calculateDistance.dart

static String calculateDistance(double lat1, double lon1, double lat2, double 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;
    double distance = 12742 * asin(sqrt(a));
    String strDistance = distance.toStringAsFixed(2);
    return strDistance + " km";
}