react-native-reanimated（React native reanimated 前端）

React Native Reanimated

Overview

UI-thread animation library for React Native. Reanimated 4.x runs animations on the native UI thread for guaranteed 60fps performance, replacing Framer Motion for mobile applications. Provides shared values, animated styles, declarative entering/exiting transitions, and gesture integration.

Install: pnpm add react-native-reanimated (included with Expo SDK 54+)

Workflows

Adding animations to a screen:

1. Import from react-native-reanimated: Animated, useSharedValue, useAnimatedStyle, withTiming
2. Create shared values for animated properties
3. Define animated styles using useAnimatedStyle
4. Apply to Animated.View, Animated.Text, etc.
5. Check useReducedMotion() and skip animations when true
6. Test on device for smooth 60fps

Adding entering/exiting transitions:

1. Import layout animations: FadeInDown, FadeOutUp, SlideInRight, etc.
2. Apply entering and exiting props on Animated.View
3. Chain modifiers: .duration(300).easing(...).delay(100)
4. Verify reduced motion handling

Guidance

Core Primitives

• useSharedValue(initialValue): Mutable value that lives on UI thread
• useAnimatedStyle(() => ({...})): Derives style object from shared values
• withTiming(toValue, config): Duration-based animation
• withSpring(toValue, config): Physics-based spring animation
• withDelay(ms, animation): Delay before animation starts
• withSequence(...animations): Run animations in sequence
• withRepeat(animation, count, reverse): Repeat animation

Standard Timing Configs

Consistent with demo-development.md durations:

Easing: Easing.out(Easing.cubic) for entrances, Easing.inOut(Easing.cubic) for transitions, Easing.in(Easing.cubic) for exits.

Declarative Entering/Exiting

Apply directly to Animated.View — no parent wrapper needed (unlike AnimatePresence):

• Entering: FadeIn, FadeInDown, FadeInUp, SlideInRight, ZoomIn
• Exiting: FadeOut, FadeOutUp, FadeOutDown, SlideOutLeft, ZoomOut
• Chain modifiers: .duration(300), .delay(100), .easing(Easing.out(Easing.cubic))
• Spring: .springify().damping(15).stiffness(150)

Stagger Pattern

Use withDelay with index multiplication:

entering={FadeInDown.delay(index * 50).duration(300)}

Or for imperative animations:

offset.value = withDelay(index * 50, withTiming(1, { duration: 300 }))

Accessibility — Reduced Motion

Always respect the user's accessibility preference:

const reducedMotion = useReducedMotion();

When reducedMotion is true:

• Skip all animations (set values instantly)
• Use duration(0) or don't apply entering/exiting props
• Still show content — just don't animate it

Animated Components

Use Animated.View, Animated.Text, Animated.ScrollView, etc. Or create animated versions:

const AnimatedPressable = Animated.createAnimatedComponent(Pressable);

Performance

• Animate only transform and opacity — these run on the UI thread
• Avoid animating width, height, margin, padding (causes layout recalculation)
• Shared values update on UI thread without JS bridge overhead
• useAnimatedStyle worklets run on UI thread — keep them pure
• For lists, use entering/exiting on items rather than animating the container

Best Practices

• Use declarative entering/exiting for mount/unmount animations — simpler than manual shared values
• Prefer withSpring for natural-feeling interactions (drag, gestures)
• Prefer withTiming for UI transitions (screen enter, fade in)
• Always check useReducedMotion() before applying animations
• Keep standard durations consistent: 150ms / 300ms / 500ms
• Use withDelay(index * 50,...) for staggered list reveals
• Combine with react-native-gesture-handler for gesture-driven animations
• Use Easing.out(Easing.cubic) as default easing for entrances

Anti-Patterns

• Using React Native's built-in Animated API instead of Reanimated
• Animating layout properties (width, height, margin) — use transform and opacity
• Forgetting useReducedMotion() accessibility check
• Running JS-thread-heavy logic inside useAnimatedStyle worklets
• Hardcoding duration values (use constants: 150/300/500ms)
• Nesting many animated views unnecessarily (performance overhead)
• Using setTimeout for stagger instead of withDelay
• Applying entering animations without testing on real device