Hydra
Lightweight full-featured Promises, Async & Await Library in Swift
...Its async and await helpers allow asynchronous tasks to be written in a more sequential style. Promises can be cancelled through invalidation tokens and executed on configurable Grand Central Dispatch queues. Operators include timeout, retry, recover, validate, map, zip, all, any, defer, pass, and always. Hydra supports Apple platforms and Linux and can be integrated through common Swift dependency-management systems.