Comment by zserge

You might like Anvil: it's a declarative react-like UI library for Android. I personally use it a lot, and I normally write single-activity applications (e.g. I have my own backstack of views inside a single activity):

https://github.com/zserge/anvil

http://zserge.com/blog/anvil-2.html

Kotlin is supported as well (http://zserge.com/blog/anvil-kotlin.html)