I want add user login/out and post comments features to my android && iOS native app. Is that possible only use discourse’s backend to develop those features?
You can use the discourse API to do pretty much all of that. AFAIK there isn’t any documentation yet so you’ll probably have to dig around yourself. The frontend uses the API, so watching how it interacts with the backend is probably the best way to start.
I’ve managed to get almost everything working in an android app, so it’s completely possible.
You might find this useful, GitHub - zeusbaba/discourse-api-client: Discourse API client in Java (created by another user here)