04Pull-distance stretch — the button's shape value stretches by an amount tied to how far the pointer's distance value has moved away, then returns when released

<main class="stage">
<div class="brs is-demo" id="brs">
  <div class="brs__field" id="brsField">
    <span class="brs__anchor" aria-hidden="true"></span>
    <button class="brs__btn" id="brsBtn" type="button">Try pulling</button>
  </div>
  <p class="brs__hint">It stretches by the distance value you pull it away, then returns when released</p>
</div>
</main>

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

// 계산 겹(저항 곡선·복귀 스프링 적분)은 JS 에 있다 — 여기는 정적 배치만.
@include stage($stage-paper);

.stage { position: relative; }

.brs {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-4;
}

.brs__field {
  position: relative;
  width: 220px;
  height: 60px;
  display: flex;
  align-items: center;
}

.brs__anchor {
  position: absolute;
  left: 0;
  top: 50%;
  width: 10px;
  height: 10px;
  margin-top: -5px;
  border-radius: $r-pill;
  background: $subject-ink;
}

.brs__btn {
  position: relative;
  margin-left: $sp-5;
  border: 0;
  border-radius: $r-control;
  padding: $sp-3 $sp-6;
  background: $subject-blue-deep;
  color: $subject-cream;
  font-size: 14px;
  font-weight: 700;
  cursor: grab;
  touch-action: none;
  // 왼쪽 고정점(anchor)에 매인 요소라 중심이 아니라 왼쪽 변에서 늘어난다 (규칙 8 예외 — 채움 막대와 같은 계열)
  transform-origin: 0% 50%;
}

.brs__hint {
  color: rgba($subject-ink, .72);
  font-size: 12px;
  text-align: center;
  max-width: 220px;
}

@media (max-width: 340px) {
  .brs__field { width: 180px; }
  .brs__btn { padding: $sp-2 $sp-5; font-size: 13px; }
}

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

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);
}

.stage {
  position: relative;
}

.brs {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 16px;
}

.brs__field {
  position: relative;
  width: 220px;
  height: 60px;
  display: flex;
  align-items: center;
}

.brs__anchor {
  position: absolute;
  left: 0;
  top: 50%;
  width: 10px;
  height: 10px;
  margin-top: -5px;
  border-radius: 999px;
  background: #17141a;
}

.brs__btn {
  position: relative;
  margin-left: 20px;
  border: 0;
  border-radius: 12px;
  padding: 12px 24px;
  background: #0a4fe8;
  color: #fff7e6;
  font-size: 14px;
  font-weight: 700;
  cursor: grab;
  touch-action: none;
  transform-origin: 0% 50%;
}

.brs__hint {
  color: rgba(23, 20, 26, 0.72);
  font-size: 12px;
  text-align: center;
  max-width: 220px;
}

@media (max-width: 340px) {
  .brs__field {
    width: 180px;
  }
  .brs__btn {
    padding: 8px 20px;
    font-size: 13px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .brs__btn {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('brs');
  var field = document.getElementById('brsField');
  var btn = document.getElementById('brsBtn');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 당긴 거리 값(dist)이 커질수록 늘어난 배율(stretch)도 커지되, 저항 상수
  // PULL_RESISTANCE 로 나눈 뒤 1-exp(-x) 를 씌워 거리가 커질수록 증가폭이 줄어드는
  // (당길수록 뻑뻑해지는) 고무줄 저항 곡선을 만든다. 놓으면 스프링(K·C)이 1로 되돌린다.
  var PULL_RESISTANCE = 130, MAX_STRETCH = 0.9, K = 260, C = 8;
  var dist = 0, stretch = 1, vel = 0, target = 1, dragging = false, startX = 0, raf = null, last = 0;

  function stretchFromDist(d) {
    var n = Math.max(0, d) / PULL_RESISTANCE;
    return 1 + MAX_STRETCH * (1 - Math.exp(-n));
  }
  function paint() { btn.style.transform = 'scaleX(' + stretch.toFixed(3) + ')'; }
  function springTick(now) {
    var dt = Math.min((now - last) / 1000, 0.032);
    last = now;
    var accel = -K * (stretch - target) - C * vel;
    vel += accel * dt;
    stretch += vel * dt;
    paint();
    if (Math.abs(stretch - target) < 0.002 && Math.abs(vel) < 0.01) { stretch = target; vel = 0; paint(); raf = null; return; }
    raf = requestAnimationFrame(springTick);
  }
  function startSpring() { if (raf === null) { last = performance.now(); raf = requestAnimationFrame(springTick); } }

  function onDown(clientX) {
    dragging = true; startX = clientX;
    if (raf !== null) { cancelAnimationFrame(raf); raf = null; }
  }
  function onMove(clientX) {
    if (!dragging) { return; }
    dist = clientX - startX;
    stretch = stretchFromDist(dist);
    vel = 0;
    paint();
  }
  function onUp() {
    if (!dragging) { return; }
    dragging = false;
    target = 1;
    if (reduce) { stretch = 1; paint(); return; }
    startSpring();
  }

  function wireReal() {
    btn.addEventListener('pointerdown', function (e) { root.classList.remove('is-demo'); stopAutoplay(); onDown(e.clientX); });
    window.addEventListener('pointermove', function (e) { onMove(e.clientX); });
    window.addEventListener('pointerup', onUp);
  }
  wireReal();
  paint();

  // 자동 루프: 실제 드래그 대신 dist 값을 단계적으로 키웠다 놓아 같은 저항·스프링 경로를 탄다.
  var autoTimer = null, autoDist = 0, growing = true;
  function autoTick() {
    if (growing) {
      autoDist += 26;
      stretch = stretchFromDist(autoDist);
      paint();
      if (autoDist >= 170) { growing = false; target = 1; startSpring(); }
    } else {
      growing = true; autoDist = 0;
    }
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 200); }
})();

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

export type ButtonPhysicsRubberStretchPullProps = {
  color?: string;
};

export default function ButtonPhysicsRubberStretchPull({
  color = "#0a4fe8",
}: ButtonPhysicsRubberStretchPullProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    const btn = root.querySelector<HTMLButtonElement>(`.${styles.btn}`);
    if (!btn) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    // 계산 겹: 당긴 거리 값(dist)이 커질수록 늘어난 배율(stretch)도 커지되, 저항 상수
    // PULL_RESISTANCE 로 나눈 뒤 1-exp(-x) 를 씌워 거리가 커질수록 증가폭이 줄어드는
    // (당길수록 뻑뻑해지는) 고무줄 저항 곡선을 만든다. 놓으면 스프링(K·C)이 1로 되돌린다.
    const PULL_RESISTANCE = 130, MAX_STRETCH = 0.9, K = 260, C = 8;
    let dist = 0, stretch = 1, vel = 0, target = 1, dragging = false, startX = 0, raf: number | null = null, last = 0;

    function stretchFromDist(d: number) {
      const n = Math.max(0, d) / PULL_RESISTANCE;
      return 1 + MAX_STRETCH * (1 - Math.exp(-n));
    }
    function paint() { btn!.style.transform = "scaleX(" + stretch.toFixed(3) + ")"; }
    function springTick(now: number) {
      const dt = Math.min((now - last) / 1000, 0.032);
      last = now;
      const accel = -K * (stretch - target) - C * vel;
      vel += accel * dt;
      stretch += vel * dt;
      paint();
      if (Math.abs(stretch - target) < 0.002 && Math.abs(vel) < 0.01) { stretch = target; vel = 0; paint(); raf = null; return; }
      raf = requestAnimationFrame(springTick);
    }
    function startSpring() { if (raf === null) { last = performance.now(); raf = requestAnimationFrame(springTick); } }

    function onPointerDown(e: PointerEvent) {
      stopAutoplay();
      dragging = true; startX = e.clientX;
      if (raf !== null) { cancelAnimationFrame(raf); raf = null; }
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      dist = e.clientX - startX;
      stretch = stretchFromDist(dist);
      vel = 0;
      paint();
    }
    function onPointerUp() {
      if (!dragging) return;
      dragging = false;
      target = 1;
      if (reduce) { stretch = 1; paint(); return; }
      startSpring();
    }
    btn.addEventListener("pointerdown", onPointerDown);
    window.addEventListener("pointermove", onPointerMove);
    window.addEventListener("pointerup", onPointerUp);
    paint();

    let autoTimer: number | null = null, autoDist = 0, growing = true;
    function autoTick() {
      if (growing) {
        autoDist += 26;
        stretch = stretchFromDist(autoDist);
        paint();
        if (autoDist >= 170) { growing = false; target = 1; startSpring(); }
      } else {
        growing = true; autoDist = 0;
      }
    }
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    if (!reduce) { autoTimer = window.setInterval(autoTick, 200); }

    return () => {
      stopAutoplay();
      if (raf !== null) cancelAnimationFrame(raf);
      btn.removeEventListener("pointerdown", onPointerDown);
      window.removeEventListener("pointermove", onPointerMove);
      window.removeEventListener("pointerup", onPointerUp);
    };
  }, []);

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.brs}>
        <div className={styles.field}>
          <span className={styles.anchor} aria-hidden="true" />
          <button className={styles.btn} type="button" style={{ background: color }}>당겨보기</button>
        </div>
        <p className={styles.hint}>멀어진 거리 값만큼 늘어났다가 놓으면 되돌아갑니다</p>
      </div>
    </div>
  );
}