ReactiveCocoa (RAC) is an opensource framework for FRP, developed by Github for the iOS and OS X platforms. RAC has been around for some years now. It started as an Objective-C framework and now it's object of an almost-complete rewrite using the brand new language introduced by Apple, Swift.
At the time of writing (may/june/july 2015), RAC 3.0 is in beta. The 3.0 version offers new APIs in Swift, that are also mostly backward compatible with version 2.0 (that is written in Objective-C).
Using Swift, the APIs now have a better and cleaner form. Infact, Swift is a language that allows to build composable abstraction pretty easily, supporting immutability (value types vs reference types), high-order functions, optionals, custom operators, etc...
In the community of iOS and OS X developers RAC is an emerging trend that is clearly gaining the attention of an increasing number of users, confirming the general trend of RP and FRP in our industry.