Practical Meta Programming in Ruby
I really recommend watching this talk. It's about metaprogramming in Ruby but the majority of things are applicable to our loved Objective-C, since both languages take advantage of the Smalltalk message passing system.
I really like Swift safeiness but I hope that we don't lose this powerful tool. In fact I've been doing some things in Swift that resulted to be more difficult that doing it in Objective-C with a little of runtime programming.