[pavlov]

I learned a lot from the Cocotron source:

https://code.google.com/p/cocotron/source/browse

It's a free cross-platform implementation of Apple's Cocoa, so there's a lot of stuff there. But the project is well organized, and almost everything is written in a minimalist oldschool Objective-C style.

I've looked at some other cross-platform frameworks, and they are often hard to understand because they have been developed by a large group of developers and include lots of complex optimizations and platform-specific code paths. Cocotron is not as finely tuned as Apple's CoreFoundation (for example), but much more readable.