flutter - Dart:即使在调用异步Setter之后，Getter也会返回null(确保该方法中的字段不为null)

Question

感觉很简单，但没有用。我在这里错过了一些非常基本的东西吗？这些getter和setter方法在AppData类中，该类具有一个静态字段:List(元素类型LatLng)_navRoute = List();由于某种原因，get总是返回null。

static void setNewRoute() async{
  List<LatLng> navRoute = await _googleMapPolyline.getCoordinatesWithLocation(
    origin: LatLng(_currentLocation.latitude, _currentLocation.longitude),
    destination: LatLng(_navDestination.latitude, _navDestination.longitude),
    mode: RouteMode.walking
  );
  _navRoute = navRoute;
  print("Nav route in set route is:" + _navRoute.toString()); //does not print null
}

static List<LatLng> getNavRoute(){
   print("Nav route in get route is:" + _navRoute.toString()); //prints null 
  return _navRoute; //returns null
}

我调用这些函数的方式是:

onPressed: (){
            setState(() {
              AppData.updateCurrentLocation();
              AppData.setNavDestination(widget._latitude, widget._longitude);
              AppData.setNewRoute();
              AppData.setNavStatus(true);
            });
            print("Current location is: " + AppData.getCurrentLocation().latitude.toString() + " , " + AppData.getCurrentLocation().longitude.toString()); //Not null
            print("Nav destination is: " + AppData.getNavDestination().latitude.toString()+ ", " + AppData.getNavDestination().longitude.toString()); //Not null
            print("Nav is: " + AppData.getNavRoute().toString()); //Always null
            Navigator.pop(context); //pop the bottomModalSheet
            Navigator.pushReplacement(
                context, 
                new MaterialPageRoute(
                  builder: (BuildContext context) => AppData.getTour(widget._tourID),
                ),
            );
          },
        ),

Answer 1

这里有几个问题。

首先，您的async方法应返回Future，因为所有async方法都应返回Future:

static Future<void> setNewRoute() async{
...
}

其次，您需要对 await进行 Future:

onPressed: (){
            setState(() async {
              AppData.updateCurrentLocation();
              AppData.setNavDestination(widget._latitude, widget._longitude);
              // This will complete once you've actually set _navRoute
              await AppData.setNewRoute();
              AppData.setNavStatus(true);
            });
            print("Current location is: " + AppData.getCurrentLocation().latitude.toString() + " , " + AppData.getCurrentLocation().longitude.toString()); //Not null
            print("Nav destination is: " + AppData.getNavDestination().latitude.toString()+ ", " + AppData.getNavDestination().longitude.toString()); //Not null
            print("Nav is: " + AppData.getNavRoute().toString()); //Always null
            Navigator.pop(context); //pop the bottomModalSheet
            Navigator.pushReplacement(
                context, 
                new MaterialPageRoute(
                  builder: (BuildContext context) => AppData.getTour(widget._tourID),
                ),
            );
          },
        ),

基本上 AppData.getNavRoute返回 null，因为实际上尚未设置 _navRoute，因为您没有等待它依赖的异步操作完成。