06押している間に色相が加速して回るボタン

<main class="stage">
<div class="bc is-demo" id="bcRoot">
  <button class="bc__btn" id="bcBtn" type="button">長押し</button>
  <p class="bc__meta"><span class="bc__tag">HUE</span> <b id="bcHue">270</b><span>deg</span> <span class="bc__tag">SPEED</span> <b id="bcSpeed">0</b></p>
</div>
</main>

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

// 핵심: 색상 각도(--hue)는 속도를 쌓아 얻는 값이다. 누르면 속도가 가속하고, 떼면 마찰로 줄어 멈춘 색에 남는다.

$hue-sat: 72%;
$hue-light: 82%;

@include stage($stage-ink);

.bc {
  width: min(100%, 320px);
  display: grid;
  gap: $sp-3;
  justify-items: center;
}

.bc__btn {
  --hue: 270;
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: $r-control;
  background: hsl(var(--hue), $hue-sat, $hue-light);
  color: $subject-ink;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
  touch-action: manipulation;
  user-select: none;
}
.bc__btn:focus-visible { outline: 3px solid $stage-paper; outline-offset: 3px; }
.bc__btn:active { transform: translateY(1px); }

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  flex-wrap: wrap;
  justify-content: center;
  gap: $sp-1;
  font-size: 12px;
  line-height: 16px;
  color: rgba($stage-paper, .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: 3ch; text-align: right; font-variant-numeric: tabular-nums; color: $stage-paper; }

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

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.bc {
  width: min(100%, 320px);
  display: grid;
  gap: 12px;
  justify-items: center;
}

.bc__btn {
  --hue: 270;
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: 12px;
  background: hsl(var(--hue), 72%, 82%);
  color: #17141a;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
  touch-action: manipulation;
  user-select: none;
}

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

.bc__btn:active {
  transform: translateY(1px);
}

.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(255, 255, 255, 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: 3ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
  color: #ffffff;
}

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

(function () {
  // 기법: 누르는 동안 색상 각속도가 ACCEL 로 커지고(상한 MAX_SPEED), 떼면 FRICTION 으로 줄어 멈춘다.
  // 색상 각도는 속도를 매 프레임 쌓아 얻는다. 시간은 rAF 타임스탬프 간격(dt)만 쓴다.
  var ACCEL = 600;
  var MAX_SPEED = 720;
  var FRICTION = 2.4;
  var HUE_START = 270;
  var HUE_STEP = 60;
  var START_MS = 200;
  var HOLD_MS = 900;
  var CYCLE_MS = 2000;

  var root = document.getElementById('bcRoot');
  var btn = document.getElementById('bcBtn');
  var hueEl = document.getElementById('bcHue');
  var speedEl = document.getElementById('bcSpeed');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var hue = HUE_START;
  var speed = 0;
  var pressed = false;
  var t0 = null;
  var lastTs = null;

  function paint() {
    btn.style.setProperty('--hue', hue.toFixed(1));
    hueEl.textContent = String(Math.round(hue));
    speedEl.textContent = String(Math.round(speed));
  }
  function stopDemo() { root.classList.remove('is-demo'); }
  function press() {
    stopDemo();
    pressed = true;
    if (reduce) { hue = (hue + HUE_STEP) % 360; paint(); }
  }
  function release() { pressed = false; }

  btn.addEventListener('pointerdown', function (e) {
    try { btn.setPointerCapture(e.pointerId); } catch (err) { /* 캡처를 못 걸어도 누름은 동작한다 */ }
    press();
  });
  btn.addEventListener('pointerup', release);
  btn.addEventListener('pointercancel', release);
  btn.addEventListener('blur', release);
  btn.addEventListener('keydown', function (e) {
    if ((e.key === ' ' || e.key === 'Enter') && !e.repeat) press();
  });
  btn.addEventListener('keyup', function (e) {
    if (e.key === ' ' || e.key === 'Enter') release();
  });

  function demoPressed(now) {
    return root.classList.contains('is-demo') && now >= START_MS && (now - START_MS) % CYCLE_MS < HOLD_MS;
  }

