Fix the QR scanner showing a black preview

apps/ios/Koins/Views/QRScannerView.swift

@@ -1,8 +1,14 @@
 import AVFoundation
 import SwiftUI
+import UIKit
 
-/// Camera QR scanner, presented as a sheet. On a simulator (no capture device)
+/// Camera QR scanner, presented as a sheet.
-/// it explains that instead of showing a dead viewfinder.
+///
+/// Notes on the shape of this code, since a black preview is easy to ship:
+/// the preview layer is only created once the camera is authorised, which can
+/// happen *after* the last layout pass — so its frame is applied at creation
+/// time as well as in `viewDidLayoutSubviews`, otherwise it stays zero-sized
+/// and renders as a black rectangle.
 struct QRScannerView: UIViewControllerRepresentable {
   var onScan: (String) -> Void
   var onCancel: () -> Void
@@ -22,9 +28,14 @@ final class ScannerViewController: UIViewController, AVCaptureMetadataOutputObje
   var onCancel: (() -> Void)?
 
   private let session = AVCaptureSession()
+  private let sessionQueue = DispatchQueue(label: "com.koins.qr.session")
   private var previewLayer: AVCaptureVideoPreviewLayer?
+  private var isConfigured = false
   private var hasScanned = false
-  private var messageLabel: UILabel?
+
+  private let statusLabel = UILabel()
+  private let settingsButton = UIButton(type: .system)
+  private let viewfinder = UIView()
 
   override func viewDidLoad() {
     super.viewDidLoad()
@@ -37,97 +48,171 @@ final class ScannerViewController: UIViewController, AVCaptureMetadataOutputObje
     cancel.addTarget(self, action: #selector(cancelTapped), for: .touchUpInside)
     cancel.translatesAutoresizingMaskIntoConstraints = false
     view.addSubview(cancel)
+
+    viewfinder.layer.borderColor = UIColor.white.withAlphaComponent(0.6).cgColor
+    viewfinder.layer.borderWidth = 2
+    viewfinder.layer.cornerRadius = 18
+    viewfinder.isUserInteractionEnabled = false
+    viewfinder.translatesAutoresizingMaskIntoConstraints = false
+    view.addSubview(viewfinder)
+
+    statusLabel.numberOfLines = 0
+    statusLabel.textAlignment = .center
+    statusLabel.textColor = .white
+    statusLabel.font = .systemFont(ofSize: 14)
+    statusLabel.translatesAutoresizingMaskIntoConstraints = false
+    view.addSubview(statusLabel)
+
+    settingsButton.setTitle("Open Settings", for: .normal)
+    settingsButton.setTitleColor(.systemOrange, for: .normal)
+    settingsButton.titleLabel?.font = .systemFont(ofSize: 14, weight: .semibold)
+    settingsButton.addTarget(self, action: #selector(openSettings), for: .touchUpInside)
+    settingsButton.isHidden = true
+    settingsButton.translatesAutoresizingMaskIntoConstraints = false
+    view.addSubview(settingsButton)
+
     NSLayoutConstraint.activate([
       cancel.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 12),
       cancel.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 20),
-    ])
 
-    let label = UILabel()
+      viewfinder.centerXAnchor.constraint(equalTo: view.centerXAnchor),
-    label.numberOfLines = 0
+      viewfinder.centerYAnchor.constraint(equalTo: view.centerYAnchor),
-    label.textAlignment = .center
+      viewfinder.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.68),
-    label.textColor = .white
+      viewfinder.heightAnchor.constraint(equalTo: viewfinder.widthAnchor),
-    label.font = .systemFont(ofSize: 14)
-    label.translatesAutoresizingMaskIntoConstraints = false
-    view.addSubview(label)
-    NSLayoutConstraint.activate([
-      label.centerYAnchor.constraint(equalTo: view.centerYAnchor),
-      label.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 32),
-      label.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -32),
-    ])
-    messageLabel = label
 
-    requestCamera()
+      statusLabel.centerYAnchor.constraint(equalTo: view.centerYAnchor),
+      statusLabel.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 32),
+      statusLabel.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -32),
+
+      settingsButton.topAnchor.constraint(equalTo: statusLabel.bottomAnchor, constant: 16),
+      settingsButton.centerXAnchor.constraint(equalTo: view.centerXAnchor),
+    ])
+  }
+
+  override func viewWillAppear(_ animated: Bool) {
+    super.viewWillAppear(animated)
+    begin()
   }
 
   override func viewDidLayoutSubviews() {
     super.viewDidLayoutSubviews()
-    previewLayer?.frame = view.bounds
+    // Keeps the preview sized even if it was created after the last layout pass.
+    previewLayer?.frame = view.layer.bounds
   }
 
   override func viewWillDisappear(_ animated: Bool) {
     super.viewWillDisappear(animated)
+    sessionQueue.async { [session] in
       if session.isRunning { session.stopRunning() }
     }
+  }
 
