geolocation - Android Geofencing (Polygon) -

how create polygon geofence multiple geo locations(long,lat values) . how track user entering geofence region or exiting region on android.

a geofence array of lat/long points form polygon. once have list of lat/long points, can use point-inside-polygon check see if location within polygon.

this code have used in own projects perform point-in-polygon checks large concave polygons (20k+ vertices):

public class polygontest {     class latlng     {         double latitude;         double longitude;          latlng(double lat, double lon)         {             latitude = lat;             longitude = lon;         }     }      bool pointisinregion(double x, double y, latlng[] thepath)     {         int crossings = 0;          latlng point = new latlng (x, y);         int count = thepath.length;         // each edge         (var i=0; < count; i++)          {             var = thepath [i];             var j = + 1;             if (j >= count)              {                 j = 0;             }             var b = thepath [j];             if (raycrossessegment(point, a, b))              {                 crossings++;             }         }         // odd number of crossings?         return (crossings % 2 == 1);     }      bool raycrossessegment(latlng point, latlng a, latlng b)     {         var px = point.longitude;         var py = point.latitude;         var ax = a.longitude;         var ay = a.latitude;         var bx = b.longitude;         var = b.latitude;         if (ay > by)         {             ax = b.longitude;             ay = b.latitude;             bx = a.longitude;             = a.latitude;         }             // alter longitude cater 180 degree crossings         if (px < 0) { px += 360; };         if (ax < 0) { ax += 360; };         if (bx < 0) { bx += 360; };          if (py == ay || py == by) py += 0.00000001;         if ((py > || py < ay) || (px > math.max(ax, bx))) return false;         if (px < math.min(ax, bx)) return true;          var red = (ax != bx) ? ((by - ay) / (bx - ax)) : float.max_value;         var blue = (ax != px) ? ((py - ay) / (px - ax)) : float.max_value;         return (blue >= red);     } } 

in terms of program flow, want background service location updates , perform check against lat/long polygon data see if location inside.