How to detect modified fields before save and conditionally block save ?

Hi Community,

I am working with Creatio Freedom UI (8.x) and I have the following validation requirement.

Scenario

I have two sets of fields:

• Set A → A group of key fields (for example: Field1, Field2, Field3)
• Set B → A group of  fields (for example: ApprovalReason, Comments)

Requirement

1. If any field from Set A is modified, then:
   • I need to check whether all fields in Set B are filled.
   • If any field in Set B is empty → block save and show validation message.
2. If the user modifies fields that are NOT part of Set A:
   • Save should be allowed normally.

My Questions

1. In Freedom UI, is there any built-in attribute that stores the list of changed/modified fields before saving the record?
2. If not, what is the recommended approach to detect whether specific fields were modified?
3. Should this logic be implemented:
   • On client side using HandleViewModelAttributeChangeRequest?
   • Or in backend using the EntitySaving event and comparing old/new values?
4. Is there a supported way to retrieve changed columns on the backend instead of manually comparing values?

I would like to follow the recommended and supported best-practice approach.

Thanks in advance!