09바탕색을 따라 그림자 색이 번지는 버튼

<main class="stage">
<div class="bc is-demo" id="bcRoot">
  <button class="bc__btn" id="bcBtn" type="button">구매하기</button>
  <p class="bc__meta"><span class="bc__tag">SHADOW</span> <span id="bcHex">#6f6099</span> <span class="bc__tag">Y</span> <b id="bcY">4</b></p>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;

// 핵심: 바탕색·그림자 색·그림자 높이를 진행도 하나가 함께 정한다. 그림자 색은 지금 바탕색을 어둡게 곱해 만든 값이다.

$glow-from: $subject-lilac;
$glow-to: $subject-pink;

@include stage($stage-paper);

.bc {
  --c-from: #{$glow-from};
  --c-to: #{$glow-to};
  width: min(100%, 320px);
  display: grid;
  gap: $sp-6;
  justify-items: center;
}

.bc__btn {
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: $r-control;
  background: $glow-from;
  box-shadow: 0 4px 10px rgba(111, 96, 153, .55);
  color: $subject-ink;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.bc__btn:focus-visible { outline: 3px solid $subject-ink; outline-offset: 3px; }

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  flex-wrap: wrap;
  justify-content: center;
  gap: $sp-1;
  font-size: 12px;
  line-height: 16px;
  color: rgba($subject-ink, .78);
}
.bc__tag { margin: 0 $sp-1 0 $sp-2; font-weight: 700; letter-spacing: .08em; }
.bc__tag:first-child { margin-left: 0; }
.bc__meta b { min-width: 2ch; text-align: right; font-variant-numeric: tabular-nums; color: $subject-ink; }

@media (prefers-reduced-motion: reduce) {
  .bc__btn { transition: none; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffffff;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.bc {
  --c-from: #b9a0ff;
  --c-to: #ff9ebb;
  width: min(100%, 320px);
  display: grid;
  gap: 24px;
  justify-items: center;
}

.bc__btn {
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: 12px;
  background: #b9a0ff;
  box-shadow: 0 4px 10px rgba(111, 96, 153, 0.55);
  color: #17141a;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

.bc__btn:focus-visible {
  outline: 3px solid #17141a;
  outline-offset: 3px;
}

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  flex-wrap: wrap;
  justify-content: center;
  gap: 4px;
  font-size: 12px;
  line-height: 16px;
  color: rgba(23, 20, 26, 0.78);
}

.bc__tag {
  margin: 0 4px 0 8px;
  font-weight: 700;
  letter-spacing: 0.08em;
}

.bc__tag:first-child {
  margin-left: 0;
}

.bc__meta b {
  min-width: 2ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
  color: #17141a;
}

@media (prefers-reduced-motion: reduce) {
  .bc__btn {
    transition: none;
  }
}

(function () {
  // 기법: 진행도 p 하나가 바탕색·그림자 색·그림자 높이를 함께 정한다. 바탕색은 from→to 로 섞고,
  // 그림자 색은 지금 바탕색의 각 채널에 SHADOW_DARKEN 을 곱해 만든다. p 는 목표로 일정 비율씩 끌린다.
  var SHADOW_DARKEN = 0.6;
  var SHADOW_ALPHA = 0.55;
  var LIFT_REST = 4;
  var LIFT_HOVER = 14;
  var BLUR_REST = 10;
  var BLUR_HOVER = 28;
  var GLOW_RATE = 6;
  var START_MS = 200;
  var ON_MS = 1000;
  var CYCLE_MS = 2000;

  var root = document.getElementById('bcRoot');
  var btn = document.getElementById('bcBtn');
  var hexEl = document.getElementById('bcHex');
  var yEl = document.getElementById('bcY');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  function readRgb(name) {
    var v = getComputedStyle(btn).getPropertyValue(name).trim().replace('#', '');
    if (v.length === 3) v = v.split('').map(function (c) { return c + c; }).join('');
    return [parseInt(v.slice(0, 2), 16), parseInt(v.slice(2, 4), 16), parseInt(v.slice(4, 6), 16)];
  }
  function toHex(rgb) {
    return '#' + rgb.map(function (c) { return ('0' + Math.round(c).toString(16)).slice(-2); }).join('');
  }

  var from = readRgb('--c-from');
  var to = readRgb('--c-to');
  var p = 0;
  var pointerIn = false;
  var kbFocus = false;
  var t0 = null;
  var lastTs = null;

  function demoOn(now) {
    return root.classList.contains('is-demo') && now >= START_MS && (now - START_MS) % CYCLE_MS < ON_MS;
  }
  function paint() {
    var fill = from.map(function (v, i) { return v + (to[i] - v) * p; });
    var shade = fill.map(function (v) { return v * SHADOW_DARKEN; });
    var lift = LIFT_REST + (LIFT_HOVER - LIFT_REST) * p;
    var blur = BLUR_REST + (BLUR_HOVER - BLUR_REST) * p;
    btn.style.background = toHex(fill);
    btn.style.boxShadow = '0 ' + lift.toFixed(1) + 'px ' + blur.toFixed(1) + 'px rgba(' +
      Math.round(shade[0]) + ', ' + Math.round(shade[1]) + ', ' + Math.round(shade[2]) + ', ' + SHADOW_ALPHA + ')';
    hexEl.textContent = toHex(shade);
    yEl.textContent = String(Math.round(lift));
  }
  function stopDemo() { root.classList.remove('is-demo'); }
  function jump() { p = pointerIn || kbFocus ? 1 : 0; paint(); }

  btn.addEventListener('pointerenter', function () { stopDemo(); pointerIn = true; if (reduce) jump(); });
  btn.addEventListener('pointerleave', function () { pointerIn = false; if (reduce) jump(); });
  btn.addEventListener('focus', function () { kbFocus = btn.matches(':focus-visible'); if (kbFocus) stopDemo(); if (reduce) jump(); });
  btn.addEventListener('blur', function () { kbFocus = false; if (reduce) jump(); });

  function tick(ts) {
    if (t0 === null) t0 = ts;
    var now = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    var g = demoOn(now) || pointerIn || kbFocus ? 1 : 0;
    p += (g - p) * (1 - Math.exp(-dt * GLOW_RATE));
    if (Math.abs(g - p) < 0.002) p = g;
    paint();
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    paint();
  } else {
    requestAnimationFrame(tick);
  }
})();