   @objc private func cancelTapped() {
     onCancel?()
   }
 
-  private func requestCamera() {
+  @objc private func openSettings() {
-    guard let device = AVCaptureDevice.default(for: .video) else {
+    guard let url = URL(string: UIApplication.openSettingsURLString) else { return }
-      showMessage("This device has no camera. Use “Photo” to pick a QR from the library instead.")
+    UIApplication.shared.open(url)
+  }
+
+  // MARK: - Session
+
+  private func begin() {
+    guard AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .back) != nil else {
+      showHelp("This device has no camera. Use “Photo” to read a QR from your library.")
       return
     }
 
     switch AVCaptureDevice.authorizationStatus(for: .video) {
     case .authorized:
-      configure(device: device)
+      start()
     case .notDetermined:
       AVCaptureDevice.requestAccess(for: .video) { [weak self] granted in
         DispatchQueue.main.async {
+          guard let self else { return }
           if granted {
-            self?.configure(device: device)
+            self.start()
           } else {
-            self?.showMessage("Camera access is off. Enable it in Settings, or use “Photo”.")
+            self.showHelp("Camera access is off. Turn it on in Settings, or use “Photo”.")
+            self.settingsButton.isHidden = false
           }
         }
       }
     default:
-      showMessage("Camera access is off. Enable it in Settings, or use “Photo”.")
+      showHelp("Camera access is off. Turn it on in Settings, or use “Photo”.")
+      settingsButton.isHidden = false
     }
   }
 
-  private func configure(device: AVCaptureDevice) {
+  private func start() {
-    guard let input = try? AVCaptureDeviceInput(device: device), session.canAddInput(input) else {
+    statusLabel.isHidden = true
-      showMessage("The camera could not be started. Use “Photo” instead.")
+    settingsButton.isHidden = true
+    if !isConfigured { configure() }
+    guard isConfigured else { return }
+    sessionQueue.async { [session] in
+      if !session.isRunning { session.startRunning() }
+    }
+  }
+
+  private func configure() {
+    session.beginConfiguration()
+    session.sessionPreset = .high
+
+    guard let device = AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .back),
+          let input = try? AVCaptureDeviceInput(device: device),
+          session.canAddInput(input) else {
+      session.commitConfiguration()
+      showHelp("The camera could not be started. Use “Photo” instead.")
       return
     }
     session.addInput(input)
 
     let output = AVCaptureMetadataOutput()
     guard session.canAddOutput(output) else {
-      showMessage("The camera could not be started. Use “Photo” instead.")
+      session.commitConfiguration()
+      showHelp("The camera could not be started. Use “Photo” instead.")
       return
     }
     session.addOutput(output)
     output.setMetadataObjectsDelegate(self, queue: .main)
+    // Reading a type that is not available raises an Objective-C exception, so
+    // check first (it is only populated once the session has an input).
+    if output.availableMetadataObjectTypes.contains(.qr) {
       output.metadataObjectTypes = [.qr]
+    } else {
+      session.commitConfiguration()
+      showHelp("This camera cannot read QR codes.")
+      return
+    }
+    session.commitConfiguration()
 
     let layer = AVCaptureVideoPreviewLayer(session: session)
     layer.videoGravity = .resizeAspectFill
-    layer.frame = view.bounds
+    layer.frame = view.layer.bounds
     view.layer.insertSublayer(layer, at: 0)
     previewLayer = layer
+    isConfigured = true
+    view.setNeedsLayout()
 
-    DispatchQueue.global(qos: .userInitiated).async { [weak self] in
+    NotificationCenter.default.addObserver(self,
-      self?.session.startRunning()
+                                          selector: #selector(sessionFailed),
-    }
+                                          name: .AVCaptureSessionRuntimeError,
+                                          object: session)
   }
 
-  private func showMessage(_ text: String) {
+  @objc private func sessionFailed(_ note: Notification) {
-    messageLabel?.text = text
+    showHelp("The camera stopped unexpectedly. Close and try again.")
   }
 
+  private func showHelp(_ text: String) {
+    statusLabel.text = text
+    statusLabel.isHidden = false
+    viewfinder.isHidden = true
+  }
+
+  // MARK: - Detection
+
   func metadataOutput(_ output: AVCaptureMetadataOutput,
                       didOutput metadataObjects: [AVMetadataObject],
                       from connection: AVCaptureConnection) {
@@ -135,7 +220,10 @@ final class ScannerViewController: UIViewController, AVCaptureMetadataOutputObje
     guard let object = metadataObjects.first as? AVMetadataMachineReadableCodeObject,
           let value = object.stringValue, !value.isEmpty else { return }
     hasScanned = true
-    session.stopRunning()
+    sessionQueue.async { [session] in
+      if session.isRunning { session.stopRunning() }
+    }
+    UINotificationFeedbackGenerator().notificationOccurred(.success)
     onScan?(value)
   }
 }