RxJava

Reactive programming has gained popularity in large part thanks to the ReactiveX project, which defines a comprehensive set of APIs around common reactive programming concepts. There are ReactiveX (Rx) implementations for many languages, including the following:

• Java—RxJava
• Kotlin—RxKotlin
• C++—RxCpp
• C#(Unity)—RxUni
• RxGroovy—Groovy
• RxDart—Dart

For more information regarding the ReactiveX project, check out http://reactivex.io/. There you will find additional resources for understanding reactive programming, as well as links to the full list of Rx implementations. Currently, there are almost 20 languages that have Rx bindings available to them.

RxJava and RxKotlin can both be used to write reactive code in Kotlin, and have gained a lot of popularity in the Android development community.