// ShadowFollowButton — vanilla 판(09-btn-color-shadow-follow-glow) 의 React 포트.
// 진행도 p 하나가 바탕색·그림자 색·그림자 높이를 함께 정한다. 그림자 색은 지금 바탕색에 SHADOW_DARKEN 을 곱한 값이다.
import { useEffect, useRef } from "react";
import styles from "./ShadowFollowButton.module.scss";

type Box<T> = { current: T };
type Rgb = [number, number, number];

const SHADOW_DARKEN = 0.6;
const SHADOW_ALPHA = 0.55;
const LIFT_REST = 4;
const LIFT_HOVER = 14;
const BLUR_REST = 10;
const BLUR_HOVER = 28;
const GLOW_RATE = 6;
const START_MS = 200;
const ON_MS = 1000;
const CYCLE_MS = 2000;

function toHex(rgb: Rgb): string {
  return "#" + rgb.map((c) => ("0" + Math.round(c).toString(16)).slice(-2)).join("");
}

export default function ShadowFollowButton() {
  const btnRef = useRef<HTMLButtonElement | null>(null) as Box<HTMLButtonElement | null>;
  const hexRef = useRef<HTMLSpanElement | null>(null) as Box<HTMLSpanElement | null>;
  const yRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;

  useEffect(() => {
    const btn = btnRef.current;
    const hexEl = hexRef.current;
    const yEl = yRef.current;
    if (!btn || !hexEl || !yEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const readRgb = (name: string): Rgb => {
      let v = getComputedStyle(btn).getPropertyValue(name).trim().replace("#", "");
      if (v.length === 3) v = v.split("").map((c) => c + c).join("");
      return [parseInt(v.slice(0, 2), 16), parseInt(v.slice(2, 4), 16), parseInt(v.slice(4, 6), 16)];
    };
    const from = readRgb("--c-from");
    const to = readRgb("--c-to");
    let p = 0;
    let demo = !reduce;
    let pointerIn = false;
    let kbFocus = false;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    const demoOn = (now: number): boolean => demo && now >= START_MS && (now - START_MS) % CYCLE_MS < ON_MS;
    const paint = (): void => {
      const fill: Rgb = [from[0] + (to[0] - from[0]) * p, from[1] + (to[1] - from[1]) * p, from[2] + (to[2] - from[2]) * p];
      const shade: Rgb = [fill[0] * SHADOW_DARKEN, fill[1] * SHADOW_DARKEN, fill[2] * SHADOW_DARKEN];
      const lift = LIFT_REST + (LIFT_HOVER - LIFT_REST) * p;
      const blur = BLUR_REST + (BLUR_HOVER - BLUR_REST) * p;
      btn.style.background = toHex(fill);
      btn.style.boxShadow = "0 " + lift.toFixed(1) + "px " + blur.toFixed(1) + "px rgba(" +
        Math.round(shade[0]) + ", " + Math.round(shade[1]) + ", " + Math.round(shade[2]) + ", " + SHADOW_ALPHA + ")";
      hexEl.textContent = toHex(shade);
      yEl.textContent = String(Math.round(lift));
    };
    const stopDemo = (): void => { demo = false; };
    const jump = (): void => { p = pointerIn || kbFocus ? 1 : 0; paint(); };
    const onEnter = (): void => { stopDemo(); pointerIn = true; if (reduce) jump(); };
    const onLeave = (): void => { pointerIn = false; if (reduce) jump(); };
    const onFocus = (): void => { kbFocus = btn.matches(":focus-visible"); if (kbFocus) stopDemo(); if (reduce) jump(); };
    const onBlur = (): void => { kbFocus = false; if (reduce) jump(); };
    btn.addEventListener("pointerenter", onEnter);
    btn.addEventListener("pointerleave", onLeave);
    btn.addEventListener("focus", onFocus);
    btn.addEventListener("blur", onBlur);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      const now = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      const g = demoOn(now) || pointerIn || kbFocus ? 1 : 0;
      p += (g - p) * (1 - Math.exp(-dt * GLOW_RATE));
      if (Math.abs(g - p) < 0.002) p = g;
      paint();
      raf = requestAnimationFrame(tick);
    }

    if (reduce) paint();
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("pointerenter", onEnter);
      btn.removeEventListener("pointerleave", onLeave);
      btn.removeEventListener("focus", onFocus);
      btn.removeEventListener("blur", onBlur);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.bc}>
        <button className={styles.bc__btn} type="button" ref={btnRef}>구매하기</button>
        <p className={styles.bc__meta}>
          <span className={styles.bc__tag}>SHADOW</span> <span ref={hexRef}>#6f6099</span> <span className={styles.bc__tag}>Y</span> <b ref={yRef}>4</b>
        </p>
      </div>
    </main>
  );
}