This claims to be a collection of libraries, but AFAICT the main thing it offers is Tasks, which is like a promise kit for async requests.  Looks nice, though.

Bolts is a collection of low-level libraries designed to make developing mobile apps easier.

Bolts Tasks is a complete implementation of futures/promises for iOS/OS X/watchOS/tvOS and any platform that supports Swift. A task represents the result of an asynchronous operation, which typically would be returned from a function