feat: Camera controller as a singleton (#1873)

* Improvements about resuming the camera

* Remove unused print call

* Singleton for the CameraController

packages/smooth_app/lib/helpers/camera_helper.dart

@@ -1,10 +1,15 @@
 import 'package:camera/camera.dart';
+import 'package:smooth_app/pages/scan/camera_controller.dart';
 
 class CameraHelper {
   const CameraHelper._();
 
   static List<CameraDescription>? _cameras;
 
+  /// Ensure we have a single instance of this controller
+  /// /!\ Lazy-loaded
+  static SmoothCameraController? _controller;
+
   /// Mandatory method to call before [findBestCamera]
   static Future<void> init() async {
     _cameras = await availableCameras();
@@ -48,4 +53,12 @@ class CameraHelper {
 
     return _cameras![cameraIndex];
   }
+
+  /// Init the controller
+  /// And prevents the redefinition of it
+  static void initController(SmoothCameraController controller) {
+    _controller ??= controller;
+  }
+
+  static SmoothCameraController? get controller => _controller;
 }

packages/smooth_app/lib/pages/scan/camera_controller.dart

@@ -0,0 +1,96 @@
+import 'package:camera/camera.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+/// A lifecycle-aware [CameraController]
+class SmoothCameraController extends CameraController {
+  SmoothCameraController(
+    CameraDescription description,
+    ResolutionPreset resolutionPreset, {
+    bool? enableAudio,
+    ImageFormatGroup? imageFormatGroup,
+  })  : _isPaused = false,
+        _isInitialized = false,
+        super(
+          description,
+          resolutionPreset,
+          enableAudio: enableAudio ?? true,
+          imageFormatGroup: imageFormatGroup,
+        );
+
+  /// Status of the preview
+  bool _isPaused;
+
+  /// Status of the controller
+  bool _isInitialized;
+
+  Future<void> init({
+    required FocusMode focusMode,
+    required Offset focusPoint,
+    required DeviceOrientation deviceOrientation,
+    required onLatestImageAvailable onAvailable,
+  }) async {
+    if (!_isInitialized) {
+      await initialize();
+      await setFocusMode(focusMode);
+      await setFocusPoint(focusPoint);
+      await setExposurePoint(focusPoint);
+      await lockCaptureOrientation(deviceOrientation);
+      await startImageStream(onAvailable);
+      _isInitialized = true;
+    }
+  }
+
+  /// Please use [init] instead
+  @protected
+  @override
+  Future<void> initialize() => super.initialize();
+
+  @override
+  Future<void> startImageStream(onLatestImageAvailable onAvailable) {
+    final Future<void> startImageStreamResult =
+        super.startImageStream(onAvailable);
+    _isPaused = false;
+    return startImageStreamResult;
+  }
+
+  @override
+  Future<void> pausePreview() {
+    final Future<void> pausePreviewResult = super.pausePreview();
+    _isPaused = true;
+    return pausePreviewResult;
+  }
+
+  Future<void> resumePreviewIfNecessary() async {
+    if (_isPaused) {
+      return resumePreview();
+    }
+  }
+
+  /// Please use [resumePreviewIfNecessary] instead
+  @protected
+  @override
+  Future<void> resumePreview() {
+    final Future<void> resumePreviewResult = super.resumePreview();
+    _isPaused = false;
+    return resumePreviewResult;
+  }
+
+  @override
+  Future<void> stopImageStream() {
+    final Future<void> stopImageStreamResult = super.stopImageStream();
+    _isPaused = false;
+    return stopImageStreamResult;
+  }
+
+  @override
+  Future<void> dispose() {
+    final Future<void> disposeResult = super.dispose();
+    _isPaused = false;
+    _isInitialized = false;
+    return disposeResult;
+  }
+
+  bool get isPaused => _isPaused;
+  bool get isInitialized => _isInitialized;
+}

packages/smooth_app/lib/pages/scan/lifecycle_manager.dart

@@ -7,21 +7,26 @@ import 'package:visibility_detector/visibility_detector.dart';
 /// [onStart] will be called only when the Widget is displayed for the first time
 /// (= during the [initState] phase)
 /// [onResume] will be called once the app is reopened (eg: the app is minimized
-/// and brought back to front) or this part of the Widget tree is visible again
-/// [onPause] will be called once the app is minimized or if this part of the
-/// tree is invisible
+/// and brought back to front)
+/// [onPause] will be called once the app is minimized
+/// [onVisible] will be called if this part of the tree is visible
+/// [onInvisible] will be called if this part of the tree is invisible
 class LifeCycleManager extends StatefulWidget {
   const LifeCycleManager({
     required this.onResume,
     required this.onPause,
     required this.child,
     this.onStart,
+    this.onVisible,
+    this.onInvisible,
     Key? key,
   }) : super(key: key);
 
   final Function() onResume;
   final Function() onPause;
   final Function()? onStart;
+  final Function()? onVisible;
+  final Function()? onInvisible;
   final Widget child;
 
