05Hanging-angle swing — a value hung from a top point swings its angle value side to side while the swing shrinks over time

<main class="stage">
<div class="bph is-demo" id="bph">
  <div class="bph__field" id="bphField" role="application" aria-label="Hanging button swing test area" tabindex="0">
    <span class="bph__pivot" aria-hidden="true"></span>
    <div class="bph__arm" id="bphArm">
      <span class="bph__rod" aria-hidden="true"></span>
      <button class="bph__btn" id="bphBtn" type="button" aria-label="Notification button">3</button>
    </div>
  </div>
  <p class="bph__hint">Pull the button sideways and release — it swings on its own</p>
</div>
</main>

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

// 계산 겹(진자 각가속도 적분)은 JS 에 있다 — 여기는 정적 배치만.
@include stage($stage-ink);

.stage { position: relative; }

.bph {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-3;
}

.bph__field {
  position: relative;
  width: 220px;
  height: 168px;
}

.bph__pivot {
  position: absolute;
  left: 50%;
  top: 0;
  width: 10px;
  height: 10px;
  margin-left: -5px;
  border-radius: $r-pill;
  background: rgba($stage-paper, .5);
}

.bph__arm {
  position: absolute;
  left: 50%;
  top: 0;
  width: 2px;
  height: 110px;
  transform-origin: 50% 0%;
  // 위쪽 한 점(pivot)에 매달려 회전하는 팔이라 원점이 위 끝에 있다 (규칙 8 예외)
}

.bph__rod {
  position: absolute;
  left: 0;
  top: 0;
  width: 2px;
  height: 100%;
  background: rgba($stage-paper, .35);
}

.bph__btn {
  position: absolute;
  left: 50%;
  bottom: -22px;
  margin-left: -22px;
  width: 44px;
  height: 44px;
  border: 0;
  border-radius: $r-pill;
  background: $subject-pink;
  color: $subject-ink;
  font-size: 15px;
  font-weight: 700;
  cursor: grab;
  touch-action: none;
  box-shadow: $shadow-raised;
}

.bph__hint {
  color: rgba($stage-paper, .72);
  font-size: 12px;
  text-align: center;
  max-width: 220px;
}

@media (max-width: 340px) {
  .bph__field { width: 160px; height: 96px; }
  .bph__arm { height: 56px; }
  .bph__btn { width: 32px; height: 32px; bottom: -16px; margin-left: -16px; font-size: 12px; }
}

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

.stage {
  position: relative;
}

.bph {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
}

.bph__field {
  position: relative;
  width: 220px;
  height: 168px;
}

.bph__pivot {
  position: absolute;
  left: 50%;
  top: 0;
  width: 10px;
  height: 10px;
  margin-left: -5px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.5);
}

.bph__arm {
  position: absolute;
  left: 50%;
  top: 0;
  width: 2px;
  height: 110px;
  transform-origin: 50% 0%;
}

.bph__rod {
  position: absolute;
  left: 0;
  top: 0;
  width: 2px;
  height: 100%;
  background: rgba(255, 255, 255, 0.35);
}

.bph__btn {
  position: absolute;
  left: 50%;
  bottom: -22px;
  margin-left: -22px;
  width: 44px;
  height: 44px;
  border: 0;
  border-radius: 999px;
  background: #ff9ebb;
  color: #17141a;
  font-size: 15px;
  font-weight: 700;
  cursor: grab;
  touch-action: none;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.bph__hint {
  color: rgba(255, 255, 255, 0.72);
  font-size: 12px;
  text-align: center;
  max-width: 220px;
}

@media (max-width: 340px) {
  .bph__field {
    width: 160px;
    height: 96px;
  }
  .bph__arm {
    height: 56px;
  }
  .bph__btn {
    width: 32px;
    height: 32px;
    bottom: -16px;
    margin-left: -16px;
    font-size: 12px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bph__arm {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('bph');
  var field = document.getElementById('bphField');
  var arm = document.getElementById('bphArm');
  var btn = document.getElementById('bphBtn');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 각가속도 = -(GRAVITY/ARM_LEN)*sin(angle) - DAMPING*avel. 각도(angle)는 도(degree) 단위라
  // sin() 에 넣기 전 라디안으로 바꾸고 결과를 다시 도로 되돌린다(RAD 변환 상수) — 단위를 안 맞추면
  // 복원력이 57배 작아져 흔들림이 멈춘 것처럼 보인다.
  var GRAVITY = 2000, ARM_LEN = 110, DAMPING = 0.42, RAD = 180 / Math.PI;
  var angle = 0, avel = 0, dragging = false, last = 0, running = false;

