05진행률을 따라 오르는 액체 수위

<main class="stage">
<div class="lq is-demo" id="lqRoot">
  <button class="lq__btn" id="lqBtn" type="button">
    <svg class="lq__wave" viewBox="0 0 240 72" preserveAspectRatio="none" aria-hidden="true"><path id="lqPath" d="M0 72H240V72Z"/></svg>
    <span class="lq__text"><span id="lqLabel">내려받기</span> <b id="lqPct">0%</b></span>
  </button>
  <p class="lq__meta"><span class="lq__tag">LEVEL</span> <b id="lqLevel">0.00</b> <span class="lq__tag">AMP</span> <b id="lqAmp">0.0</b></p>
</div>
</main>

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

// 핵심: 수면은 SVG path 의 d 를 JS 가 매 프레임 다시 계산한다. 속성 트윈이 아니라 파동 합성이다.

$lq-face: $subject-cream;
$lq-water: $subject-sky;

@include stage($stage-ink);

.lq {
  width: min(100%, 280px);
  display: grid;
  gap: $sp-3;
  justify-items: center;
}

.lq__btn {
  position: relative;
  box-sizing: border-box;
  width: 240px;
  max-width: 100%;
  height: 72px;
  overflow: hidden;
  border: 0;
  border-radius: $r-card;
  background: $lq-face;
  color: $subject-ink;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.lq__btn:focus-visible { outline: 3px solid $stage-paper; outline-offset: 3px; }
.lq__wave {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  fill: $lq-water;
}
.lq__text { position: relative; }
.lq__text b { font-variant-numeric: tabular-nums; }

.lq__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: $sp-2;
  font-size: 12px;
  line-height: 16px;
  color: $stage-paper;
}
.lq__tag { font-weight: 700; letter-spacing: .08em; }
.lq__meta b { min-width: 4ch; text-align: right; font-variant-numeric: tabular-nums; }

@media (prefers-reduced-motion: reduce) {
  .lq__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: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.lq {
  width: min(100%, 280px);
  display: grid;
  gap: 12px;
  justify-items: center;
}

.lq__btn {
  position: relative;
  box-sizing: border-box;
  width: 240px;
  max-width: 100%;
  height: 72px;
  overflow: hidden;
  border: 0;
  border-radius: 16px;
  background: #fff7e6;
  color: #17141a;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

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

.lq__wave {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  fill: #6ec8ff;
}

.lq__text {
  position: relative;
}

.lq__text b {
  font-variant-numeric: tabular-nums;
}

.lq__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 8px;
  font-size: 12px;
  line-height: 16px;
  color: #ffffff;
}

.lq__tag {
  font-weight: 700;
  letter-spacing: 0.08em;
}

.lq__meta b {
  min-width: 4ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
}

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

(function () {
  // 기법: 수위는 목표 진행률을 향해 지수 보간(LEVEL_LERP)으로 따라가고, 수면은 사인 두 개를 합성한 곡선이다.
  // 파동 진폭에 (1 - 0.7 x 수위) 를 곱해서 물이 찰수록 수면이 가라앉는다.
  var WAVE_AMP = 6;
  var WAVE_FREQ = 2;
  var LEVEL_LERP = 6;
  var RUN_MS = 1500;
  var START_MS = 200;
  var CYCLE_MS = 2000;
  var W = 240;
  var H = 72;
  var POINTS = 48;

  var root = document.getElementById('lqRoot');
  var btn = document.getElementById('lqBtn');
  var pathEl = document.getElementById('lqPath');
  var labelEl = document.getElementById('lqLabel');
  var pctEl = document.getElementById('lqPct');
  var levelEl = document.getElementById('lqLevel');
  var ampEl = document.getElementById('lqAmp');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var clock = 0;
  var runStart = null;
  var level = 0;
  var target = 0;
  var t0 = null;
  var lastTs = null;

  function stopDemo() { root.classList.remove('is-demo'); }

  function progressAt(now) {
    var s = root.classList.contains('is-demo')
      ? (now < START_MS ? null : START_MS + Math.floor((now - START_MS) / CYCLE_MS) * CYCLE_MS)
      : runStart;
    return s === null ? 0 : Math.max(0, Math.min(1, (now - s) / RUN_MS));
  }

  function draw(phase) {
    var reach = WAVE_AMP * 1.5;
    var base = -reach + (H + 2 * reach) * (1 - level);
    var amp = WAVE_AMP * (1 - 0.7 * level);
    var d = 'M0 ' + H;
    for (var i = 0; i <= POINTS; i += 1) {
      var x = (i / POINTS) * W;
      var a = 2 * Math.PI * WAVE_FREQ * (x / W);
      var y = base + amp * (Math.sin(a + phase) + 0.5 * Math.sin(2 * a - 1.3 * phase));
      d += 'L' + x.toFixed(1) + ' ' + y.toFixed(2);
    }
    d += 'L' + W + ' ' + H + 'Z';
    pathEl.setAttribute('d', d);
    var pct = Math.round(level * 100);
    pctEl.textContent = pct + '%';
    labelEl.textContent = level > 0.995 ? '완료' : (level > 0.01 ? '받는 중' : '내려받기');
    levelEl.textContent = level.toFixed(2);
    ampEl.textContent = amp.toFixed(1);
  }

