Build animations in React Native and Expo, making the decisions in the order that determines whether they feel right — should it animate, which thread it runs on, which properties, spring or timing, how the gesture hands off, how it degrades. Writes the implementation with Reanimated, Gesture Handle

Building Animations in Expo 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 Vector illustration, celebration, empty state Lottie — for illustration only, never for UI state A huge animated scene, freeform drawing @shopify/react native skia — a canvas, for when the view hierarchy itself is the bottleneck Reach for a shared value only when the value is continuous or interruptible. A press scale is a CSS transition; a drag is a shared value. Using a worklet for a two state toggle is the mobile equivalent of installing a motion library for a fade. Dependencies. Install with npx expo install <package — it resolves the version that matches the project's SDK, which plain npm install won't: Need Package Animation react native reanimated + react native worklets Gestures react native gesture handler Navigation, sheets, native tabs, menus expo router Haptics expo haptics Keyboard following UI react native keyboard controller (needs KeyboardProvider at the root — see the keyboard recipe) Illustration, celebration lottie react native Very large animated scenes, custom drawing @shopify/react native skia 4. Pick the properties transform and opacity are free. Everything else is a layout pass. width , height , margin , padding , flex , top , left , gap re run Yoga on every frame for that node and its siblings . The one exception: an absolutely positioned element with no children — a tab pill, a progress bar fill. It's out of flow, so nothing else re lays out, and animating width keeps the corner radius that scaleX would smear. Never scale(0) . Start from scale(0.9–0.97) + opacity: 0 . Nothing in the real world appears from nothing. transform is an array and order matters — [{ translateY }, { scale }] scales after moving; reversed, the translate gets scaled too. Keep translate first unless you want the multiplication. Android shadows are elevation , and animating elevation re renders the shadow every frame. Animate opacity of a pre shadowed layer instead. Never animate BlurView intensity. On Android it re renders the blur each frame. Crossfade the opacity of a static BlurView instead. Percentages work in translate and are relative to the element's own size — translateY('100%') moves a sheet by its own height whatever its content. 5. Timing or spring If a finger was involved, use a spring. Springs carry velocity through an interruption; timing curves restart. Everything else uses timing. Reanimated's spring takes Apple's two designer parameters directly — use this form, not mass/stiffness/damping: Interaction Config Default settle, no overshoot { duration: 400, dampingRatio: 1 } Reposition / snap back after a drag { duration: 400, dampingRatio: 0.8, velocity } Sheet, drawer { duration: 300, dampingRatio: 0.8, velocity } Must not pass a hard edge add overshootClamping: true Bounce only when the gesture carried momentum. Overshoot on a menu that faded in feels wrong; overshoot on a card you flicked feels right. Easing , for everything without a finger on it: Situation Easing Entering or exiting ease out Moving / morphing on screen ease in out Constant motion (progress, marquee) linear Default ease out Never ease in on UI. It starts slow, delaying the exact moment the user is watching. Reanimated's built ins are as weak as CSS's — use these: Duration: Element Duration Press feedback 100–150ms Toggle, chip, small state change 150–200ms Sheet, modal, drawer spring, ~300ms perceived Screen transition the platform default — don't override it Mobile UI animations stay under 300ms, same as web. The platform's own transitions are longer (iOS push is 350ms); match the platform for navigation, beat it everywhere else. 6. Keep it off the JS thread This is the mobile specific craft, and it's where most React Native motion dies. Never setState from a gesture or scroll handler. One React render per frame is the single biggest cause of jank in RN apps. Shared value → useAnimatedStyle , and React never re renders at all. Never schedule back to the RN runtime inside onUpdate or a scroll handler. scheduleOnRN(fn, ...args) from react native worklets — the Reanimated 4 replacement for the deprecated runOnJS(fn)(...args) — queues an RN runtime call, and in onUpdate that's 60–120× per second. It belongs in onEnd , or in a useAnimatedReaction that fires when a value crosses a threshold. Never read a shared value during render ( translateY.get() in JSX). It's a snapshot that never updates and it silently desyncs. Never write one during render either — it fires mid reconciliation, and a re render you didn't cause replays the write. Touch shared values only in worklets, handlers, and effects. Use .get() / .set() , not .value . Same API, but direct .value access is the form the React Compiler can't see through — the Reanimated docs call get / set the compiler safe way. set also takes a functional update: sv.set((v) = v + 1) . Functions called from a worklet need 'worklet' as their first line, or they throw at runtime on device while working fine in the debugger. 7. Press, not hover Every hover affordance from the web has to be redesigned, not ported. Feedback on press in, commit on press out. Waiting for the tap to complete before showing anything feels dead — this is the latency the user actually perceives. scale: 0.97 in 100–150ms on any pressable, Pressable + a CSS transition. scale takes the label and icons with it, which is what makes it read as physical. 44×44pt minimum touch target (48dp Android). If the visual is smaller, add hitSlop — don't grow the visual. pressRetentionOffset so a finger drifting a few pixels doesn't cancel a press the user meant. Android ripple only in a Material styled app. In a custom designed app, the same scale on both platforms is more coherent than a ripple on one. 8. Haptics Mobile has a sense the web doesn't. Use it sparingly and it becomes the thing that makes the app feel expensive; use it everywhere and users turn it off. Moment Call A value ticks past a step — picker, slider detent, segmented control Haptics.selectionAsync() Something snaps home, a sheet detent catches, a drag commits Haptics.impactAsync(ImpactFeedbackStyle.Light) A heavy object lands, a destructive action fires Haptics.impactAsync(ImpactFeedbackStyle.Medium) Operation succeeded or failed Haptics.notificationAsync(NotificationFeedbackType.Success / Error) Three rules, and they're absolute: Same frame as the visual. A haptic that lags its animation reads as a glitch, not as feedback. Fire it at the causal moment — the detent catching — not when the animation finishes. One per user action. Never on scroll, never per frame, never on an entrance animation the user didn't cause. Never the only feedback. Haptics are off system wide for many users, and silent on most Android hardware. The visual has to stand alone. From a worklet, haptics must be scheduled back to the RN runtime: scheduleOnRN(Haptics.selectionAsync) . 9. Reduced moti

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