I want to share values among classes

The following article by Momo is easy to understand in detail
https://medium.com/flutter-jp/state-1daa7fd66b94

I recommend the official document because it is explained in a very easy-to-understand way^^
https://flutter.dev/docs/development/data-and-backend/state-mgmt/options

·InheritedWidget
·BLoC
·Provider+StateNotifier/ChangeNotifier

and so on…
Too many state management techniques lol

In my personal opinion, the in-screen processing is done with setState or RiverPod.
As for the cross-screen condition, I think StateNotifier+Provider+freezed is good for now
Flutter still has a lot of destructive changes, so I can't say it's true if I use it
Provider is officially recommended, and is somewhat easy to understand, so I recommend it

Here's an example code: https://flutter.dev/docs/development/data-and-backend/state-mgmt/simple