  function activate() {
    stopDemo();
    if (reduce) { level = 1; draw(0); return; }
    if (runStart !== null && clock - runStart < RUN_MS) return;
    runStart = clock;
  }
  btn.addEventListener('click', activate);

  function tick(ts) {
    if (t0 === null) t0 = ts;
    clock = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    target = progressAt(clock);
    level += (target - level) * (1 - Math.exp(-dt * LEVEL_LERP));
    draw(clock * 0.006);
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    draw(0);
  } else {
    requestAnimationFrame(tick);
  }
})();

// LiquidLevelButton — vanilla 판(05-dlbtn-liquid-level-wave) 의 React 포트.
// 수위는 목표 진행률을 지수 보간(LEVEL_LERP)으로 따라가고, 수면은 사인 두 개의 합성이며 물이 찰수록 진폭이 줄어든다.
import { useEffect, useRef } from "react";
import styles from "./LiquidLevelButton.module.scss";

const WAVE_AMP = 6;
const WAVE_FREQ = 2;
const LEVEL_LERP = 6;
const RUN_MS = 1500;
const START_MS = 200;
const CYCLE_MS = 2000;
const W = 240;
const H = 72;
const POINTS = 48;

export default function LiquidLevelButton() {
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const pathRef = useRef<SVGPathElement | null>(null);
  const labelRef = useRef<HTMLSpanElement | null>(null);
  const pctRef = useRef<HTMLElement | null>(null);
  const levelRef = useRef<HTMLElement | null>(null);
  const ampRef = useRef<HTMLElement | null>(null);

  useEffect(() => {
    const btn = btnRef.current;
    const pathEl = pathRef.current;
    const labelEl = labelRef.current;
    const pctEl = pctRef.current;
    const levelEl = levelRef.current;
    const ampEl = ampRef.current;
    if (!btn || !pathEl || !labelEl || !pctEl || !levelEl || !ampEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let clock = 0;
    let runStart: number | null = null;
    let level = 0;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    const progressAt = (now: number): number => {
      const s = demo
        ? now < START_MS ? null : START_MS + Math.floor((now - START_MS) / CYCLE_MS) * CYCLE_MS
        : runStart;
      return s === null ? 0 : Math.max(0, Math.min(1, (now - s) / RUN_MS));
    };
    const draw = (phase: number): void => {
      const reach = WAVE_AMP * 1.5;
      const base = -reach + (H + 2 * reach) * (1 - level);
      const amp = WAVE_AMP * (1 - 0.7 * level);
      let d = "M0 " + H;
      for (let i = 0; i <= POINTS; i += 1) {
        const x = (i / POINTS) * W;
        const a = 2 * Math.PI * WAVE_FREQ * (x / W);
        const y = base + amp * (Math.sin(a + phase) + 0.5 * Math.sin(2 * a - 1.3 * phase));
        d += "L" + x.toFixed(1) + " " + y.toFixed(2);
      }
      d += "L" + W + " " + H + "Z";
      pathEl.setAttribute("d", d);
      pctEl.textContent = Math.round(level * 100) + "%";
      labelEl.textContent = level > 0.995 ? "완료" : level > 0.01 ? "받는 중" : "내려받기";
      levelEl.textContent = level.toFixed(2);
      ampEl.textContent = amp.toFixed(1);
    };
    const activate = (): void => {
      demo = false;
      if (reduce) { level = 1; draw(0); return; }
      if (runStart !== null && clock - runStart < RUN_MS) return;
      runStart = clock;
    };
    btn.addEventListener("click", activate);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      clock = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      level += (progressAt(clock) - level) * (1 - Math.exp(-dt * LEVEL_LERP));
      draw(clock * 0.006);
      raf = requestAnimationFrame(tick);
    }

    if (reduce) draw(0);
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("click", activate);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.lq}>
        <button className={styles.lq__btn} type="button" ref={btnRef}>
          <svg className={styles.lq__wave} viewBox="0 0 240 72" preserveAspectRatio="none" aria-hidden="true">
            <path d="M0 72H240V72Z" ref={pathRef} />
          </svg>
          <span className={styles.lq__text}>
            <span ref={labelRef}>내려받기</span> <b ref={pctRef}>0%</b>
          </span>
        </button>
        <p className={styles.lq__meta}>
          <span className={styles.lq__tag}>LEVEL</span> <b ref={levelRef}>0.00</b> <span className={styles.lq__tag}>AMP</span> <b ref={ampRef}>0.0</b>
        </p>
      </div>
    </main>
  );
}