Event-driven binding and MVVM

Default: explicit push updates

1. Gameplay model exposes a typed change delegate/event.
2. Local presenter subscribes and converts raw state into display state.
3. Presenter calls a small widget setter such as SetHealth(Current, Max).
4. Widget updates only the affected controls.
5. Unbind in the matching lifecycle hook.

This keeps gameplay independent of widget classes and avoids per-frame polling.

MVVM path

Use the UMG Viewmodel plugin when reusable view state and declarative bindings justify a Beta dependency.

1. Enable UMG Viewmodel.
2. Create a UMVVMViewModelBase subclass or implement INotifyFieldValueChanged in C++.
3. Expose display fields with FieldNotify.
4. Keep fields private/protected and mutate through setters that broadcast only on change.
5. Prefer one-way-to-widget for authoritative/display state.
6. Use two-way binding only for genuine editable form state; submit validated intent separately.
7. Test cooked builds and reassignment paths. Epic documents a current binding-invalidity caveat when a designer-bound Viewmodel is reassigned in the Details panel.

Record that MVVM is Beta in UE 5.8 in the technical-risk register.

Replicated state to UI

server changes replicated property
  -> client RepNotify/delegate
  -> local presenter or Viewmodel setter
  -> FieldNotify/delegate
  -> widget update

Do not poll the replicated actor from a widget binding. Do not play irreversible one-shot VFX solely because initial replication invoked a state OnRep; distinguish current state from a newly observed event.

Rules

• Raw UMG property/function binding is not the event-driven MVVM path.
• Directly setting a widget property can break a classic property binding.
• A Viewmodel formats and exposes presentation state; it does not become gameplay authority.
• Keep async asset/data readiness explicit with loading, ready, empty, and error states.
• Store list item identity in data objects, not recycled entry-widget instances.