[firebase_ml_vision] visionText gets always empty for iOS

Describe the bug TextDetectorPainter always gets visonText as empty list for iOS. What should I do to solve this problem.

To Reproduce Dart source files in directory 'lib' is the same as follow: flutterfire/packages/firebase_ml_vision/example/lib/ iOS 13.4 (iPhone 8 Plus) Flutter 1.12.13+hotfix.9 Xcode 11.4

pubspec.yaml:

environment:
  sdk: ">=2.7.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter

  cupertino_icons: ^0.1.2
  firebase_ml_vision: ^0.9.3+8
  camera: ^0.5.7+4
  path_provider: ^1.6.7
  image_picker: ^0.6.5+3

detector_painters.dart:

class TextDetectorPainter extends CustomPainter {
  TextDetectorPainter(this.absoluteImageSize, this.visionText);

  final Size absoluteImageSize;
  final VisionText visionText;

  @override
  void paint(Canvas canvas, Size size) {
    final double scaleX = size.width / absoluteImageSize.width;
    final double scaleY = size.height / absoluteImageSize.height;

    Rect scaleRect(TextContainer container) {
      return Rect.fromLTRB(
        container.boundingBox.left * scaleX,
        container.boundingBox.top * scaleY,
        container.boundingBox.right * scaleX,
        container.boundingBox.bottom * scaleY,
      );
    }

    final Paint paint = Paint()
      ..style = PaintingStyle.stroke
      ..strokeWidth = 2.0;

    print('visionText:' + visionText.blocks.toString());
    for (TextBlock block in visionText.blocks) {
      for (TextLine line in block.lines) {
        for (TextElement element in line.elements) {
          paint.color = Colors.green;
          canvas.drawRect(scaleRect(element), paint);
        }

        paint.color = Colors.yellow;
        canvas.drawRect(scaleRect(line), paint);
      }

      paint.color = Colors.red;
      canvas.drawRect(scaleRect(block), paint);
    }
  }

camera_preview_scanner.dart:

class _CameraPreviewScannerState extends State<CameraPreviewScanner> {
  dynamic _scanResults;
  CameraController _camera;
  Detector _currentDetector = Detector.text;
  bool _isDetecting = false;
  CameraLensDirection _direction = CameraLensDirection.back;

  final BarcodeDetector _barcodeDetector =
      FirebaseVision.instance.barcodeDetector();
  final FaceDetector _faceDetector = FirebaseVision.instance.faceDetector();
  final ImageLabeler _imageLabeler = FirebaseVision.instance.imageLabeler();
  final ImageLabeler _cloudImageLabeler =
      FirebaseVision.instance.cloudImageLabeler();
  final TextRecognizer _recognizer = FirebaseVision.instance.textRecognizer();
  final TextRecognizer _cloudRecognizer =
      FirebaseVision.instance.cloudTextRecognizer();

  @override
  void initState() {
    super.initState();
    _initializeCamera();
  }

  void _initializeCamera() async {
    final CameraDescription description =
        await ScannerUtils.getCamera(_direction);

    _camera = CameraController(
      description,
      defaultTargetPlatform == TargetPlatform.iOS
          ? ResolutionPreset.high
          : ResolutionPreset.medium,
    );
    await _camera.initialize();

    _camera.startImageStream((CameraImage image) {
      if (_isDetecting) return;

      _isDetecting = true;

      ScannerUtils.detect(
        image: image,
        detectInImage: _getDetectionMethod(),
        imageRotation: description.sensorOrientation,
      ).then(
        (dynamic results) {
          if (_currentDetector == null) return;
          setState(() {
            _scanResults = results;
          });
        },
      ).whenComplete(() => _isDetecting = false);
    });
  }

  Future<dynamic> Function(FirebaseVisionImage image) _getDetectionMethod() {
    switch (_currentDetector) {
      case Detector.text:
        return _recognizer.processImage;
      case Detector.cloudText:
        return _cloudRecognizer.processImage;
      case Detector.barcode:
        return _barcodeDetector.detectInImage;
      case Detector.label:
        return _imageLabeler.processImage;
      case Detector.cloudLabel:
        return _cloudImageLabeler.processImage;
      case Detector.face:
        return _faceDetector.processImage;
    }

    return null;
  }

  Widget _buildResults() {
    const Text noResultsText = Text('No results!');

    if (_scanResults == null ||
        _camera == null ||
        !_camera.value.isInitialized) {
      return noResultsText;
    }

    CustomPainter painter;

    final Size imageSize = Size(
      _camera.value.previewSize.height,
      _camera.value.previewSize.width,
    );

    switch (_currentDetector) {
      case Detector.barcode:
        if (_scanResults is! List<Barcode>) return noResultsText;
        painter = BarcodeDetectorPainter(imageSize, _scanResults);
        break;
      case Detector.face:
        if (_scanResults is! List<Face>) return noResultsText;
        painter = FaceDetectorPainter(imageSize, _scanResults);
        break;
      case Detector.label:
        if (_scanResults is! List<ImageLabel>) return noResultsText;
        painter = LabelDetectorPainter(imageSize, _scanResults);
        break;
      case Detector.cloudLabel:
        if (_scanResults is! List<ImageLabel>) return noResultsText;
        painter = LabelDetectorPainter(imageSize, _scanResults);
        break;
      default:
        assert(_currentDetector == Detector.text ||
            _currentDetector == Detector.cloudText);
        if (_scanResults is! VisionText) return noResultsText;
        painter = TextDetectorPainter(imageSize, _scanResults);
    }

    return CustomPaint(
      painter: painter,
    );
  }

Output

flutter: visionText:[]
flutter: visionText:[]
flutter: visionText:[]
flutter: visionText:[]
flutter: visionText:[]
flutter: visionText:[]

firebase / flutterfire

[firebase_ml_vision] visionText gets always empty for iOS #2502