09Elastic snap-back value toggle — when a collapsed value turns on, a shape value shrinks, stretches slightly the other way, then returns to form

<main class="stage">
<div class="eb is-demo" id="eb">
  <button type="button" class="eb__btn" id="ebBtn" aria-pressed="false">
    <span class="eb__label">Collapse filter value</span>
  </button>
  <div class="eb__chipRow" id="ebRow" aria-hidden="true">
    <span class="eb__chip">Filter value 1</span>
    <span class="eb__chip">Filter value 2</span>
    <span class="eb__chip">Filter value 3</span>
  </div>
</div>
</main>

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

// 계산 겹: 형태 값(scaleX)이 CSS transition 이 아니라 index.html 의 스프링 감쇠 계산(K·D)으로
// 매 프레임 결정된다 — 줄어들었다가 반대로 살짝 늘었다가 제 형태로 되돌아온다.
@include stage($stage-paper);

.eb {
  display: grid;
  gap: $sp-4;
  justify-items: center;
}

.eb__btn {
  display: inline-grid;
  place-items: center;
  width: 220px;
  height: 56px;
  border: 0;
  border-radius: $r-pill;
  background: $subject-blue-deep;
  color: $subject-cream;
  font-weight: 700;
  font-size: 15px;
  cursor: pointer;
  box-shadow: $shadow-raised;
  will-change: transform;
  transform-origin: center;
}

.eb__label {
  pointer-events: none;
}

.eb__chipRow {
  display: flex;
  gap: $sp-2;
  opacity: calc(1 - var(--press, 0));
  transform: translateY(calc(var(--press, 0) * 8px));
  transition: opacity $dur-quick $ease-out;
}

.eb__chip {
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba($subject-blue-deep, .12);
  color: $subject-ink;
  font-weight: 600;
  font-size: 12px;
}

@media (max-width: 340px) {
  .eb__btn { width: 190px; height: 50px; }
  .eb__chipRow { flex-wrap: wrap; justify-content: center; }
}

@media (prefers-reduced-motion: reduce) {
  .eb__chipRow { 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);
}

.eb {
  display: grid;
  gap: 16px;
  justify-items: center;
}

.eb__btn {
  display: inline-grid;
  place-items: center;
  width: 220px;
  height: 56px;
  border: 0;
  border-radius: 999px;
  background: #0a4fe8;
  color: #fff7e6;
  font-weight: 700;
  font-size: 15px;
  cursor: pointer;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  will-change: transform;
  transform-origin: center;
}

.eb__label {
  pointer-events: none;
}

.eb__chipRow {
  display: flex;
  gap: 8px;
  opacity: calc(1 - var(--press, 0));
  transform: translateY(calc(var(--press, 0) * 8px));
  transition: opacity 220ms ease-out;
}

.eb__chip {
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(10, 79, 232, 0.12);
  color: #17141a;
  font-weight: 600;
  font-size: 12px;
}

@media (max-width: 340px) {
  .eb__btn {
    width: 190px;
    height: 50px;
  }
  .eb__chipRow {
    flex-wrap: wrap;
    justify-content: center;
  }
}
@media (prefers-reduced-motion: reduce) {
  .eb__chipRow {
    transition: none;
  }
}

(function () {
  var root = document.getElementById('eb');
  var btn = document.getElementById('ebBtn');
  var K = 190, D = 14; // 계산 겹: 스프링 상수·감쇠 — 눌렀다 놓는 순간의 되튐 값을 정한다

  var collapsed = false;
  var cur = 1, vel = 0, target = 1;
  var raf = null;

  // 스프링 감쇠 계산: 매 프레임 a = -K*(cur-target) - D*vel — 줄어들었다가 반대로 살짝 늘었다가 제 형태
  function tick() {
    var a = -K * (cur - target) * 0.001 - D * vel * 0.01;
    vel += a;
    cur += vel;
    if (Math.abs(cur - target) < 0.004 && Math.abs(vel) < 0.005) { cur = target; vel = 0; }
    btn.style.transform = 'scaleX(' + cur.toFixed(3) + ')';
    var p = Math.max(0, Math.min(1, (1 - cur) / 0.28));
    root.style.setProperty('--press', p.toFixed(3));
    raf = requestAnimationFrame(tick);
  }

