ReactiveX is a library provided as an extension for creating asynchronous event-based applications for JVM. ReactiveX doesn't use any typical design pattern; however, it's a combination of the Observer design pattern, the Iterator design pattern, and functional programming. ReactiveX is for frontend managing the UI, and the backend, as well as for business logic components developed through Java, .Net, Scala, C Sharp, C++, Python, and many more. ReactiveX extensively works with Observable Streams.
We have already discussed functional programming in the earlier chapter, so before moving ahead let's discuss the Observer and Iterator design pattern.