ReactiveX

http://reactivex.io 

The real power comes with the “reactive extensions” (hence “ReactiveX”) — operators that allow you to transform, combine, manipulate, and work with the sequences of items emitted by Observables.

These Rx operators allow you to compose asynchronous sequences together in a declarative manner with all the efficiency benefits of callbacks but without the drawbacks of nesting callback handlers that are typically associated with asynchronous systems.