此函数为CameraPosition中的zoom参数返回null而不是正确的值，我无法弄明白

Posted 2023-01-05

我有一个函数，根据2个Google地图标记之间的距离计算所需的CameraPosition缩放量。调用CameraPosition缩放参数（zoomIn）中指定的方法，该方法又调用另一个异步方法，该方法返回两点之间的距离（getDistance）。两种方法都正常运行，但尽管print语句显示正确的计算值，但zoomIn始终返回null。这是同步问题吗？

  @override
  Widget build(BuildContext context) 
    return Column(
      children: <Widget>[
        Container(
          height: MediaQuery.of(context).size.height,
          width: MediaQuery.of(context).size.width,
          child: GoogleMap(
            onMapCreated: addMarkers,
            cameraTargetBounds: CameraTargetBounds(createTargetBounds()),
            initialCameraPosition: CameraPosition(
                target:
                    LatLng((currentLocation.latitude + latitudesArr[selectedDestination]) / 2,
                          (currentLocation.longitude + longitudesArr[selectedDestination]) / 2),
                zoom: zoomIn(bounds: createTargetBounds(), padding: 200.0)),
          ),
        )
      ],
    );
  

  double zoomIn(LatLngBounds bounds, double padding) 
    double zoom;
    getDistance(bounds).then((double distance)
      print('Distance: $distance');
      if (distance > 200)
        zoom = 16.0;
       else 
        zoom = 18.0;
      
      print ('Zoom: $zoom');
    );
    return zoom;
  

  Future<double> getDistance(LatLngBounds bounds) async 
    double distance = await Geolocator().distanceBetween(
        bounds.southwest.latitude,
        bounds.southwest.longitude,
        bounds.northeast.latitude,
        bounds.northeast.longitude);
    print('Distance: $distance');
    return distance;
  


The zoomIn function correctly populates the zoom variable with 16.0 or 18.0 depending on the distance between the two markers. However, null is always returned.

答案

好吧，我认为这样做的正确方法是在声明Google地图窗口小部件时对缩放值进行硬编码，例如：

class MapState extends State<MapRoute> 

  @override
  Widget build(BuildContext context) 
    return Stack(
      children: <Widget>[
          GoogleMap(
            onMapCreated: addMarkers,
            myLocationEnabled: true,
            cameraTargetBounds: CameraTargetBounds(createTargetBounds()),
            initialCameraPosition: CameraPosition(
                target:
                    LatLng((currentLocation.latitude + latitudesArr[selectedDestination]) / 2,
                          (currentLocation.longitude + longitudesArr[selectedDestination]) / 2),
                zoom: 18.0),
          ),

然后，在这个例子中，在addMarkers方法上，下面的代码将一直等到GeoLocator返回一个距离，然后才能确定所需的缩放级别。之后只是调用地图控制器的moveCamera(CameraUpdate.zoomTo(zoom))如下：

    await Geolocator().distanceBetween(
        bounds.southwest.latitude,
        bounds.southwest.longitude,
        bounds.northeast.latitude,
        bounds.northeast.longitude)
        .then((distance) 
          print('The Distance: $distance');
          double zoom;
          if (distance > 300) 
            zoom = 17.0;
           else if (distance > 100)
            zoom = 18.0;
           else 
            zoom = 19.0;
          
          print ('Zoom: $zoom');
          controller.moveCamera(CameraUpdate.zoomTo(zoom));
        );