move media stream track creation to the MobileScannerWeb implementation instead …

…of inside the barcode reader

@@ -111,13 +111,15 @@ abstract class BarcodeReader {
     throw UnimplementedError('setTorchState() has not been implemented.');
   }
 
-  /// Start the barcode reader and initialize the video stream.
+  /// Start the barcode reader and initialize the [videoStream].
   ///
   /// The [options] are used to configure the barcode reader.
   /// The [containerElement] will become the parent of the video output element.
+  /// The [videoStream] is the input for the barcode reader and video preview element.
   Future<void> start(
     StartOptions options, {
     required HTMLElement containerElement,
+    required MediaStream videoStream,
   }) {
     throw UnimplementedError('start() has not been implemented.');
   }

 import 'dart:async';
+import 'dart:js_interop';
 import 'dart:ui_web' as ui_web;
 
 import 'package:flutter/widgets.dart';
 import 'package:flutter_web_plugins/flutter_web_plugins.dart';
+import 'package:mobile_scanner/src/enums/camera_facing.dart';
 import 'package:mobile_scanner/src/enums/mobile_scanner_error_code.dart';
 import 'package:mobile_scanner/src/enums/torch_state.dart';
 import 'package:mobile_scanner/src/mobile_scanner_exception.dart';
@@ -69,6 +71,93 @@ class MobileScannerWeb extends MobileScannerPlatform {
     _settingsController.add(settings);
   }
 
+  /// Prepare a [MediaStream] for the video output.
+  ///
+  /// This method requests permission to use the camera.
+  ///
+  /// Throws a [MobileScannerException] if the permission was denied,
+  /// or if using a video stream, with the given set of constraints, is unsupported.
+  Future<MediaStream> _prepareVideoStream(
+    CameraFacing cameraDirection,
+  ) async {
+    if ((window.navigator.mediaDevices as JSAny?).isUndefinedOrNull) {
+      throw const MobileScannerException(
+        errorCode: MobileScannerErrorCode.unsupported,
+        errorDetails: MobileScannerErrorDetails(
+          message: 'This browser does not support displaying video from the camera.',
+        ),
+      );
+    }
+
+    final MediaTrackSupportedConstraints capabilities = window.navigator.mediaDevices.getSupportedConstraints();
+
+    final MediaStreamConstraints constraints;
+
+    if ((capabilities as JSAny).isUndefinedOrNull || !capabilities.facingMode) {
+      constraints = MediaStreamConstraints(video: true.toJS);
+    } else {
+      final String facingMode = switch (cameraDirection) {
+        CameraFacing.back => 'environment',
+        CameraFacing.front => 'user',
+      };
+
+      constraints = MediaStreamConstraints(
+        video: MediaTrackConstraintSet(facingMode: facingMode.toJS) as JSAny,
+      );
+    }
+
+    try {
+      // Retrieving the video track requests the camera permission.
+      // If the completer is not null, the permission was never requested before.
+      _cameraPermissionCompleter ??= Completer<void>();
+
+      final MediaStream? videoStream =
+          await window.navigator.mediaDevices.getUserMedia(constraints).toDart as MediaStream?;
+
+      // At this point the permission is granted.
+      if (!_cameraPermissionCompleter!.isCompleted) {
+        _cameraPermissionCompleter!.complete();
+      }
+
+      if (videoStream == null) {
+        throw const MobileScannerException(
+          errorCode: MobileScannerErrorCode.genericError,
+          errorDetails: MobileScannerErrorDetails(
+            message: 'Could not create a video stream from the camera with the given options. '
+                'The browser might not support the given constraints.',
+          ),
+        );
+      }
+
+      return videoStream;
+    } on DOMException catch (error, stackTrace) {
+      // At this point the permission request completed, although with an error,
+      // but the error is irrelevant for the completer.
+      if (!_cameraPermissionCompleter!.isCompleted) {
+        _cameraPermissionCompleter!.complete();
+      }
+
+      final String errorMessage = error.toString();
+
+      MobileScannerErrorCode errorCode = MobileScannerErrorCode.genericError;
+
+      // Handle both unsupported and permission errors from the web.
+      if (errorMessage.contains('NotFoundError') || errorMessage.contains('NotSupportedError')) {
+        errorCode = MobileScannerErrorCode.unsupported;
+      } else if (errorMessage.contains('NotAllowedError')) {
+        errorCode = MobileScannerErrorCode.permissionDenied;
+      }
+
+      throw MobileScannerException(
+        errorCode: errorCode,
+        errorDetails: MobileScannerErrorDetails(
+          message: errorMessage,
+          details: stackTrace.toString(),
+        ),
+      );
+    }
+  }
+
   @override
   Widget buildCameraView() {
     if (!_barcodeReader.isScanning) {
@@ -111,6 +200,11 @@ class MobileScannerWeb extends MobileScannerPlatform {
       );
     }
 
