Discriminated unions

Use a discriminated union when a form section has mutually exclusive shapes, such as a personal or business account. A variant reference is valid only while the value has that variant.

Render the active variant#

Inside a row widget, fields is that row's generated ActivityFieldRefs. Read its current discriminator with a selector, then render the matching generated variant reference. A VariantRef resolves only while the row contains that variant; after a switch, the old branch's fields stop resolving instead of reading stale data.

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, controller) => TextField(
        controller: controller,
        decoration: InputDecoration(errorText: field.errorText),
      ),
    ),
    'meal' => KeyedFormField.text<ItinerarySchema>(
      field: fields.asMeal.restaurant,
      builder: (context, field, controller) => TextField(
        controller: controller,
        decoration: InputDecoration(errorText: field.errorText),
      ),
    ),
    _ => const SizedBox.shrink(),
  },
);

activityList.updateById(
  activity.clientId,
  (current) => kind == 'sightseeing'
      ? SightseeingActivitySchema.create(clientId: current.clientId)
      : MealActivitySchema.create(clientId: current.clientId),
);

The selector reads the row's current discriminator because switching variants keeps the same clientId; the outer list's row snapshot may therefore stay unchanged. updateById replaces the row with the selected generated shape while preserving its identity. The schema uses ks.discriminatedUnion('kind', ...) and the generator creates variant accessors such as fields.asSightseeing and fields.asMeal.

Keep shared fields outside the union and validate each branch in its own schema. See how it works for the role of VariantRef.