I stumbled upon this ReactiveX web page that provides a list of reactive libraries for various languages. Julia is notably missing in the list. Has anyone thought about building something and put it on that list? 
The first paragraph says:
ReactiveX is a library for composing asynchronous and event-based programs by using observable sequences.
It extends the observer pattern to support sequences of data and/or events and adds operators that allow you to compose sequences together declaratively while abstracting away concerns about things like low-level threading, synchronization, thread-safety, concurrent data structures, and non-blocking I/O.