What problem does this feature solve?

I want to clear some effect when app unmount. but I can't find the app api to do this. now i only hack rootProps to add onVnodeBeforeUnmount prop to achieve it. but i find vue2 has event dispatch

https://github.com/vuejs/vue-router/blob/dev/src/index.js#L98

What does the proposed API look like?

app maybe should expose some function to achieve it

Your plugin can expose a bootstrapping function / mixin that can be added to the root component.

That would be the usual way so far. Personally I'm not a friend of further increasing the Plugin API surface.

There's currently no API to do this. For now wrapping the app's unmount function would probably be the cleanest way to do it.

I think an easy improvement is allowing a plugin function to return a "cleanup" callback.

I think an easy improvement is allowing a plugin function to return a "cleanup" callback.

yes. perfect design