断言失败：第 20 行第 16 行：'经度！= null'：不正确

Question

我正在尝试构建一个出租车应用程序，乘客应该在其中搜索下车地点，但每次用户搜索一个位置并使用 Google 自动完成点击它时，它都会不断带来这个错误：[ERROR:flutter/lib/ui/ui_dart_state.cc(199)] 未处理的异常：'package:google_maps_flutter_platform_interface/src/types/location.dart'：断言失败：第 20 行 pos 16：'longitude != null'：不是真的。

那个错误指向这个方法

Future<void> getPlaceDirection() async
  {
    var initialPos = Provider.of<AppData>(context, listen: false).pickUpLocation;
    var finalPos = Provider.of<AppData>(context, listen: false).dropOffLocation;

    var pickUpLatLng = LatLng(initialPos.latitude, initialPos.longitude);
    var dropOffLatLng = LatLng(finalPos.latitude, finalPos.longitude);

    showDialog(
      context: context, 
      builder: (BuildContext content) => ProgressDialog(message: "Please wait...",)
    );
    var details = await AssistantMethods.obtainPlaceDirectionDetails(pickUpLatLng, dropOffLatLng);
    setState(() {
          tripDirectionDetails = details;
        });

    Navigator.pop(context);

    print("This is the encoded points:: ");
    print(details.encodedPoints);

    PolylinePoints polylinePoints = PolylinePoints();
    List<PointLatLng> decodedPolyLinePointsResult = polylinePoints.decodePolyline(details.encodedPoints);
    pLineCoordinates.clear();

    if (decodedPolyLinePointsResult.isNotEmpty)
    {
      decodedPolyLinePointsResult.forEach((PointLatLng pointLatLng)
      {
         pLineCoordinates.add(LatLng(pointLatLng.latitude, pointLatLng.longitude));
      });
    }

    polylineSet.clear();

    setState(() {
     Polyline polyline = Polyline(
      color: orange,
      polylineId: PolylineId("PolylineID"),
      jointType: JointType.round,
      points: pLineCoordinates,
      width: 5,
      startCap: Cap.roundCap,
      endCap: Cap.roundCap,
      geodesic: true,
       );

       polylineSet.add(polyline);
    });

    LatLngBounds latLngBounds;
      if(pickUpLatLng.latitude > dropOffLatLng.latitude && pickUpLatLng.longitude > dropOffLatLng.longitude)
      {
        latLngBounds = LatLngBounds(southwest: dropOffLatLng, northeast: pickUpLatLng);
      }
      else if(pickUpLatLng.longitude> dropOffLatLng.longitude)
      {
        latLngBounds = LatLngBounds(southwest: LatLng(pickUpLatLng.latitude, dropOffLatLng.longitude), 
        northeast: LatLng(dropOffLatLng.latitude, pickUpLatLng.longitude));
      }
      else if(pickUpLatLng.latitude > dropOffLatLng.latitude)
      {
        latLngBounds = LatLngBounds(southwest: LatLng(dropOffLatLng.latitude, pickUpLatLng.longitude), 
        northeast: LatLng(pickUpLatLng.latitude, dropOffLatLng.longitude));
      } 
      else 
      {
        latLngBounds = LatLngBounds(southwest: pickUpLatLng, northeast: dropOffLatLng);
      }

      newGoogleMapController.animateCamera(CameraUpdate.newLatLngBounds(latLngBounds, 70));

      Marker pickUpLocMarker = Marker(
        icon: BitmapDescriptor.defaultMarkerWithHue(BitmapDescriptor.hueBlue),
        infoWindow: InfoWindow(title: initialPos.placeName, snippet: "My location"),
        position:  pickUpLatLng,
        markerId: MarkerId("pickUpId"),
      );

      Marker dropOffLocMarker = Marker(
        icon: BitmapDescriptor.defaultMarkerWithHue(BitmapDescriptor.hueRed),
        infoWindow: InfoWindow(title: finalPos.placeName, snippet: "Drop-off location"),
        position:  dropOffLatLng,
        markerId: MarkerId("dropOffId"),
      );

      setState(() {
              markerSet.add(pickUpLocMarker);
              markerSet.add(dropOffLocMarker);
            });

      Circle pickUpLocCircle = Circle(
        fillColor: Colors.blue,
        center: pickUpLatLng,
        radius: 12,
        strokeWidth: 4,
        strokeColor: Colors.blueAccent,
        circleId: CircleId("pickUpId")
      );

      Circle dropOffLocCircle = Circle(
        fillColor: Colors.red,
        center: dropOffLatLng,
        radius: 12,
        strokeWidth: 4,
        strokeColor: Colors.redAccent,
        circleId: CircleId("dropOffId"),
      ); 

      setState(() {
              circleSet.add(pickUpLocCircle);
              circleSet.add(dropOffLocCircle);
            }); 
  } 

Answer 1

了解问题

您需要使用FutureBuilder 小部件，因为您的应用程序依赖于等待另一个操作完成。否则纬度值将是NULL。

检查这个 - https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html

---

解决方案

你必须用FutureBuilder 包裹你的 Scaffold 主体（你实际调用主地图小部件的地方）。您尚未添加调用地图小部件的主要功能，但以下是执行此操作的伪代码 -

 @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        iconTheme: IconThemeData(color: Colors.white),
        title: Text('Map'),
        centerTitle: true,
        backgroundColor: Colors.blue,
      ),
     //this mapWidget should be wrapped in future builder
      body: mapWidget(),
    );
  }

 Widget mapWidget() {
    return FutureBuilder(
                
                future: getPlaceDirection,//adding your main location widget in future
                builder: (context, state) {
                  if (state.connectionState == ConnectionState.active ||
                      state.connectionState == ConnectionState.waiting) {
                      return SpinKitRipple(
                        itemBuilder: (BuildContext context, int index) {
                          return DecoratedBox(
                               decoration: BoxDecoration(
                               color: index.isEven ? Colors.grey : 
                                    Color(0xffffb838),
                               ),);},);
                  } 
                  else {

//here you should write your main logic to be shown once latitude ! = NULL
               
                   }
}