Is there anywhere I can browse the source?

I've been diving into Xamarin.Forms a bit, but there isn't too much documentation out there beyond the few pages on the main documentation site and some of the class libraries. In particular I'm looking for more information about how the integration happens with the base platform (so I can see where I'd run into the edges of the abstraction).

The most immediate example I can think of is needing to create a custom UITreeView and wanting to convert Xamarin.Forms Cells into UICells.

Thanks!