public void OnLocationChanged(Android.Locations.Location location)
        {
            _currentLocation = location;

            if (_currentLocation == null) {
                Global.currentPosition = new Xamarin.Forms.Maps.Position (0, 0);
                Global.PositionChangedNumber = 1;
                //_locationText.Text = "Unable to determine your location.";
            } else {
                Global.currentPosition = new Xamarin.Forms.Maps.Position (location.Latitude, location.Longitude);
                var x = new platformSpecific_Android ();
                x.userlatitude = location.Latitude;
                x.userlongitude = location.Longitude;
                Global.PositionChangedNumber += 1;
                Global.calculateDistance (false);
                Global.LocationUpdatePending = true;
                //_locationText.Text = String.Format ("{0},{1}", _currentLocation.Latitude, _currentLocation.Longitude);
            }
        }
        protected override void OnResume()
        {
            base.OnResume ();
            try {
                //posso provenire dalla finestra settaggi
                if (_locationProvider == "" || _locationProvider == "passive") {
                    InitializeLocationManager ();
                }
                if (_locationProvider == "" || _locationProvider == "passive") {
                    Global.currentPosition = new Xamarin.Forms.Maps.Position (Global.defaultPosition.Latitude, Global.defaultPosition.Longitude);
                    var x = new platformSpecific_Android ();
                    x.userlatitude = Global.defaultPosition.Latitude;
                    x.userlongitude = Global.defaultPosition.Longitude;
                    Global.PositionChangedNumber += 1;
                    Global.calculateDistance (false);
                    Global.LocationUpdatePending = true;
                    return;
                }

                _locationManager.RequestLocationUpdates (_locationProvider, 60000, 500, this); //60 sec e 500 metri
            } catch (Exception ex) {

            }
        }