motion-foundations

name	motion-foundations
description	动效基础模式——过渡、基础动画、缓动曲线和动画系统设计原则。
version	1
tags	["motion","animation","performance","accessibility"]
category	frontend
author	jeff

Motion Foundations

The base layer of the motion system. Defines every value, constraint, and rule that downstream skills (motion-patterns, motion-advanced) inherit. Load this skill before any animation work begins.

When to Activate

• Starting any animated component from scratch
• Setting up tokens, spring presets, or easing values
• Implementing prefers-reduced-motion support
• Debugging hydration mismatches from animation initial states
• Evaluating whether an animation should exist at all

Outputs

This skill produces:

• A shared motionTokens object (duration, easing, distance, scale)
• A shared springs preset map (5 named configs)
• A shouldAnimate() gate used by all components
• Accessibility-compliant animation defaults via useReducedMotion
• SSR-safe initial states with zero hydration warnings

Principles

Motion must do at least one of the following or it must be removed:

• Guide attention
• Communicate state
• Preserve spatial continuity

Responsiveness always outranks smoothness. A 60 fps animation that causes input delay is worse than no animation.

Rules

These are non-negotiable. They apply to every component in the system.

1. Use motion/react only. Never import from framer-motion. Never mix the two in the same tree.
2. initial must match server output. If the server renders opacity: 1, the initial prop must also be opacity: 1. No exceptions.
3. Reduced motion overrides everything. When useReducedMotion() returns true or prefersReduced is true, all transforms are disabled. Opacity-only fades at ≤ 0.2s are the only permitted fallback.
4. Never animate layout properties. width, height, top, left, margin, padding are banned from animate. Use transform and opacity only.
5. All token values come from motionTokens. Hardcoded durations and easings in component files are forbidden.
6. All spring configs come from the springs map. Inline stiffness/damping values are forbidden.
7. "use client" is required on every file that imports from motion/react.
8. Never read window or navigator at module level. Always guard with typeof window !== "undefined".

Decision Guidance

Choosing a duration

Token	Use when
instant	Tooltip show/hide, focus ring, badge update
fast	Button feedback, icon swap, chip toggle
normal	Modal open, card expand, page element enter
slow	Hero entrance, full-page transition
crawl	Deliberate storytelling; use sparingly

Choosing a spring

Preset	Use when
snappy	Default UI — buttons, chips, nav items
gentle	Cards, modals, panels landing softly
bouncy	Playful moments — empty states, onboarding
instant	Tooltips, popovers, dropdowns
release	Drag release — natural physics feel

When to disable animation entirely

Disable (make shouldAnimate() return false) when:

• prefersReduced is true
• isLowEnd is true and the animation is non-essential
• The element is off-screen and will never enter the viewport
• The animation is purely decorative with no UX purpose

Core Concepts

Token system

// lib/motion-tokens.ts
export const motionTokens = {
  duration: {
    instant: 0.08,
    fast:    0.18,
    normal:  0.35,
    slow:    0.6,
    crawl:   1.0,
  },
  easing: {
    smooth: [0.22, 1, 0.36, 1],
    sharp:  [0.4, 0, 0.2, 1],
    bounce: [0.34, 1.56, 0.64, 1],
    linear: [0, 0, 1, 1],
  },
  distance: {
    xs: 4,
    sm: 8,
    md: 16,
    lg: 24,
    xl: 48,
  },
  scale: {
    subtle: 0.98,
    press:  0.95,
    pop:    1.04,
  },
}

export const springs = {
  snappy:  { type: , : , :  },
  :  { : , : , :  },
  :  { : , : , :  },
  : { : , : , :  },
  : { : , : , : , :  },
}

Runtime flags

// lib/motion-config.ts
export const motionConfig = {
  isLowEnd() {
    return (
      typeof navigator !== "undefined" &&
      navigator.hardwareConcurrency <= 4
    )
  },

  prefersReduced() {
    return (
      typeof window !== "undefined" &&
      window.matchMedia("(prefers-reduced-motion: reduce)").matches
    )
  },

  shouldAnimate({ essential = false } = {}) {
    if (this.prefersReduced()) return false
    if (!essential && this.isLowEnd()) return false
    return true
  },

  duration() {
    return this.isLowEnd() || this.prefersReduced()
      ? motionTokens.duration.instant
      : motionTokens.duration.normal
  },
}

Accessibility

Priority order (highest to lowest):

1. prefers-reduced-motion: reduce — disables all transforms, limits opacity transitions to ≤ 0.2s
2. Low-end device detection — reduces duration, removes non-essential animations
3. Design preference — everything else

Motion must degrade gracefully. It must never disappear abruptly in a way that causes layout shift or confuses orientation.

