$ show pmndrs/react-spring
react-spring is a spring-physics-first animation library for React, providing a cross-platform animation engine that targets both the DOM (via @react-spring/web) and React Three Fiber (via @react-spring/three). The library is built on a layered architecture: a custom requestAnimationFrame scheduler (rafz), a shared utilities layer (FluidValue observables, color/string interpolation, easing functions), an animated value system (Animated, AnimatedValue, AnimatedString), a core animation engine (Controller, SpringValue, Interpolation), and target-specific adapters that inject renderer behavior via Globals.assign and createHost. The public API consists of declarative hooks (useSpring, useSprings, useTrail, useTransition, usePresence, useChain, useScroll, useResize, useInView), imperative controls (SpringRef), and low-level components (Spring, Trail, Transition). The library is published as a monorepo with 12 npm packages, all version-locked and released via changesets. The repository also includes a comprehensive documentation site built with React Router 7, MDX, and Vanilla Extract, plus a demo app with 30+ interactive sandboxes. The codebase is extensively tested with unit tests (Vitest browser mode), type-level tests, and E2E tests, using a custom mock-raf for deterministic animation testing. The architecture emphasizes performance on the hot path, target-agnostic core logic, and a strict bottom-up dependency graph (rafz → shared → animated → core → targets).
The LLM identified 5 files worth highlighting for reuse:
A custom requestAnimationFrame scheduler that supports demand-mode (pausing the loop when idle) and timeout-based scheduling. It is a standalone, zero-dependency module (<700 bytes) that can be reused outside react-spring for any animation or polling loop that needs efficient frame management. The implementation demonstrates non-obvious skill in handling edge cases like restarting the loop after a demand pause and batching multiple callbacks per frame.
Implements the FluidValue base class and observer pattern that underpins react-spring's reactive value tracking. FluidValues can be attached to each other (e.g., one spring tracking another) and automatically notify observers when their value changes. The pattern is cleanly separated from the animation engine and could be extracted for any reactive state management system that needs efficient change propagation without React re-renders.
A memoized computed value that tracks its fluid dependencies and automatically enters/leaves the frameloop when dependencies change. It supports range mapping (like d3.interpolate) and can be chained. The implementation demonstrates advanced understanding of reactive dependency tracking and lazy evaluation, making it reusable for any scenario requiring efficient recomputation of derived values from observable sources.
Handles interpolation of complex strings containing colors (hex, rgb, rgba, hsl, named), CSS units (px, %, deg, etc.), transform functions (translate, rotate, scale), and CSS custom properties (var(--name)). It preserves decimal precision and correctly handles mixed-type strings. This is a non-trivial parsing and interpolation engine that could be reused in any animation or data-binding library that needs to animate CSS string properties.
A generic diff algorithm for computing enter, update, and leave transitions in a list of items. It supports mode: 'wait' (sequential transitions) and an expires mechanism for delayed removal. The algorithm is independent of the animation engine and could be reused in any UI framework that needs to manage list transitions with lifecycle phases (enter/update/leave).