Geolocator使用distanceBetween（）Flutter和链式等待异步方法给出错误的距离

Question

我希望大家安全。我编写了一种方法来获取用户位置，家庭位置和商品位置之间的距离，以进行购买和销售应用程序。我能够调试并获得三个值的打印。当我调用该方法计算距离时，它会返回预期的更大值。如果将动态值替换为静态值，则该方法有效。我不确定哪里出了问题：帮助...还应如何清除distanceList（将其传递给Text小部件。时间上它会显示项目在4096Km处，即使使用的值也应显示为0米或120米。尝试了所有内容，没有尝试。

    Future<List> calculateDistance() async {
    double itemAddressLat;
    double itemAddressLong;
    await FirebaseFirestore.instance
        .collection(Str.ITEMS)
        .get()
        .then((QuerySnapshot querySnapshot) => {
              querySnapshot.docs.forEach((doc) {
                itemAddressLat =
                    double.tryParse(doc.data()[Str.ITEM_LATITUDE].toString()) ??
                        0.0;
                itemAddressLong = double.tryParse(
                        doc.data()[Str.ITEM_LONGITUDE].toString()) ??
                    0.0;

                print("ITEMS LOCATIONS....: " +
                    itemAddressLat.toString() +
                    ", " +
                    itemAddressLong.toString());
                //sample print below for itemAddressLat and itemAddressLong
                // I/flutter (14429): ITEMS LOCATIONS....: -1.2675001, 22.7865
                // I/flutter (14429): ITEMS LOCATIONS....: -1.2675001, 0.3299
                // I/flutter (14429): ITEMS LOCATIONS....: -1.278937, 2.278937
                // I/flutter (14429): ITEMS LOCATIONS....: 0.4565, 11.3422
                // I/flutter (14429): ITEMS LOCATIONS....: -1.278937, 11.278937
                // I/flutter (14429): ITEMS LOCATIONS....: -1.2675001, 32.3456
                // I/flutter (14429): ITEMS LOCATIONS....: 0.1212, 21.2222
                // I/flutter (14429): ITEMS LOCATIONS....: 0.0, 0.6765
                // I/flutter (14429): ITEMS LOCATIONS....: 1.2921, 31.8219
                // I/flutter (14429): ITEMS LOCATIONS....: 0.0, 0.0
                // I/flutter (14429): ITEMS LOCATIONS....: null, null

                //Error handling for null and empty for same fields printed above
                if (itemAddressLat.toString().isNotEmpty ||
                    itemAddressLat.toString() != null &&
                        itemAddressLong.toString().isNotEmpty ||
                    itemAddressLong != null) {
                  if (usersCurrentLocationLat.toString().isNotEmpty ||
                      usersCurrentLocationLat.toString() != null &&
                          usersCurrentLocationLong.toString().isNotEmpty ||
                      usersCurrentLocationLong.toString() != null) {
                    distance = distanceBetween(itemAddressLat, itemAddressLong,
                            usersCurrentLocationLat, usersCurrentLocationLong) /
                        1000;
                    
                    distance = distance.roundToDouble();
                    //distanceList.add(distance);
                  }
                }
                distanceList.add(distance);
              })
            });
    print(distanceList);
    return distanceList;
  }
    Future<void> setItemAndUserLocationLatLong() async {
    await getLocationFromLocationPackage();
    await getUserHomeAddress();
    print("MY LOCATION COORDINATES=========:  " +
        usersCurrentLocationLat.toString() +
        " , " +
        usersCurrentLocationLong
            .toString()); //I/flutter (14429): MY LOCATION COORDINATES=========:  0.3379671 , 32.6681845

    print("MY HOME ADDRESS COORDINATES=========:  " +
        usersCurrentLocationLatFromDb.toString() +
        " , " +
        usersCurrentLocationLongFromDb
            .toString()); //I/flutter (14429): MY HOME ADDRESS COORDINATES=========:  -1.278937 , 36.7777984

    print(distanceList); //prints I/flutter (14429): []
    print(distanceList.length); //I/flutter (14429): 0
  }

  getLocationFromLocationPackage() async {
    Location location = new Location();

    bool _serviceEnabled;
    PermissionStatus _permissionGranted;
    LocationData _locationData;

    _serviceEnabled = await location.serviceEnabled();
    if (!_serviceEnabled) {
      _serviceEnabled = await location.requestService();
      if (!_serviceEnabled) {
        usersCurrentLocationLat = -1.2921;
        usersCurrentLocationLong = 36.8219;
        return;
      }
    }

    _permissionGranted = await location.hasPermission();
    if (_permissionGranted == PermissionStatus.denied) {
      _permissionGranted = await location.requestPermission();
      if (_permissionGranted != PermissionStatus.granted) {
        return;
      }
    }

    _locationData = await location.getLocation();
    setState(() {
      usersCurrentLocationLat = _locationData.latitude;
      usersCurrentLocationLong = _locationData.longitude;
    });

    print(usersCurrentLocationLat.toString() +
        " NEW WAY " +
        usersCurrentLocationLong.toString());
  }

  Future<void> getUserHomeAddress() async {
    var firebaseUser = FirebaseAuth.instance.currentUser;

    DocumentSnapshot snapshot = await FirebaseFirestore.instance
        .collection(Str.USERS)
        .doc(firebaseUser.uid)
        .get();
    var userAddressLatFromDB = snapshot.data()[Str.LAT];
    var userAddressLongFromDB = snapshot.data()[Str.LNG];
    setState(() {
      usersCurrentLocationLatFromDb = userAddressLatFromDB;
      usersCurrentLocationLongFromDb = userAddressLongFromDB;
    });
  }