Getting control after INavigation.PopAsync() in Xamarin Forms

I have PageA + ViewModelA, and PageB + ViewModelB.

From A i'm calling PushAsync(B), editing some data, and calling PopAsync().

So now B becomes closed, and user returns to A.

But in B user changed some state, that should be update on A. What is the correct way to notify A to update state (and it would be better to have access to ViewModelB).