React Native

React Native (app code)

Hand-off — the app vs its motion. Animation and gestures in a React Native app — Reanimated worklets on the UI runtime, gesture handoff, sheets, press feedback, haptics — are ../motion-craft/SKILL.md's, whose guidance is platform-agnostic and applies to bare React Native as well as Expo.

You write the code that runs inside the app: render, navigate, animate, list, persist, and author native modules. Mobile is split across two skills by verb — run the verb test before doing anything:

Assume the New Architecture is on (Fabric + Turbo Modules). It is mandatory from RN 0.82 / Expo SDK 55 — RN 0.82 (2025-10-08) ignores any disable flag, and SDK 55 is New-Arch-only. SDK 54 (RN 0.81) was the last that could opt out. Never write newArchEnabled: false to dodge a bug; fix the bug. Why: the flag is a no-op on current versions, so the "fix" silently does nothing.

Reference SDK→RN map (accessed 2026-06-02): SDK 53 = RN 0.79, SDK 54 = RN 0.81 (React Compiler GA), SDK 55 = RN 0.83.

Project shape — pick a router first

This is the one real branch at scaffold time:

Default file tree for an Expo Router app:

app/                  # routes — file system IS the navigation
  (auth)/             # group: unauthenticated screens
  (app)/              # group: authenticated screens
    (tabs)/           # nested tab navigator
  _layout.tsx         # root layout + providers
components/           # dumb, reusable UI
features/<domain>/    # feature-scoped screens, hooks, components
lib/                  # clients, query setup, storage, utils

Keep route files thin: a route file wires params and renders a feature component. Put logic in features/. Why: routes get re-mounted by navigation; business logic in them is hard to test and re-runs unexpectedly.

Navigation

Type your routes. With Expo Router, enable typed routes and let useLocalSearchParams<{ id: string }>() carry the contract. Why: deep links and back-stack restoration pass strings you will otherwise misread.

Gate auth at the group layout, declaratively — not imperatively inside an effect:

// app/(app)/_layout.tsx — Bad: imperative nav in an effect races the first paint
export default function AppLayout() {
  const { user } = useAuth();
  useEffect(() => {
    if (!user) router.replace('/(auth)/sign-in'); // flashes protected UI first
  }, [user]);
  return <Stack />;
}

// app/(app)/_layout.tsx — Good: redirect before children mount
import { Redirect, Stack } from 'expo-router';
export default function AppLayout() {
  const { user, loading } = useAuth();
  if (loading) return null;          // or a splash
  if (!user) return <Redirect href="/(auth)/sign-in" />;
  return <Stack />;
}

Modals/sheets: declare presentation on the screen, not by pushing a styled full-screen route — <Stack.Screen options={{ presentation: 'modal' }} />. Configure deep links via the scheme + the linking config so a cold-start link resolves to the right nested route, not just the root. A deep link landing on the wrong screen is almost always a group/segment mismatch between the URL and the app/ tree.

Lists & performance

Switch from FlatList to FlashList (Shopify) once a list crosses ~100 items, has variable row heights, or renders images. Why: FlashList recycles views instead of mounting one per row, so memory and scroll jank stay flat as the list grows.

Three things every long list needs:

1. A stable keyExtractor — return a real id, never the index. Index keys defeat recycling and reorder rows on insert.
2. getItemType when rows differ structurally (header vs product vs ad). Why: it lets FlashList recycle within a type instead of remeasuring.
3. A memoized row component with no inline closures or style objects in renderItem.

// Bad: new fn + new style object every parent render -> every visible row re-renders
<FlashList
  data={items}
  renderItem={({ item }) => (
    <Pressable style={{ padding: 12 }} onPress={() => open(item.id)}>
      <Text>{item.title}</Text>
    </Pressable>
  )}
/>

// Good: stable refs, memoized row, typed rows
const Row = memo(function Row({ item }: { item: Product }) {
  const open = useOpenProduct();          // stable from context/store
  return (
    <Pressable style={styles.row} onPress={() => open(item.id)}>
      <Text>{item.title}</Text>
    </Pressable>
  );
});

<FlashList
  data={items}
  keyExtractor={(it) => it.id}
  getItemType={(it) => it.kind}
  renderItem={({ item }) => <Row item={item} />}
/>

FlashList caveat: rows can flash into the wrong position for a split second if mounted while entry animations run. Disable entry animations on the initial mount.

React Compiler (GA in SDK 54) auto-memoizes components and reduces manual useMemo/useCallback — but it does not remove the need for virtualization, stable keys, or getItemType. Don't delete your list discipline because the compiler is on. For re-render hunting and profiling, see references/performance-debugging.md.

Animation & gestures

Reanimated worklets run on the UI thread, so animations keep going at 60fps even when JS is busy. Reanimated 4 (stable Oct 2025) requires the New Architecture / RN 0.76+ and adds CSS-style animations while keeping the worklet API. Many teams stay on Reanimated 3 for dependency compatibility — match the project's installed major.

Two rules that fix most gesture bugs:

• Wrap gesture objects in useMemo. Why: a new gesture object each render reattaches the recognizer and drops in-flight touches.
• Cross to JS only at the boundary with runOnJS. Why: calling a JS-thread function directly from a worklet crashes; everything inside the worklet must stay on the UI thread.

// Good: memoized gesture, UI-thread shared value, JS only at the end
const x = useSharedValue(0);
const pan = useMemo(
  () =>
    Gesture.Pan()
      .onUpdate((e) => { x.value = e.translationX; })            // UI thread
      .onEnd(() => { runOnJS(onSwiped)(); x.value = withSpring(0); }), // boundary
  [onSwiped],
);

Use Reanimated 4 CSS animations for simple declarative cases (fades, simple transitions); use worklets for gesture-driven or physics-based motion. Never animate layout on the JS thread when a shared value will do it on the UI thread.