Resizing Custom MKAnnotationView According to MapKit Zoom Level

Question

Here is the situation: I am displaying images on the map by using custom annotations. Everything works fine but I would like to improve it visually by resizing the AnnotationView so that it shrinks as the map is zoomed out. I want to do this because, obviously, more annotations fill up the screen and need to be shrunk so that they don't overlap each other too much and the user can see more annotations. (Of course, I would like to enlarge AnnotationViews when the map is zoomed in again).

I have achieved this by removing all annotations from the map and adding them back again, whenever regionDidChangeAnimated is called. However, these images are loaded from the internet, so downloading the images again (since the annotations have been removed) doesn't seem to make sense.

I hope I have explained the situation well, and hope to contribute to SO in the future.

Thanks in advance.

P.S. I'm developing with iOS 4.0

Answer 1

Since you are using iOS 4.0 I would suggest using a class that conforms to the MKOverlay protocol instead of MKAnnotationView. Overlays zoom with the map and will scale appropriately. You can probably use 1 overlay to handle all your annotations. Check out the HazardMap demo Apple provided in the 2010 WWDC talks on http://developer.apple.com

Alternatively, you could use something like the k nearest neighbors algorithm to group annotations per zoom scale. However, that can be kind of slow if you have a lot of annotations. I tried it once with several thousand annotations and did not like the performance hit that was incurred. I think < 1000 annotations might have decent performance with this method though.

Answer 2

It sounds like you're already on the right path and that the only real problem is the downloading of the images. Is there any particular reason you can't cache them by writing them to the documents directory (or the cache directory if you want them to go way when the app quits) when you receive them and look there first before displaying them?