A barebones demo app that demonstrates the use of some new MapKit functionality in iOS 4.0.:

The MKOverlay protocol and MKOverlayView class and how to use these to render custom tile layers a la Google Maps API. (i.e. Google Maps-compatible tile layers from Mapnik/Tilecache, gheat, etc.) Examples include tilesets from OpenStreetMap and MapBox.

(Caveat: the OpenStreetMap renders opaquely over the default Google layer, it does not replace it.)

Comments and feedback are welcome.

I refactored django-gheat as a side project and I’ve been working on iOS development a bit lately and was seeking a slick way to plug the django-gheat demo tileserver into the iPhone. The Google Maps API just doesn’t have enough performance in the browser to make for a decent user experience.

Some conversation threads recommended combining a UIScrollView with CATiledLayer, but this seemed unwieldy to couple with the existing MapKit framework.

Lo and behold, iOS 4.0 added custom overlay functionality to MapKit, though I have not seen any mention or example of it’s use to date.

The code is generally commented thoroughly. I am not an expert in cartography, so my knowledge of map projections and some of the calculations within the code are to be taken with a grain of salt.

CustomOverlayView compensates for [[UIScreen mainScreen] scale], or the iPhone 4’s pixel doubling. The view applies the screen scale factor, creating a situation as if the screen were twice as large. Because MapKit loads pixel-doubled tiles but maintains the same viewport, CustomOverlayView renders tiles of the next (or scale-appropriate) zoom level to maintain a one-to-one mapping with MapKit’s own tiles.

The [MKOverlayView -canDrawMapRect…] and [MKOverlayView -drawMapRect…] methods rely on asynchronously downloading tiles as requested (by -canDrawMapRect) into cache, and then using that cache in the -drawMapRect method after the success callback notifies MapKit to try the tile again.