   @override
@@ -68,9 +73,9 @@ class LifeCycleManagerState extends State<LifeCycleManager>
 
   void _onVisibilityChanged(bool visible) {
     if (visible) {
-      widget.onResume();
+      widget.onVisible?.call();
     } else {
-      widget.onPause();
+      widget.onInvisible?.call();
     }
   }
 

packages/smooth_app/lib/pages/scan/ml_kit_scan_page.dart

@@ -11,6 +11,7 @@ import 'package:smooth_app/data_models/user_preferences.dart';
 import 'package:smooth_app/helpers/camera_helper.dart';
 import 'package:smooth_app/helpers/collections_helper.dart';
 import 'package:smooth_app/pages/preferences/user_preferences_dev_mode.dart';
+import 'package:smooth_app/pages/scan/camera_controller.dart';
 import 'package:smooth_app/pages/scan/lifecycle_manager.dart';
 import 'package:smooth_app/pages/scan/mkit_scan_helper.dart';
 import 'package:smooth_app/widgets/lifecycle_aware_widget.dart';
@@ -70,7 +71,6 @@ class MLKitScannerPageState
 
   late ContinuousScanModel _model;
   late UserPreferences _userPreferences;
-  CameraController? _controller;
   CameraDescription? _camera;
   double _previewScale = 1.0;
 
@@ -106,8 +106,10 @@ class MLKitScannerPageState
     // all entry points
     return LifeCycleManager(
       onStart: _startLiveFeed,
-      onResume: _startLiveFeed,
-      onPause: () => _stopImageStream(fromPauseEvent: true),
+      onResume: _onResumeImageStream,
+      onVisible: () => _onResumeImageStream(forceStartPreview: true),
+      onPause: _onPauseImageStream,
+      onInvisible: _onPauseImageStream,
       child: _buildScannerWidget(),
     );
   }
@@ -146,28 +148,17 @@ class MLKitScannerPageState
     );
   }
 
-  bool get isCameraNotInitialized {
-    return _controller == null ||
-        _controller!.value.isInitialized == false ||
-        stoppingCamera ||
-        _controller!.value.isPreviewPaused ||
-        !_controller!.value.isStreamingImages;
-  }
+  bool get isCameraNotInitialized => _controller?.isInitialized != true;
 
   Future<void> _startLiveFeed() async {
-    if (_controller != null || _camera == null) {
+    if (_camera == null) {
       return;
+    } else if (_controller != null) {
+      return _onResumeImageStream();
     }
 
     stoppingCamera = false;
 
-    _controller = CameraController(
-      _camera!,
-      ResolutionPreset.medium,
-      enableAudio: false,
-      imageFormatGroup: ImageFormatGroup.yuv420,
-    );
-
     // If the controller is initialized update the UI.
     _barcodeDecoder ??= MLKitScanDecoder(
       camera: _camera!,
@@ -177,47 +168,49 @@ class MLKitScannerPageState
         ),
       ),
     );
-    _controller?.addListener(_cameraListener);
 
-    // Restart the subscription if necessary
-    if (_streamSubscription?.isPaused == true) {
-      _streamSubscription!.resume();
-    } else {
-      _subject
-          .throttleTime(
-            Duration(
-              milliseconds:
-                  _averageProcessingTime.average(_defaultProcessingTime) *
-                      _processingTimeWindows,
-            ),
-          )
-          .asyncMap((CameraImage image) async {
-            final DateTime start = DateTime.now();
+    CameraHelper.initController(
+      SmoothCameraController(
+        _camera!,
+        ResolutionPreset.medium,
+        enableAudio: false,
+        imageFormatGroup: ImageFormatGroup.yuv420,
+      ),
+    );
 
-            final List<String?>? res =
-                await _barcodeDecoder?.processImage(image);
+    _controller!.addListener(_cameraListener);
 
