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It depends what you mean 'on top of'. If you mean adding some RN to an existing app, that is conceptually easy but practically quite messy (https://facebook.github.io/react-native/docs/integration-wit...).

If you mean adding native modules into an essentially RN app, that's practically very simple (https://facebook.github.io/react-native/docs/native-modules-...) but still requires some conceptual care (like any API design).

Nvm! It is possible:

You can use 5 line of code to push uiviewcontroller from xib or storyboard. Easier than encapsulate native code to library rn. Xamarin cannot do that

Is truly hard. And not even thing in use it for Xamarin or something else.

I love to have a react-native-like for just the UI and do my code on swift/F# for the rest.

It's definitely possible but hard to do well; dependency management is a pain.