Question

我对Flutter / Dart非常陌生，并且正在使用flutter_bloc块/存储库模式。

我正在努力从存储库方法获取值，而在bloc方法中却得到null。

块部分工作正常，但从表单存储库方法获得的LatLng期望值为空。

bloc的值和“事件到状态”方法：

LatLng locationStream;    

Stream<MapState> _mapGetLocationStreamToState(
          GetLocationStream event) async* {
        print('_mapGetLocationStreamToState event received : $event');

        locationStream = await _mapRepository.getLocationStream();
        print('_mapGetLocationStreamToState() locationStream is: $locationStream ');
        yield LocationStream(locationStream);
      }

打印：

颤振：_mapGetLocationStreamToState（）locationStream为：null

flutter：交易正在过渡{currentState：LocationStream {location：null}，事件：“ GetLocationStream”的实例，nextState： LocationStream {location：null}}

我尝试使用T和Future<T>的存储库方法返回类型，都从不同的范围返回值（请参见注释掉的返回值），但是无论从何处返回值，我总是得到null。您能找出问题所在吗？应该使用getLocation()方法吧？

非常感谢您的时间和帮助。

T：

    LatLng getLocationStream() {
    print('getLocationStream() called');
    LatLng location;
    LocationOptions locationOptions = LocationOptions(
        accuracy: LocationAccuracy.bestForNavigation, distanceFilter: 0);
    try {
      if (isTracking == true) {
        _positionStreamSubsciption.cancel();
        isTracking = !isTracking;
      } else {
        _positionStreamSubsciption = locationManager
            .getPositionStream(locationOptions)
            .listen((Position position) {
          if (position != null) {
            location = LatLng(position.latitude, position.longitude);
//            return location; // stops the stream
          }
          isTracking = !isTracking;
          print('getLocationStream() location is : $location');
//          return location; // returns null
        });
//        return location; // returns null
      }
//      return location; // returns null
    } catch (error) {
      print('startTracking error: $error');
    }
    return location; // returns null
  }

打印：

flutter：getLocationStream（）的位置是：LatLng（latitude：37.33022，经度：-122.023788）抖动：getLocationStream（）的位置是： LatLng（纬度：37.330219，经度：-122.023748）抖动： getLocationStream（）的位置是：LatLng（纬度：37.330219，经度：-122.023708）抖动：getLocationStream（）的位置是： LatLng（纬度：37.330221，经度：-122.023668）

未来：

Future<LatLng> getLocationStream() {
    print('getLocationStream() called');
    print('isTracking was : $isTracking');
    Future<LatLng> location;
    LocationOptions locationOptions = LocationOptions(
        accuracy: LocationAccuracy.bestForNavigation, distanceFilter: 0);
    try {
      if (isTracking == true) {
        _positionStreamSubsciption.cancel();
//        isTracking = !isTracking;
//        print('isTracking was ${!isTracking} and now is : $isTracking');
      } else {
        _positionStreamSubsciption = locationManager
            .getPositionStream(locationOptions)
            .listen((Position position) {
          if (position != null) {
            location =
                LatLng(position.latitude, position.longitude) as Future<LatLng>;
//            return location;

          }
          print('getLocationStream() location is : $location');
//          return location;
        });
//        return location;
      }

      isTracking = !isTracking;
      print('isTracking is : $isTracking');
//      return location;
    } catch (error) {
      print('startTracking error: $error');
    }
  }

打印：

flutter：getLocationStream（）的位置是：LatLng（latitude：37.324746，经度：-122.021542）抖动：getLocationStream（）位置为： LatLng（纬度：37.324746，经度：-122.021573）抖动： getLocationStream（）的位置是：LatLng（latitude：37.324746，经度：-122.021605）

Answer 1

稍微摆弄代码后，我决定更改方法，似乎以前选择了错误的代码。我决定不转换存储库方法的LatLng，而是决定转换Stream<Position> coming from Geolocator API directly into a Stream，并从bloc收听它。

现在状态如预期般流动，并带有新的位置值。

要解决的唯一问题（我实际上以为我没有）是MapScren BlocBuilder不能容纳来自新状态的值，而当像_mapController.move(userLocation, 16);中一样使用它。

LatLng userLocation = (state as LocationStream).location;不是正确的方法吗？

更新的代码是：

BlocBuilder：

bloc: MapBloc(mapRepository: _mapRepository),
        builder: (BuildContext context, MapState state) {
          LatLng userLocation = (state as LocationStream).location;
          return Scaffold(
..

存储库：

Stream<LatLng> getLocationStream() {
    print('getLocationStream() called');
    print('isTracking was : $isTracking');
    Stream<LatLng> locationStream;
    Stream<Position> _positionStream;
    LocationOptions locationOptions = LocationOptions(
        accuracy: LocationAccuracy.bestForNavigation, distanceFilter: 0);
    try {
      if (isTracking == true) {
        _positionStreamSubsciption.cancel();
      } else {
        _positionStream = locationManager.getPositionStream(locationOptions);

        handleData(Position position, EventSink<LatLng> sink) =>
            sink.add(LatLng(position.latitude, position.longitude));

        final transformer = StreamTransformer<Position, LatLng>.fromHandlers(
            handleData: handleData);
        locationStream = _positionStream.transform(transformer);
        return locationStream;
      }

      isTracking = !isTracking;
      print('isTracking is : $isTracking');
    } catch (error) {
      print('startTracking error: $error');
    }
  }

bloc：

    MapState get initialState => LocationStream(locationStream);

  @override
  Stream<MapState> mapEventToState(MapEvent event) async* {
    // user location
    if (event is GetLocationStream) {
      print('MapBloc event received : $event');
      yield* _mapGetLocationStreamToState(event);
    }
    if (event is LocationUpdated) {
      yield* _mapLocationUpdatedToState(event);
    }
  }


  Stream<MapState> _mapGetLocationStreamToState(
      GetLocationStream event) async* {
    print('_mapGetLocationStreamToState event received : $event');

    _locationStreamSubscription =
        _mapRepository.getLocationStream().listen((LatLng location) {
          locationStream = location;
          add(LocationUpdated(locationStream));
          print(
              '_mapGetLocationStreamToState() locationStream is: $locationStream ');
        });

//    yield LocationStream(locationStream);
  }

控制台打印：

颤振：事件为LocationUpdated {更新位置： LatLng（纬度：37.3246，经度：-122.02457）}颤动： _mapGetLocationStreamToState（）locationStream是：LatLng（纬度：37.3246，经度：-122.02457）抖动：交易记录为 Transition {currentState：LocationStream {location： LatLng（纬度：37.324603，经度：-122.024523）}，事件： LocationUpdated {更新位置：LatLng（latitude：37.3246，经度：-122.02457）}，nextState：LocationStream {location： LatLng（纬度：37.3246，经度：-122.02457）}}

我从方法中获得了空返回值。扑

1 个答案: