This is a variation on Apple's PhotoScroller sample app that loads the tiles over a network connection rather than storing them locally.
Unfortunately it requires use of a couple of undocumented methods on CATiledLayer that would prevent you from deploying it to the App Store.
You can use it in your own internal ad-hoc apps and possibly (with the obvious modifications) for Mac apps not distributed through the Mac App Store.
Because this code demonstrates the impossibility of fetching tiles asynchronously in a CATiledLayer, my hope is that I can get a few more developers to file duplicate bug reports at http://bugreport.apple.com/ to "vote" for publishing the required APIs.
The tiles are hosted on my personal account. If you plan to do any heavy testing or (God forbid) distribution of this example, please download the original project from Apple and host a copy of the images on your own server.
This project incorporates the excellent AFNetworking project as a submodule. Once you have cloned the project,
run git submodule init && git submodule update to pull down a copy into your project.
In a CATiledLayer, any time you return from -drawRect: or -drawLayer:inContext: without having drawn anything, the tile (previously containing a blurry zoomed-in version of the previous content) is replaced by solid black. This is somewhat jarring.
One solution is to use a blocking network call inside of these methods to fetch a tile over the net, but the OS only fetches one tile at a time, so you're limited to one concurrent network request. The performance is abysmal.
CATiledLayer has a couple of undocumented methods. The first is -canDrawRect:levelOfDetail:, which lets you tell the layer whether or not a tile has been loaded, and serves as a hint that you should start downloading it. The second is -setNeedsDisplayInRect:levelOfDetail: which lets you tell the layer that you've finished downloading the tile (for a particular rectangular region and level of detail) and that it should go ahead and call one of the drawing methods.
LSNetTiledLayerDataSourceis a singleton, whereas it should probably be passed down the chain from the app delegate to the view- The call to grab the image metadata is blocking. It should be done asynchronously.
- There seem to be some drawing glitches where CATiledLayer is displaying invalid cached tiles.
My modifications are licensed under Creative Commons by attribution (CC BY 3.0), with the following disclaimer courtesy of MIT:
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