Streaming the map image like MKMapView does

I'm developing a map application for both iPhone OS and OS X. This uses Core Graphics to render a map, but rendering each frame is too slow to scroll in 60fps, and consumes the battery.

So I want to cache the map with rendering the map on bitmap tiles(256x256), but what开发者_如何转开发 kind of bitmap should I use? (like CGImage)