logistaice_distance_calculation.dart

 getAllTheDistanceCover(String addressOne, String addressTwo) async {
    isCalculating = true;
    update();
    List<Location> startAddress =
        await locationFromAddress(addressOne, localeIdentifier: "en");
    List<Location> destinationAddress =
        await locationFromAddress(addressTwo, localeIdentifier: "en");

    await _createPolylines(startAddress[0], destinationAddress[0]);
    double sum = totalDistanceCovered();
    isCalculating = false;
    distanceCal = sum;
    totalDistance = sum.ceil();
    logisticsTotal(sum);
    update();
  }

  // Sets the delivery type and update the UI
  void selectDeliveryType(bool value) {
    isRegular = value;
    if (isRegular) {
      service.deliveryType = "Regular";
    } else {
      service.deliveryType = "Premium";
    }
    logisticsType();
    update();
  }

  _createPolylines(Location start, Location destination) async {
    // Initializing PolylinePoints
    polylinePoints = PolylinePoints();

    // Generating the list of coordinates to be used for
    // drawing the polylines
    PolylineResult result = await polylinePoints.getRouteBetweenCoordinates(
      ApiKeys.MapsKey, // Google Maps API Key
      PointLatLng(start.latitude, start.longitude),
      PointLatLng(destination.latitude, destination.longitude),
      travelMode: TravelMode.driving,
    );

    if (result.points.isNotEmpty) {
      polylineCoordinates = [];
      result.points.forEach((PointLatLng point) {
        polylineCoordinates.add(LatLng(point.latitude, point.longitude));
      });

      update();
    }
  }

  double _coordinateDistance(lat1, lon1, lat2, lon2) {
    var p = 0.017453292519943295;
    var c = cos;
    var a = 0.5 -
        c((lat2 - lat1) * p) / 2 +
        c(lat1 * p) * c(lat2 * p) * (1 - c((lon2 - lon1) * p)) / 2;
    return 12742 * asin(sqrt(a));
  }

  double totalDistanceCovered() {
    double totalDistance = 0;
    for (int i = 0; i < polylineCoordinates.length - 1; i++) {
      totalDistance += _coordinateDistance(
        polylineCoordinates[i].latitude,
        polylineCoordinates[i].longitude,
        polylineCoordinates[i + 1].latitude,
        polylineCoordinates[i + 1].longitude,
      );
    }
    return totalDistance;
  }

  void logisticsTotal(double distance) {
    totalPrice = (flatRateValue * logBaseValue).toDouble();
    print("totalPrice");
    if (distance > logBaseValue) {
      totalPrice = (flatRateValue * logBaseValue).toDouble() +
          ((distance.ceil().toInt() - logBaseValue) * logNextPrice);
    }

    if (!isRegular) {
      totalPrice = totalPrice + premiumCharge;
    }
    update();
  }