Expo Animation

# Building Animations in Expo

This skill was created in collaboration with [Emil Kowalski](https://github.com/emilkowalski) and can also be found in the [emilkowalski/skills](https://github.com/emilkowalski/skills) repository, along with other useful animation skills.

A construction skill for React Native. It turns a request for motion into an implementation that survives a strict review on a real device — not in the simulator, not on a flagship phone in dev mode.

Mobile changes three things about animation, and everything in this skill follows from them:

1. **There is no hover.** Every affordance the web puts in hover has to live in press, position, or nothing. 2. **There are two runtimes.** Worklets (Reanimated 4) makes this explicit: the React Native runtime, where React renders and your app logic runs, and the UI runtime, where worklets run every frame (plus optional worker runtimes for background work). An animation that touches the RN runtime stutters the moment the app does anything else. The whole craft is keeping motion on the UI runtime. 3. **The user's finger is on the element.** Gestures are the primary input, so interruptibility and velocity handoff aren't polish — they're the baseline.

## Operating Posture

You are a senior mobile engineer building the animation yourself. Make the call, state the reasoning in one line, write the code. Never present motion options as a menu.

Two failure modes, and the first is worse:

1. **Animating something that shouldn't animate.** The gate below exists to produce zero lines of code sometimes. 2. **Animating the right thing on the wrong thread** — a `setState` per frame, a `PanResponder`, an animated `height`. It looks fine in dev on your phone and drops to 20fps on a three-year-old Android.

## Hard Rules

1. **Run the sequence in order.** Steps 1 and 2 gate everything. 2. **Reanimated, not core `Animated`.** Core `Animated` can't be driven by a gesture without crossing the bridge, and `useNativeDriver` refuses anything but transform and opacity anyway. Reanimated worklets run on the UI thread and keep running while JS is busy. 3. **No approximated values.** Curves and spring configs come from the tables below. 4. **Reduced motion ships with the animation**, not as a follow-up. 5. **Feel is judged on a release build on the slowest device you support.** Nothing else counts as verified.

## The Build Sequence

### 1. Should this animate at all?

| Frequency | Decision | | --- | --- | | 100+ times/day — tab switches, keyboard open/close, scrolling, toggles in settings | **No animation.** Platform default or nothing. Stop here. | | Tens of times/day — press feedback, list navigation, row selection | Near-imperceptible only: under 150ms, or nothing | | Occasional — sheets, modals, toasts, onboarding steps | Standard animation | | Rare / first-time — success states, empty-state illustrations, celebration | The delight budget lives here |

**Tab switches never slide.** Tabs are peers, not a hierarchy — sliding implies depth that isn't there, and the user pays for it dozens of times a session. `animation: 'none'`.

If the request fails this gate, say so and don't write it.

### 2. What is the purpose?

Name it in one word before continuing: **feedback**, **spatial consistency**, **state indication**, **preventing a jarring change**, **explanation**, or **delight** (rare tier only).

Can't name it? Don't build it.

### 3. Pick the tool — cheapest that works

Walk down; stop at the first that fits.

| Need | Tool | | --- | --- | | A state-driven change with no gesture — press, toggle, color, a value flipping | **Reanimated CSS transition** (`transitionProperty` in the style) | | Loop, multi-stage, or plays on mount with no state change | **Reanimated CSS animation** (`animationName` keyframes) | | An element mounting or unmounting, or a list reflowing | **Layout animations** (`entering` / `exiting` / `itemLayoutAnimation`) | | Anything a finger touches, or anything derived from scroll | **`useSharedValue` + `Gesture` + `useAnimatedStyle`** | | Screen to screen | **Native stack options in Expo Router.** Never hand-roll this | | A bottom sheet that is its own screen | **`presentation: 'formSheet'`** — it's a real UISheetPresentationController, free and correct | | Tab bar | **`NativeTabs`** (from `expo-router/unstable-native-tabs`) — the platform's real tab bar, its behaviors and transitions included | | Context menu, press-and-hold preview | **`Link.Menu` / `Link.Preview`** (Expo Router, iOS-only) — native menus and peek, never rebuilt in JS | | Header that collapses into a large title | **`headerLargeTitleEnabled`** on the native stack (iOS-only; `headerLargeTitle` is deprecated) — not a scroll worklet | | Pull to refresh | **`RefreshControl`** — hand-roll only when it's a signature interaction (see the threshold recipe) | | UI that tracks the keyboard | **`react-native-keyboard-controller`** — the keyboard's real position, frame by frame, on the UI thread | |