Reactive Pattern

The Reactive Manifesto
https://en.wikipedia.org/wiki/Reactive_programming
reactive programming is a programming paradigm oriented around data flows and the propagation of change. This means that it should be possible to express static or dynamic data flows with ease in the programming languages used, and that the underlying execution model will automatically propagate changes through the data flow.

http://stackoverflow.com/questions/16652773/what-is-difference-between-observer-pattern-and-reactive-programming

Reactive programming is the general term behind easily propagating changes through the execution of a program. It's not a specific pattern or entity per-se, it's an idea, or style of programming. 
It's the concept that when x changes in one location, the things that depend on the value of x are recalculated and updated in various other locations with a minimum of fuss.

The observer pattern (at least in OO languages) is a common method for providing a "trigger" to allow information to be updated whenever such a change is made (or, in more common OO terms, when an "event" is fired.) In that sense, it provides a mechanism for allowing the concept of reactive programming to happen in OO (and sometimes other) style languages.

https://gist.github.com/staltz/868e7e9bc2a7b8c1f754
https://codewords.recurse.com/issues/two/an-introduction-to-reactive-programming

Related: http://massivetechinterview.blogspot.com/2015/07/rxjava-reactive-programming.html