Question

This Monotouch code below will not cause an exception, but it also won’t show the polygon on the map.

var coords = new CLLocationCoordinate2D()
{
new CLLocationCoordinate2D(32.67, -81.9),
new CLLocationCoordinate2D(32.77, -81.9),
new CLLocationCoordinate2D(32.61, -81.9),
new CLLocationCoordinate2D(32.43, -81.9),
new CLLocationCoordinate2D(32.67, -81.9)
};

var mkp = MKPolygon.FromCoordinates(coords);

mapView.AddOverlay(mkp);

I’ve centered the map on coords[0] and that works fine, as does a SetRegion with coords[0]. Using the coords[0], I have no problem adding an Annotation to the MapView.

I also tried this with same results (no errors, but no overlay displayed):

var mkc = MKCircle.Circle(coords[0], 100);

mapView.AddOverlay(mkc);

Thinking that perhaps the MapView wasn’t "ready" yet, I even tried a thread that waited a second, and then created the Polygon, via the InvokeOnMainThread. The map did the animated move to the coord, but still just showed the map and no overlays.

If I add this:

var mkp = new MKPolygonView(mkp);

mapView.AddOverlay(mkpv);

I get this exception:

"Objective-C exception thrown. Name: NSInvalidArgumentException Reason: – [MKPolygonView boundingMapRect] unrecognized selector sent to instance 0x7579100"

Is there perhaps a problem with my coordinate list? I’ve tried with the first coordinate at the end of the list to close the polygon and without that, with the same results.

Thanks

Answer 1

Adding an overlay object to the map view is not enough. You have to create a delegate object and implement the GetViewForOverlay method:

public class MapDelegate : MKMapViewDelegate
{

    public override MKOverlayView GetViewForOverlay (MKMapView mapView, NSObject overlay)
    {

        MKPolygon polygon = overlay as MKPolygon;
        if (null != polygon) // "overlay" is the overlay object you added
        {

            MKPolygonView polyView = new MKPolygonView(polygon);
            // customize code for the MKPolygonView
            return polyView;
        } 

        return null;

    }
}

You then assign a new instance of this delegate object to your map view’s Delegate property:

myMapView.Delegate = new MapDelegate();

Every annotation or overlay you add to a map view, needs a corresponding view.

The exception you are getting is because you are adding an MKOverlayView derived object where an MKOverlay derived is expected.