manage media stream constraints listener

@@ -95,6 +95,13 @@ abstract class BarcodeReader {
     await completer.future;
   }
 
+  /// Set a listener for the media stream constraints.
+  void setMediaTrackConstraintsListener(void Function(MediaTrackConstraints) listener) {
+    throw UnimplementedError(
+      'setMediaTrackConstraintsListener() has not been implemented.',
+    );
+  }
+
   /// Set the torch state for the active camera to the given [value].
   Future<void> setTorchState(TorchState value) {
     throw UnimplementedError('setTorchState() has not been implemented.');

@@ -33,6 +33,9 @@ class MobileScannerWeb extends MobileScannerPlatform {
   /// This container element is used by the barcode reader.
   HTMLDivElement? _divElement;
 
+  /// The stream controller for the media track constraints stream.
+  final StreamController<MediaTrackConstraints> _constraintsController = StreamController.broadcast();
+
   /// The view type for the platform view factory.
   final String _viewType = 'MobileScannerWeb';
 
@@ -43,6 +46,14 @@ class MobileScannerWeb extends MobileScannerPlatform {
   @override
   Stream<BarcodeCapture?> get barcodesStream => _barcodesController.stream;
 
+  void _handleMediaTrackConstraintsChange(MediaTrackConstraints constraints) {
+    if (_constraintsController.isClosed) {
+      return;
+    }
+
+    _constraintsController.add(constraints);
+  }
+
   @override
   Widget buildCameraView() {
     if (!_barcodeReader.isScanning) {
@@ -86,6 +97,9 @@ class MobileScannerWeb extends MobileScannerPlatform {
       // Clear the existing barcodes.
       _barcodesController.add(const BarcodeCapture());
 
+      // Listen for changes to the media track constraints.
+      _barcodeReader.setMediaTrackConstraintsListener(_handleMediaTrackConstraintsChange);
+
       await _barcodeReader.start(
         startOptions,
         containerElement: _divElement!,
@@ -170,5 +184,6 @@ class MobileScannerWeb extends MobileScannerPlatform {
 
     await stop();
     await _barcodesController.close();
+    await _constraintsController.close();
   }
 }

@@ -19,7 +19,7 @@ final class ZXingBarcodeReader extends BarcodeReader {
   ZXingBarcodeReader();
 
   /// The internal media stream track constraints delegate.
-  final MediaTrackConstraintsDelegate _flashlightDelegate = const MediaTrackConstraintsDelegate();
+  final MediaTrackConstraintsDelegate _mediaTrackConstraintsDelegate = const MediaTrackConstraintsDelegate();
 
   /// The internal barcode reader.
   ZXingBrowserMultiFormatReader? _reader;
@@ -180,7 +180,7 @@ final class ZXingBarcodeReader extends BarcodeReader {
       return Future<bool>.value(false);
     }
 
-    return _flashlightDelegate.hasFlashlight(mediaStream);
+    return _mediaTrackConstraintsDelegate.hasFlashlight(mediaStream);
   }
 
   @override
@@ -196,7 +196,7 @@ final class ZXingBarcodeReader extends BarcodeReader {
           return Future<void>.value();
         }
 
-        return _flashlightDelegate.setFlashlightState(mediaStream, value);
+        return _mediaTrackConstraintsDelegate.setFlashlightState(mediaStream, value);
     }
   }