reimplement mobile_scanner web with platform interface

+import 'dart:async';
+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/mobile_scanner_error_code.dart';
+import 'package:mobile_scanner/src/enums/torch_state.dart';
+import 'package:mobile_scanner/src/mobile_scanner_exception.dart';
 import 'package:mobile_scanner/src/mobile_scanner_platform_interface.dart';
+import 'package:mobile_scanner/src/mobile_scanner_view_attributes.dart';
+import 'package:mobile_scanner/src/objects/barcode_capture.dart';
+import 'package:mobile_scanner/src/objects/start_options.dart';
+import 'package:mobile_scanner/src/web/barcode_reader.dart';
+import 'package:mobile_scanner/src/web/zxing/zxing_barcode_reader.dart';
+import 'package:web/web.dart';
 
 /// A web implementation of the MobileScannerPlatform of the MobileScanner plugin.
 class MobileScannerWeb extends MobileScannerPlatform {
   /// Constructs a [MobileScannerWeb] instance.
   MobileScannerWeb();
 
+  /// The internal barcode reader.
+  final BarcodeReader _barcodeReader = ZXingBarcodeReader();
+
+  /// The stream controller for the barcode stream.
+  final StreamController<BarcodeCapture> _barcodesController = StreamController.broadcast();
+
+  /// The subscription for the barcode stream.
+  StreamSubscription<Object?>? _barcodesSubscription;
+
+  /// The container div element for the camera view.
+  ///
+  /// This container element is used by the barcode reader.
+  HTMLDivElement? _divElement;
+
+  /// The view type for the platform view factory.
+  final String _viewType = 'MobileScannerWeb';
+
   static void registerWith(Registrar registrar) {
     MobileScannerPlatform.instance = MobileScannerWeb();
   }
+
+  @override
+  Widget buildCameraView() {
+    if (!_barcodeReader.isScanning) {
+      throw const MobileScannerException(
+        errorCode: MobileScannerErrorCode.controllerUninitialized,
+        errorDetails: MobileScannerErrorDetails(
+          message: 'The controller was not yet initialized. Call start() before calling buildCameraView().',
+        ),
+      );
+    }
+
+    return HtmlElementView(viewType: _viewType);
+  }
+
+  @override
+  Future<MobileScannerViewAttributes> start(StartOptions startOptions) async {
+    await _barcodeReader.maybeLoadLibrary();
+
+    // Setup the view factory & container element.
+    if (_divElement == null) {
+      _divElement = (document.createElement('div') as HTMLDivElement)
+        ..style.width = '100%'
+        ..style.height = '100%';
+
+      ui_web.platformViewRegistry.registerViewFactory(
+        _viewType,
+        (int id) => _divElement,
+      );
+    }
+
+    if (_barcodeReader.isScanning) {
+      throw const MobileScannerException(
+        errorCode: MobileScannerErrorCode.controllerAlreadyInitialized,
+        errorDetails: MobileScannerErrorDetails(
+          message: 'The scanner was already started. Call stop() before calling start() again.',
+        ),
+      );
+    }
+
+    try {
+      await _barcodeReader.start(
+        startOptions,
+        containerElement: _divElement!,
+      );
+    } 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,
+        errorDetails: MobileScannerErrorDetails(
+          message: errorMessage,
+          details: stackTrace.toString(),
+        ),
+      );
+    }
+
+    try {
+      _barcodesSubscription = _barcodeReader.detectBarcodes().listen((BarcodeCapture barcode) {
+        if (_barcodesController.isClosed) {
+          return;
+        }
+
+        _barcodesController.add(barcode);
+      });
+
+      final bool hasTorch = _barcodeReader.hasTorch;
+
+      if (hasTorch && startOptions.torchEnabled) {
+        await _barcodeReader.setTorchState(TorchState.on).catchError((_) {
+          // If the torch could not be turned on, continue the camera session anyway.
+        });
+      }
+
+      return MobileScannerViewAttributes(
+        hasTorch: hasTorch,
+        size: _barcodeReader.outputSize,
+      );
+    } catch (error, stackTrace) {
+      throw MobileScannerException(
+        errorCode: MobileScannerErrorCode.genericError,
+        errorDetails: MobileScannerErrorDetails(
+          message: error.toString(),
+          details: stackTrace.toString(),
+        ),
+      );
+    }
+  }
+
+  @override
+  Future<void> stop() async {
+    if (_barcodesController.isClosed) {
+      return;
+    }
+
+    _barcodesSubscription?.cancel();
+    _barcodesSubscription = null;
+
+    // Clear the existing barcodes.
+    _barcodesController.add(const BarcodeCapture());
+
+    await _barcodeReader.stop();
+  }
+
+  @override
+  Future<void> updateScanWindow(Rect? window) {
+    // A scan window is not supported on the web,
+    // because the scanner does not expose size information for the barcodes.
+    return Future<void>.value();
+  }
+
+  @override
+  Future<void> dispose() async {
+    if (_barcodesController.isClosed) {
+      return;
+    }
+
+    await stop();
+    await _barcodeReader.dispose();
+    await _barcodesController.close();
+  }
 }