How to call the MainPage.xaml.cs to MainActivity.cs method in xamarin forms

As per the title, please let me know if you know how to call the MainPage.xaml.cs method from Android on xamarin forms.
What I want to do is to process things that take a long time on Android, and if there is a CallBack, I would like to inform MainPage.xaml.cs.
I think MainPage.xaml.cs side of Android can be called by DependencyService from MainActivity.cs, but it is the opposite direction.
Is it possible to register a delivery and call it?
Or is there any interface available?