-            _averageProcessingTime.add(
-              DateTime.now().difference(start).inMilliseconds,
-            );
+    _subject
+        .throttleTime(
+          Duration(
+            milliseconds:
+                _averageProcessingTime.average(_defaultProcessingTime) *
+                    _processingTimeWindows,
+          ),
+        )
+        .asyncMap((CameraImage image) async {
+          final DateTime start = DateTime.now();
 
-            return res;
-          })
-          .where(
-            (List<String?>? barcodes) => barcodes?.isNotEmpty == true,
-          )
-          .cast<List<String>>()
-          .listen(_onNewBarcodeDetected);
-    }
+          final List<String?>? res = await _barcodeDecoder?.processImage(image);
+
+          _averageProcessingTime.add(
+            DateTime.now().difference(start).inMilliseconds,
+          );
+
+          return res;
+        })
+        .where(
+          (List<String?>? barcodes) => barcodes?.isNotEmpty == true,
+        )
+        .cast<List<String>>()
+        .listen(_onNewBarcodeDetected);
 
     try {
-      await _controller?.initialize();
-      await _controller?.setFocusMode(FocusMode.auto);
-      await _controller?.setFocusPoint(_focusPoint);
-      await _controller?.setExposurePoint(_focusPoint);
-      await _controller?.lockCaptureOrientation(DeviceOrientation.portraitUp);
-      await _controller?.startImageStream(
-        (CameraImage image) => _subject.add(image),
+      await _controller?.init(
+        focusMode: FocusMode.auto,
+        focusPoint: _focusPoint,
+        deviceOrientation: DeviceOrientation.portraitUp,
+        onAvailable: (CameraImage image) => _subject.add(image),
       );
     } on CameraException catch (e) {
       if (kDebugMode) {
@@ -238,33 +231,55 @@ class MLKitScannerPageState
   }
 
   void _cameraListener() {
+    _redrawScreen();
+
     if (_controller?.value.hasError == true) {
       // TODO(M123): Handle errors better
       debugPrint(_controller!.value.errorDescription);
     }
   }
 
-  Future<void> _stopImageStream({bool fromPauseEvent = false}) async {
+  Future<void> _onPauseImageStream() async {
     if (stoppingCamera) {
       return;
     }
 
+    _controller?.pausePreview();
+    _streamSubscription?.pause();
+  }
+
+  Future<void> _onResumeImageStream({bool forceStartPreview = false}) async {
+    if (stoppingCamera ||
+        (!forceStartPreview && ScreenVisibilityDetector.invisible(context))) {
+      return;
+    }
+
+    _controller?.resumePreviewIfNecessary();
+    stoppingCamera = false;
+
+    if (_streamSubscription?.isPaused == true) {
+      _streamSubscription!.resume();
+    }
+  }
+
+  Future<void> _stopImageStream() async {
+    if (stoppingCamera) {
+      return;
+    }
+
+    _controller?.pausePreview();
+
     stoppingCamera = true;
     _redrawScreen();
 
     _controller?.removeListener(_cameraListener);
 
-    if (fromPauseEvent) {
-      _streamSubscription?.pause();
-    } else {
-      await _streamSubscription?.cancel();
-    }
+    await _streamSubscription?.cancel();
 
     await _controller?.dispose();
     await _barcodeDecoder?.dispose();
 
     _barcodeDecoder = null;
-    _controller = null;
 
     _restartCameraIfNecessary();
   }
@@ -306,7 +321,7 @@ class MLKitScannerPageState
   void dispose() {
     // /!\ This call is a Future, which may leads to some issues.
     // This should be handled by [_restartCameraIfNecessary]
-    _stopImageStream(fromPauseEvent: false);
+    _stopImageStream();
     super.dispose();
   }
 
@@ -321,4 +336,6 @@ class MLKitScannerPageState
       return Offset(0.5, 0.25 / _previewScale);
     }
   }
+
+  SmoothCameraController? get _controller => CameraHelper.controller;
 }

packages/smooth_app/lib/widgets/screen_visibility.dart

@@ -16,6 +16,8 @@ class ScreenVisibilityDetector extends StatefulWidget {
 
   static bool visible(BuildContext context) =>
       context.read<ScreenVisibility>().isVisible;
+
+  static bool invisible(BuildContext context) => !visible(context);
 }
 
 class _ScreenVisibilityDetectorState extends State<ScreenVisibilityDetector> {