flutter - 如何在 child 中使用此条件语句(消费者)

Question

我无法获得if(绘图)Draw(onDrawEnd:_onDrawPolygon);出于某种原因通过的声明。有人知道怎么做这个吗？请指教。

body: Stack(children: <Widget>[
  SizedBox(
    width:
        MediaQuery.of(context).size.width, // or use fixed size like 200
    height: MediaQuery.of(context).size.height,
    child: Consumer<ProviderMaps>(builder: (context, Provmap, widget) {
      return GoogleMap(
          myLocationEnabled: true,
          compassEnabled: true,
          mapToolbarEnabled: true,
          zoomControlsEnabled: true,
          zoomGesturesEnabled: true,
          onLongPress: _addMarkerLongPressed,
          myLocationButtonEnabled: false,
          mapType: maptype,
          onTap: (latLng) {
            _selectCenterCircle(latLng);
          },
          onMapCreated: (GoogleMapController controller) {
            mapController = controller;
            _controller.complete(controller);
            isMapCreated = true;
            changeMapMode();
            subscription.pause();
            changeMapType();
            setState(() {});
          },
          initialCameraPosition:
              CameraPosition(target: LatLng(lat, lng), zoom: 10.0),
          circles: _getCircles(),
          markers: _getMarkers(),
          polygons: _getPolygons()
        (() {
        if (drawing) Draw(onDrawEnd: _onDrawPolygon);
      }()));
    }),
  ),

  Column(children: <Widget>[
    if (enableFreeDraw)
      FloatingActionButton(

上面的代码段运行后，我看到的错误是， NoSuchMethodError _CompactLinkedHashSet<Polygon> has no instance method call..我猜它正在尝试从多边形中提取:_getPolygons()。我不知道如何使用((){包装器，或者即使那是我应该使用的包装器。
编辑:
抱歉，不清楚。我对Dart还是很陌生，我无法解决以上问题为何无法解决的问题。但是，这确实有效:

Expanded(
              child: Stack(
                alignment: Alignment.bottomRight,
                children: <Widget>[
                  GoogleMap(
                    mapType: MapType.normal,
                    initialCameraPosition: CameraPosition(
                      target: centerCamera,
                      zoom: zoomCamera,
                    ),
                    myLocationEnabled: true,
                    myLocationButtonEnabled: false,
                    onMapCreated: (GoogleMapController controller) {
                      googleMapController = controller;
                    },
                    onTap: (latLng) {
                      _selectCenterCircle(latLng);
                    },
                    onCameraMove: (position) {
                      centerCamera = position.target;
                      zoomCamera = position.zoom;
                    },
                    circles: _getCircles(),
                    markers: _getMarkers(),
                    polygons: _getPolygons(),
                  ),
//                  if (!drawing) _mapButtons(),
                  if (drawing)
                    Draw(
                      onDrawEnd: _onDrawPolygon,
                    ),
                ],
              ),
            ),

我在第一组代码中尝试使用的部分是:
if(绘图)Draw(onDrawEnd:_onDrawPolygon);
它无需消费者声明即可工作。否则无法正常工作。

Answer 1

您应该写:

if (dawing != null)...[
    Draw(onDrawEnd: _onDrawPolygon);
],