  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;
    if (demoPressed(now) || pressed) {
      speed = Math.min(MAX_SPEED, speed + ACCEL * dt);
    } else {
      speed *= Math.exp(-FRICTION * dt);
      if (speed < 0.5) speed = 0;
    }
    hue = (hue + speed * dt) % 360;
    paint();
    requestAnimationFrame(tick);
  }

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

// HueHoldButton — vanilla 판(06-btn-hue-hold-inertia-ramp) 의 React 포트.
// 누르는 동안 색상 각속도가 가속하고(MAX_SPEED 상한), 떼면 FRICTION 으로 줄어 멈춘다.
import { useEffect, useRef } from "react";
import styles from "./HueHoldButton.module.scss";

type Box<T> = { current: T };
type PointerLike = Event & { pointerId: number };
type KeyLike = Event & { key: string; repeat: boolean };

const ACCEL = 600;
const MAX_SPEED = 720;
const FRICTION = 2.4;
const HUE_START = 270;
const HUE_STEP = 60;
const START_MS = 200;
const HOLD_MS = 900;
const CYCLE_MS = 2000;

export default function HueHoldButton() {
  const btnRef = useRef<HTMLButtonElement | null>(null) as Box<HTMLButtonElement | null>;
  const hueRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;
  const speedRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;

  useEffect(() => {
    const btn = btnRef.current;
    const hueEl = hueRef.current;
    const speedEl = speedRef.current;
    if (!btn || !hueEl || !speedEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let hue = HUE_START;
    let speed = 0;
    let pressed = false;
    let demo = !reduce;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    const paint = (): void => {
      btn.style.setProperty("--hue", hue.toFixed(1));
      hueEl.textContent = String(Math.round(hue));
      speedEl.textContent = String(Math.round(speed));
    };
    const press = (): void => {
      demo = false;
      pressed = true;
      if (reduce) { hue = (hue + HUE_STEP) % 360; paint(); }
    };
    const release = (): void => { pressed = false; };
    const onDown = (e: Event): void => {
      try { btn.setPointerCapture((e as PointerLike).pointerId); } catch (err) { /* 캡처를 못 걸어도 누름은 동작한다 */ }
      press();
    };
    const onKeyDown = (e: Event): void => {
      const k = e as KeyLike;
      if ((k.key === " " || k.key === "Enter") && !k.repeat) press();
    };
    const onKeyUp = (e: Event): void => {
      const k = e as KeyLike;
      if (k.key === " " || k.key === "Enter") release();
    };
    btn.addEventListener("pointerdown", onDown);
    btn.addEventListener("pointerup", release);
    btn.addEventListener("pointercancel", release);
    btn.addEventListener("blur", release);
    btn.addEventListener("keydown", onKeyDown);
    btn.addEventListener("keyup", onKeyUp);

    const demoPressed = (now: number): boolean => demo && now >= START_MS && (now - START_MS) % CYCLE_MS < HOLD_MS;

    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;
      if (demoPressed(now) || pressed) {
        speed = Math.min(MAX_SPEED, speed + ACCEL * dt);
      } else {
        speed *= Math.exp(-FRICTION * dt);
        if (speed < 0.5) speed = 0;
      }
      hue = (hue + speed * dt) % 360;
      paint();
      raf = requestAnimationFrame(tick);
    }

    if (reduce) paint();
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("pointerdown", onDown);
      btn.removeEventListener("pointerup", release);
      btn.removeEventListener("pointercancel", release);
      btn.removeEventListener("blur", release);
      btn.removeEventListener("keydown", onKeyDown);
      btn.removeEventListener("keyup", onKeyUp);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.bc}>
        <button className={styles.bc__btn} type="button" ref={btnRef}>꾹 누르기</button>
        <p className={styles.bc__meta}>
          <span className={styles.bc__tag}>HUE</span> <b ref={hueRef}>270</b><span>deg</span> <span className={styles.bc__tag}>SPEED</span> <b ref={speedRef}>0</b>
        </p>
      </div>
    </main>
  );
}