Modular android app without RX java or RX android

I had gone through this github app, Buffer app GitHub Link

I loved this project and devoted my lot of time to understand it quite nicely. But the only problem is that i want to do it without RX java,

I am trying to understand my questions,

In the domain section, Rxjava is the thing which redirects the result from respective data source to corresponding source or presenter for success or Error

So, I only want to understand how can I do it without RXJava? It will be great if you guys can provide me some sample app. with same modular architecture or how can we achieve it with same github app(will prefer this one).

I just want to know, how can we replace RX with custom listener or Minimal code, so that it can map listener to corresponding datasource or vice versa to presenter once data is received.

I had also opened an Issue on this GitHub app, with following url, My Open issue in this app.

but I think developer is little busy.