Programming iOS 6, 3rd Edition

Overlays

An overlay differs from an annotation in being drawn entirely with respect to points on the surface of the earth. Thus, whereas an annotation’s size is always the same, an overlay’s size is tied to the zoom of the map view.

Overlays are implemented much like annotations. You provide an object that adopts the MKOverlay protocol (which itself conforms to the MKAnnotation protocol) and add it to the map view. When the map view delegate method mapView:viewForOverlay: is called, you provide an MKOverlayView and hand it the overlay object; the overlay view then draws the overlay on demand. As with annotations, this architecture means that the overlay itself is a lightweight object, and the overlay view is needed only if the part of the earth that the overlay covers is actually being displayed in the map view. An MKOverlayView has no reuse identifier.

Some built-in MKShape subclasses adopt the MKOverlay protocol: MKCircle, MKPolygon, and MKPolyline. In parallel to those, MKOverlayView has built-in subclasses MKCircleView, MKPolygonView, and MKPolylineView, ready to draw the corresponding shapes. Thus, as with annotations, you can base your overlay entirely on the power of existing classes.

In this example, I’ll use MKPolygonView to draw an overlay triangle pointing up the road from the parking place annotated in our earlier examples (Figure 34-4). We add the MKPolygon as an overlay to our map view, and derive the MKPolygonView from it in our implementation of mapView:viewForOverlay: ...