This is a RubyMotion conversion of the TrackMix application you develop during the "Your First Mac App" getting started guide.
It's done without Interface Builder and so uses Auto Layout instead, which is basically the only complicated part to it.
You can read the blog posts for it here:
- Building OS X apps with RubyMotion
- OS X apps with Ruby: Part 2
- OS X apps with Ruby: Part 3
- OS X apps with Ruby: Part 4 (Coming soon)