How can I detect that the application is in the background?Disposable Effect not working for me

Generally, you'd want to use

Lifecycle

events for that kind of thing: https://kotlinlang.slack.com/archives/CJLTWPH7S/p1630080009198100?thread_ts=1630078038.194200&cid=CJLTWPH7S

I hava a simple counter app.I want to update database when user closed app from recent background app but i cant detect that.Is this solve my problems?

You'll have to be specific about what interaction you're trying to track. If you mean you want a callback when the user makes your app no longer visible (either by hitting the home button, system back button, receiving a phone call, anything that causes your app to no longer be visible to the user), then yes, this would be what you'd use