iOS 4 and Map Kit Overlays with MonoTouch

One of the "over 100 new features" in iOS 4 that I've been especially interested in are Map Kit overlays. Previously, developers were limited to using annotations to express points on a map. If you wanted to draw a shape of some sort you were more or less up a creek.

Overlays are a special kind of annotation designed to represent an area on a map. iOS 4's Map Kit comes with some common shapes built in (rectangles, circles, polygons, etc). As I understand it it's also possible to make your own custom shapes.

Each overlay object holds data to represent the shape and has a corresponding view that tells the MKMapView's delegate how to draw the overlay. As an example here's how one might draw a circle overlay with a 100 meter radius around the Empire State Building. The code below shows how to do this in C# via MonoTouch.

CLLocationCoordinate2D empireStBld = new CLLocationCoordinate2D(40.748433, -73.985656);
double radiusInMeters = 100d;    
MKCircle circle = MKCircle.Circle(empireStBld, radiusInMeters);            
MapView.Delegate = new MapViewDelegate(circle); 
MapView.AddOverlay(circle);

The only thing left is that the MapViewDelegate class needs to be set up to give an appropriate view for the overlay:

public class MapViewDelegate : MKMapViewDelegate
{
  private MKCircle _circle = null;
  private MKCircleView _circleView = null;
   
  public MapViewDelegate(MKCircle circle)
  {
    _circle = circle;
  }
   
  public override MKOverlayView GetViewForOverlay(MKMapView mapView, NSObject overlay)
  {
    if ((_circle != null) && (_circleView == null))
    {
      _circleView = new MKCircleView(_circle);
      _circleView.FillColor = UIColor.Cyan; 
    }
      return _circleView;
  } 
}