  function paint() { arm.style.transform = 'rotate(' + angle.toFixed(2) + 'deg)'; }
  function tick(now) {
    var dt = Math.min((now - last) / 1000, 0.032);
    last = now;
    avel += (-(GRAVITY / ARM_LEN) * Math.sin(angle / RAD) * RAD - DAMPING * avel) * dt;
    angle += avel * dt;
    paint();
    if (Math.abs(angle) < 0.2 && Math.abs(avel) < 0.6) { angle = 0; avel = 0; paint(); running = false; return; }
    raf = requestAnimationFrame(tick);
  }
  var raf = null;
  function start() { if (!running) { running = true; last = performance.now(); raf = requestAnimationFrame(tick); } }

  function angleFromClientX(clientX) {
    var r = field.getBoundingClientRect();
    var px = clientX - (r.left + r.width / 2);
    var deg = Math.atan2(px, ARM_LEN) * RAD;
    return Math.max(-70, Math.min(70, deg));
  }
  function onDown(clientX) {
    dragging = true;
    if (raf !== null) { cancelAnimationFrame(raf); raf = null; }
    running = false;
    angle = angleFromClientX(clientX);
    avel = 0;
    paint();
  }
  function onMove(clientX) {
    if (!dragging) { return; }
    angle = angleFromClientX(clientX);
    avel = 0;
    paint();
  }
  function onUp() {
    if (!dragging) { return; }
    dragging = false;
    if (reduce) { angle = 0; avel = 0; paint(); return; }
    start();
  }

  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 각도를 한 번 밀어 두고 같은 감쇠 진동 경로를 반복 재현한다.
  var autoTimer = null;
  function autoTick() { angle = 30; avel = 0; running = false; start(); }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTick(); autoTimer = setInterval(autoTick, 1700); }
})();

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

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

export default function ButtonPhysicsPendulumHangSwing({
  color = "#ff9ebb",
}: ButtonPhysicsPendulumHangSwingProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

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

    // 계산 겹: 각가속도 = -(GRAVITY/ARM_LEN)*sin(angle) - DAMPING*avel. 각도(angle)는 도(degree) 단위라
    // sin() 에 넣기 전 라디안으로 바꾸고 결과를 다시 도로 되돌린다(RAD 변환 상수) — 단위를 안 맞추면
    // 복원력이 57배 작아져 흔들림이 멈춘 것처럼 보인다.
    const GRAVITY = 2000, ARM_LEN = 110, DAMPING = 0.42, RAD = 180 / Math.PI;
    let angle = 0, avel = 0, dragging = false, last = 0, running = false, raf: number | null = null;

    function paint() { arm!.style.transform = "rotate(" + angle.toFixed(2) + "deg)"; }
    function tick(now: number) {
      const dt = Math.min((now - last) / 1000, 0.032);
      last = now;
      avel += (-(GRAVITY / ARM_LEN) * Math.sin(angle / RAD) * RAD - DAMPING * avel) * dt;
      angle += avel * dt;
      paint();
      if (Math.abs(angle) < 0.2 && Math.abs(avel) < 0.6) { angle = 0; avel = 0; paint(); running = false; return; }
      raf = requestAnimationFrame(tick);
    }
    function start() { if (!running) { running = true; last = performance.now(); raf = requestAnimationFrame(tick); } }

    function angleFromClientX(clientX: number) {
      const r = field!.getBoundingClientRect();
      const px = clientX - (r.left + r.width / 2);
      const deg = Math.atan2(px, ARM_LEN) * RAD;
      return Math.max(-70, Math.min(70, deg));
    }
    function onPointerDown(e: PointerEvent) {
      stopAutoplay();
      dragging = true;
      if (raf !== null) { cancelAnimationFrame(raf); raf = null; }
      running = false;
      angle = angleFromClientX(e.clientX);
      avel = 0;
      paint();
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      angle = angleFromClientX(e.clientX);
      avel = 0;
      paint();
    }
    function onPointerUp() {
      if (!dragging) return;
      dragging = false;
      if (reduce) { angle = 0; avel = 0; paint(); return; }
      start();
    }
    btn.addEventListener("pointerdown", onPointerDown);
    window.addEventListener("pointermove", onPointerMove);
    window.addEventListener("pointerup", onPointerUp);
    paint();

    let autoTimer: number | null = null;
    function autoTick() { angle = 30; avel = 0; running = false; start(); }
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    if (!reduce) { autoTick(); autoTimer = window.setInterval(autoTick, 1700); }

    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.bph}>
        <div className={styles.field} role="application" aria-label="매달린 버튼 흔들기 실험판" tabIndex={0}>
          <span className={styles.pivot} aria-hidden="true" />
          <div className={styles.arm}>
            <span className={styles.rod} aria-hidden="true" />
            <button className={styles.btn} type="button" aria-label="알림 버튼" style={{ background: color }}>3</button>
          </div>
        </div>
        <p className={styles.hint}>버튼을 옆으로 당겼다 놓으면 스스로 흔들립니다</p>
      </div>
    </div>
  );
}