// hooks/use-reduced-motion.tsx
"use client"
import { useReducedMotion } from "motion/react"

export function useSafeMotion(fullY: number = 16) {
  const reduce = useReducedMotion()
  return {
    initial: { opacity: 0, y: reduce ? 0 : fullY },
    animate: { opacity: 1, y: 0 },
    exit:    { opacity: 0, y: reduce ? 0 : -fullY },
  }
}

/* globals.css */
@media (prefers-reduced-motion: reduce) {
  .motion-safe-transition  { transition: opacity 0.15s; }
  .motion-reduce-transform { transform: none !important; }
}

<!-- Tailwind -->
<div class="motion-safe:animate-fade motion-reduce:opacity-100"></div>

SSR / hydration safety

Rule: initial must always match what the server renders.

// WRONG — server renders opacity:1 but initial says 0 → hydration mismatch
<motion.div initial={{ opacity: 0 }} animate={{ opacity: 1 }} />

// CORRECT — use AnimatePresence or defer to client mount
"use client"
const [mounted, setMounted] = useState(false)
useEffect(() => setMounted(true), [])

<motion.div
  initial={{ opacity: mounted ? 0 : 1 }}
  animate={{ opacity: 1 }}
/>

Code Examples

End-to-end: tokens + springs + accessibility + SSR guard

// components/fade-in-card.tsx
"use client"

import { useState, useEffect } from "react"
import { motion } from "motion/react"
import { motionTokens, springs } from "@/lib/motion-tokens"
import { useSafeMotion } from "@/hooks/use-reduced-motion"
import { motionConfig } from "@/lib/motion-config"

interface FadeInCardProps {
  children: React.ReactNode
  delay?: number
}

export function FadeInCard({ children, delay = 0 }: FadeInCardProps) {
  // SSR guard — initial must match server output (opacity: 1)
  const [mounted, setMounted] = useState(false)
  useEffect(() => setMounted(true), [])

  // Accessibility — disables transform when reduced motion is preferred
  const safeMotion = useSafeMotion(motionTokens.distance.md)

  // Device gate — skip animation on low-end hardware
  if (!motionConfig.shouldAnimate() || !mounted) {
    return 
  }

   (
    
  )
}

Constraints / Non-Goals

This skill does not cover:

• UI component patterns (button, modal, stagger) → see motion-patterns
• Drag, gestures, SVG, text animations, custom hooks → see motion-advanced
• CSS-only animations or Tailwind animate-* classes without motion/react
• Third-party animation libraries (GSAP, anime.js, etc.)
• Motion design decisions (when to animate, what to emphasize) — that is a design concern, not a code constraint

Anti-Patterns

Anti-pattern	Rule violated	Fix
import { motion } from "framer-motion"	Rule 1	Use motion/react
initial={{ opacity: 0 }} on SSR component	Rule 2	Add mount guard
Skipping useReducedMotion check	Rule 3	Use useSafeMotion hook
animate={{ width: "100%" }}	Rule 4	Use scaleX transform instead
transition={{ duration: 0.4 }} inline	Rule 5	Use motionTokens.duration.normal
{ stiffness: 300, damping: 30 } inline	Rule 6	Use springs.snappy
Missing "use client" directive	Rule 7	Add to top of file
navigator.hardwareConcurrency at module level	Rule 8	Wrap in typeof navigator !== "undefined"

Related Skills

• motion-patterns — consumes tokens and springs defined here to build button, modal, stagger, page transition, and scroll patterns. Does not redefine any values.
• motion-advanced — consumes tokens and springs defined here for drag, SVG, text, and gesture patterns. Adds useAnimate sequences and custom hooks on top of this foundation.