView { view as! CameraPreviewView }
@@ -139,6 +140,13 @@ final class QRScannerController: UIViewController, AVCaptureMetadataOutputObject
                        name: .AVCaptureSessionRuntimeError, object: session)
         nc.addObserver(self, selector: #selector(sessionInterrupted(_:)),
                        name: .AVCaptureSessionWasInterrupted, object: session)
+        // Recover the session once it's allowed to run again (interruption cleared, or the app
+        // returned to the foreground) — the reason-1 interruption at presentation time left it
+        // stopped with no retry.
+        nc.addObserver(self, selector: #selector(startSessionIfReady),
+                       name: .AVCaptureSessionInterruptionEnded, object: session)
+        nc.addObserver(self, selector: #selector(startSessionIfReady),
+                       name: UIApplication.didBecomeActiveNotification, object: nil)
 
         // Gate on camera authorization first — without this the prompt may never appear and the
         // feed silently fails (and on a denied device it would just stay black).
@@ -192,23 +200,47 @@ final class QRScannerController: UIViewController, AVCaptureMetadataOutputObject
             output.metadataObjectTypes = output.availableMetadataObjectTypes.contains(.qr)
                 ? [.qr] : output.availableMetadataObjectTypes
 
-            self.session.startRunning()
-            scannerLog.notice("📷 startRunning -> isRunning=\(self.session.isRunning, privacy: .public) inputs=\(self.session.inputs.count, privacy: .public) outputs=\(self.session.outputs.count, privacy: .public)")
-            self.report(.scanning)
+            self.configured = true
+            scannerLog.notice("📷 configured inputs=\(self.session.inputs.count, privacy: .public) outputs=\(self.session.outputs.count, privacy: .public)")
+            self.startSessionIfReady()
+        }
+    }
+
+    override func viewDidAppear(_ animated: Bool) {
+        super.viewDidAppear(animated)
+        startSessionIfReady()
+    }
+
+    /// Start the camera only once it can actually run — configured, on screen, and
+    /// foreground-active. Starting during the sheet's presentation made iOS interrupt the
+    /// session (`videoDeviceNotAvailableInBackground`, reason 1) and leave it stopped → black.
+    /// Every start trigger (config finished, view appeared, interruption ended, app foregrounded)
+    /// funnels here; the session-queue hop no-ops if already running, so it's idempotent.
+    @objc private func startSessionIfReady() {
+        DispatchQueue.main.async { [weak self] in
+            guard let self, self.isViewLoaded, self.view.window != nil,
+                  UIApplication.shared.applicationState == .active else { return }
+            self.sessionQueue.async {
+                guard self.configured, !self.session.isRunning else { return }
+                self.session.startRunning()
+                scannerLog.notice("📷 startRunning -> isRunning=\(self.session.isRunning, privacy: .public)")
+            }
         }
     }
 
     @objc private func sessionDidStart() {
         DispatchQueue.main.async { [weak self] in
             guard let self else { return }
+            self.report(.scanning)
             let connection = self.previewView.previewLayer.connection
-            scannerLog.notice("📷 didStartRunning: previewConnection=\(connection != nil, privacy: .public) active=\(connection?.isActive ?? false, privacy: .public) enabled=\(connection?.isEnabled ?? false, privacy: .public) viewBounds=\(self.view.bounds.debugDescription, privacy: .public) layerBounds=\(self.previewView.previewLayer.bounds.debugDescription, privacy: .public)")
+            scannerLog.notice("📷 didStartRunning: previewConnection=\(connection != nil, privacy: .public) active=\(connection?.isActive ?? false, privacy: .public) viewBounds=\(self.view.bounds.debugDescription, privacy: .public)")
         }
     }
 
     @objc private func sessionRuntimeError(_ note: Notification) {
         let error = note.userInfo?[AVCaptureSessionErrorKey] as? NSError
         scannerLog.error("📷 runtimeError=\(error?.description ?? "nil", privacy: .public)")
+        startSessionIfReady()  // e.g. media-services reset — try to bring it back
     }
 
     @objc private func sessionInterrupted(_ note: Notification) {