keyed_form

Rendering union fields

Select the active generated variant and update union rows without changing their stable identity.

A discriminated-union reference resolves only when the current value has its matching variant. Observe the discriminator, then render that variant's generated fields; after switching, controls for the old branch no longer resolve. The schema and generated variant model are described in Schema unions.

dart 
KeyedFormSelector<ItinerarySchema, String?>(
  selector: (form) => fields.getOrNull(form.value)?.kind,
  builder: (context, kind, _) => switch (kind) {
    'sightseeing' => KeyedFormField.text<ItinerarySchema>(
      field: fields.asSightseeing.place,
      builder: (context, field, text) => TextField(controller: text),
    ),
    'meal' => KeyedFormField.text<ItinerarySchema>(
      field: fields.asMeal.restaurant,
      builder: (context, field, text) => TextField(controller: text),
    ),
    _ => const SizedBox.shrink(),
  },
)

When a union is inside a keyed list, update the row by clientId and preserve that ID while replacing its variant shape. The selector is important: the row identity remains stable across the variant change, while the selected discriminator changes the rendered branch. Generated APIs such as asSightseeing and asMeal depend on the declared variant names; use the refs emitted for your schema rather than inventing a variant accessor. See virtualized lists and selective rebuilds. For a complete repository example, browse the tour builder source.