Rebuild only the required Flutter value

Use UseCaseBuilder when the widget needs the complete UseCaseSnapshot. Use UseCaseSelector when it needs one derived value and should ignore unrelated Snapshot changes.

UseCaseSelector<CartUseCase, CartState, int>(
  selector: (snapshot) => snapshot.state.items.length,
  builder: (context, itemCount) => Badge(label: Text('$itemCount')),
)

The selector runs for every emitted Snapshot, but the builder runs only when the selected value changes by ==. Select a small immutable value with meaningful equality.

Use buildWhen on UseCaseBuilder when the decision needs both the previous and current Snapshot. Do not copy State into a second widget-owned model merely to avoid rebuilds; that creates competing sources of truth.