I’ve posted the source code to a basic implementation of the Cocoa Shaders I talked about here. There’s code for conditional shaders, shader lists, clip shaders, and affine transform shaders. As far as shaders that actually draw there’s a solid color shader and one for drawing an image. The image shader lets you tweak the compositing operation, source rect, and set a drawing scale.
Included is a sample app to show you how you could use this stuff in practice. The app has a custom view which draws using a shader. The demo shader will change its background from blue to red when the view is clicked. Its implemented using only the simple shaders provided.
Fancier gradient fill shaders, CoreImage based shaders and that kind of thing are left, for now, as an exercise for the reader. I mentioned it in the original post but if you’re looking to do gradients you’d do yourself a favour by looking at CTGradient.