+    // Request camera permissions and prepare the video stream.
+    final MediaStream videoStream = await _prepareVideoStream(
+      startOptions.cameraDirection,
+    );
+
     try {
       // Clear the existing barcodes.
       if (!_barcodesController.isClosed) {
@@ -125,25 +219,13 @@ class MobileScannerWeb extends MobileScannerPlatform {
       await _barcodeReader.start(
         startOptions,
         containerElement: _divElement!,
+        videoStream: videoStream,
       );
     } catch (error, stackTrace) {
-      final String errorMessage = error.toString();
-
-      MobileScannerErrorCode errorCode = MobileScannerErrorCode.genericError;
-
-      if (error is DOMException) {
-        if (errorMessage.contains('NotFoundError') ||
-            errorMessage.contains('NotSupportedError')) {
-          errorCode = MobileScannerErrorCode.unsupported;
-        } else if (errorMessage.contains('NotAllowedError')) {
-          errorCode = MobileScannerErrorCode.permissionDenied;
-        }
-      }
-
       throw MobileScannerException(
-        errorCode: errorCode,
+        errorCode: MobileScannerErrorCode.genericError,
         errorDetails: MobileScannerErrorDetails(
-          message: errorMessage,
+          message: error.toString(),
           details: stackTrace.toString(),
         ),
       );

@@ -3,7 +3,6 @@ import 'dart:js_interop';
 import 'dart:ui';
 
 import 'package:mobile_scanner/src/enums/barcode_format.dart';
-import 'package:mobile_scanner/src/enums/camera_facing.dart';
 import 'package:mobile_scanner/src/enums/torch_state.dart';
 import 'package:mobile_scanner/src/objects/barcode_capture.dart';
 import 'package:mobile_scanner/src/objects/start_options.dart';
@@ -106,43 +105,6 @@ final class ZXingBarcodeReader extends BarcodeReader {
     return hints;
   }
 
-  /// Prepare the [web.MediaStream] for the barcode reader video input.
-  ///
-  /// This method requests permission to use the camera.
-  Future<web.MediaStream?> _prepareMediaStream(
-    CameraFacing cameraDirection,
-  ) async {
-    if ((web.window.navigator.mediaDevices as JSAny?).isUndefinedOrNull) {
-      return null;
-    }
-
-    final capabilities =
-        web.window.navigator.mediaDevices.getSupportedConstraints();
-
-    final web.MediaStreamConstraints constraints;
-
-    if ((capabilities as JSAny).isUndefinedOrNull || !capabilities.facingMode) {
-      constraints = web.MediaStreamConstraints(video: true.toJS);
-    } else {
-      final String facingMode = switch (cameraDirection) {
-        CameraFacing.back => 'environment',
-        CameraFacing.front => 'user',
-      };
-
-      constraints = web.MediaStreamConstraints(
-        video: web.MediaTrackConstraintSet(
-          facingMode: facingMode.toJS,
-        ) as JSAny,
-      );
-    }
-
-    final JSAny? mediaStream = await web.window.navigator.mediaDevices
-        .getUserMedia(constraints)
-        .toDart;
-
-    return mediaStream as web.MediaStream?;
-  }
-
   /// Prepare the video element for the barcode reader.
   ///
   /// The given [videoElement] is attached to the DOM, by attaching it to the [containerElement].
@@ -150,31 +112,25 @@ final class ZXingBarcodeReader extends BarcodeReader {
   /// and the video element (to display the camera output).
   Future<void> _prepareVideoElement(
     web.HTMLVideoElement videoElement, {
-    required CameraFacing cameraDirection,
+    required web.MediaStream videoStream,
     required web.HTMLElement containerElement,
   }) async {
     // Attach the video element to the DOM, through its parent container.
     containerElement.appendChild(videoElement);
 
-    // Set up the camera output stream.
-    // This will request permission to use the camera.
-    final web.MediaStream? stream = await _prepareMediaStream(cameraDirection);
-
-    if (stream != null) {
-      final JSPromise? result = _reader?.attachStreamToVideo.callAsFunction(
-        _reader as JSAny?,
-        stream as JSAny,
-        videoElement as JSAny,
-      ) as JSPromise?;
+    final JSPromise? result = _reader?.attachStreamToVideo.callAsFunction(
+      _reader as JSAny?,
+      videoStream as JSAny,
+      videoElement as JSAny,
+    ) as JSPromise?;
 
-      await result?.toDart;
+    await result?.toDart;
 
-      final web.MediaTrackSettings? settings =
-          _mediaTrackConstraintsDelegate.getSettings(stream);
+    final web.MediaTrackSettings? settings =
+        _mediaTrackConstraintsDelegate.getSettings(videoStream);
 
-      if (settings != null) {
-        _onMediaTrackSettingsChanged?.call(settings);
-      }
+    if (settings != null) {
+      _onMediaTrackSettingsChanged?.call(settings);
     }
   }
 
@@ -261,6 +217,7 @@ final class ZXingBarcodeReader extends BarcodeReader {
   Future<void> start(
     StartOptions options, {
     required web.HTMLElement containerElement,
+    required web.MediaStream videoStream,
   }) async {
     final int detectionTimeoutMs = options.detectionTimeoutMs;
     final List<BarcodeFormat> formats = options.formats;
@@ -279,7 +236,7 @@ final class ZXingBarcodeReader extends BarcodeReader {
 
     await _prepareVideoElement(
       videoElement,
-      cameraDirection: options.cameraDirection,
+      videoStream: videoStream,
       containerElement: containerElement,
     );
   }