  function toggle() {
    collapsed = !collapsed;
    target = collapsed ? 0.72 : 1;
    btn.setAttribute('aria-pressed', String(collapsed));
    root.classList.toggle('eb--collapsed', collapsed);
  }

  btn.addEventListener('pointerdown', function () {
    root.classList.remove('is-demo');
    stopAutoplay();
    target = Math.min(target, 0.86); // 누르는 순간 살짝 줄어든다
  });
  btn.addEventListener('pointerup', function () {
    root.classList.remove('is-demo');
    stopAutoplay();
    toggle();
  });
  btn.addEventListener('click', function () {
    root.classList.remove('is-demo');
    stopAutoplay();
    toggle();
  });

  raf = requestAnimationFrame(tick);

  var autoTimer = null;
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (!reduce) { autoTimer = setInterval(toggle, 1200); }
  else { collapsed = true; target = 0.72; cur = 0.72; btn.style.transform = 'scaleX(0.72)'; root.classList.add('eb--collapsed'); btn.setAttribute('aria-pressed', 'true'); }
})();

import { useEffect, useRef } from "react";
import styles from "./ElasticSnapBack.module.scss";

type Box<T> = { current: T };

const K = 190, D = 14; // 계산 겹: 스프링 상수·감쇠 — 눌렀다 놓는 순간의 되튐 값을 정한다

export default function ElasticSnapBack() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement | null>;

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    const btn = root.querySelector(`.${styles.btn}`) as HTMLButtonElement | null;
    if (!btn) return;

    let collapsed = false;
    let cur = 1, vel = 0, target = 1;
    let raf: number | null = null;

    function tick() {
      const a = -K * (cur - target) * 0.001 - D * vel * 0.01;
      vel += a;
      cur += vel;
      if (Math.abs(cur - target) < 0.004 && Math.abs(vel) < 0.005) { cur = target; vel = 0; }
      btn!.style.transform = "scaleX(" + cur.toFixed(3) + ")";
      const p = Math.max(0, Math.min(1, (1 - cur) / 0.28));
      root!.style.setProperty("--press", p.toFixed(3));
      raf = requestAnimationFrame(tick);
    }

    function toggle() {
      collapsed = !collapsed;
      target = collapsed ? 0.72 : 1;
      btn!.setAttribute("aria-pressed", String(collapsed));
      root!.classList.toggle(styles["root--collapsed"], collapsed);
    }

    function onDown() {
      root!.classList.remove(styles["is-demo"]);
      stopAutoplay();
      target = Math.min(target, 0.86); // 누르는 순간 살짝 줄어든다
    }
    function onUp() {
      root!.classList.remove(styles["is-demo"]);
      stopAutoplay();
      toggle();
    }
    function onClick() {
      root!.classList.remove(styles["is-demo"]);
      stopAutoplay();
      toggle();
    }
    btn.addEventListener("pointerdown", onDown);
    btn.addEventListener("pointerup", onUp);
    btn.addEventListener("click", onClick);

    raf = requestAnimationFrame(tick);

    let autoTimer: number | null = null;
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (!reduce) { autoTimer = window.setInterval(toggle, 1200); }
    else {
      collapsed = true; target = 0.72; cur = 0.72;
      btn!.style.transform = "scaleX(0.72)";
      root!.classList.add(styles["root--collapsed"]);
      btn!.setAttribute("aria-pressed", "true");
    }

    return () => {
      stopAutoplay();
      if (raf !== null) cancelAnimationFrame(raf);
      btn.removeEventListener("pointerdown", onDown);
      btn.removeEventListener("pointerup", onUp);
      btn.removeEventListener("click", onClick);
    };
  }, []);

  return (
    <div ref={rootRef} className={`${styles.stage} ${styles.eb} ${styles["is-demo"]}`}>
      <button type="button" className={styles.btn} aria-pressed="false">
        <span className={styles.label}>필터 값 접기</span>
      </button>
      <div className={styles.chipRow} aria-hidden="true">
        <span className={styles.chip}>값 걸러내기 1</span>
        <span className={styles.chip}>값 걸러내기 2</span>
        <span className={styles.chip}>값 걸러내기 3</span>
      </div>
    </div>
  );
}