onLocationchange.listen((event){}) enter into loop flutter

I have this function in flutter to calculate distance between particular geometry point and user loaction when user start moving any way every thing goes right but I want to make function take new values even if the the user doesn't arrive to distance so that I want to stop the onLocationChange on old values but I can't make cancel() from outside the listen itself this is my code calculateDistance(String? taskname,LatLng tasklatlong,bool start)async { FirebaseFirestore.instance .collection("tasks") .doc(empname) .collection("tasks") .doc(taskname) .update({ 'start time': DateTime.now().toString().substring(10, 16), 'estimated time ': double.parse((duration / 60).toStringAsFixed(2)) });

locationsubscription =
    locationChange.onLocationChanged.listen((event) async {
      if(start==false){
      locationsubscription?.cancel();}
      else{
      await CacheHelper.sharedPreferences!.reload();
     locationChange.enableBackgroundMode(enable: true);
  print('after initilize');
  var R = 6371; // Radius of the earth in km
  var dLat = deg2rad(event.latitude!-tasklatlong.latitude);  // deg2rad below
  var dLon = deg2rad(event.longitude!-tasklatlong.longitude);
  var a =
      sin(dLat/2) * sin(dLat/2) +
          cos(deg2rad(event.latitude)) * cos(deg2rad(tasklatlong.latitude)) *
              sin(dLon/2) * sin(dLon/2);
  var c = 2 *atan2(sqrt(a), sqrt(1-a));
  var d = R * c*1000; // Distance in km
  print('distance is $d');
  if (d <= 50) {
    print('arrived to location');
    FirebaseFirestore.instance
        .collection("tasks")
        .doc(empname)
        .collection("tasks")
        .doc(taskname)
        .update(
            {'arrived time': DateTime.now().toString().substring(10, 16)});
    Fluttertoast.showToast(
        msg: 'You arrived to location', backgroundColor: approvecolor);
    locationsubscription?.cancel();
    setState(() {
      myLocationEnabled=false;
    });
    calculateWastedTime();
  }
  else {
    print('not arrived ');
  }}
});

} how can I use locationsubscription.cancel() from outside the function I tried to use it but it just still in the loop until it cancel from inside

flutter-mapbox-gl / maps

onLocationchange.listen((event){}) enter into loop flutter #1307