09Snap-to-anchor bounce — the released position value is pulled toward the nearest of several preset anchor values and settles with a bounce

<main class="stage">
<div class="bma is-demo" id="bma">
  <div class="bma__field" id="bmaField">
    <span class="bma__anchor" style="left:30px"></span>
    <span class="bma__anchor" style="left:110px"></span>
    <span class="bma__anchor" style="left:190px"></span>
    <button class="bma__btn" id="bmaBtn" type="button" aria-label="Position-snapping toggle button">●</button>
  </div>
  <p class="bma__hint">Release it and it snaps to the nearest anchor value with a bounce</p>
</div>
</main>

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

// 계산 겹(가까운 고정 지점 값 탐색 + 스프링 튕김)은 JS 에 있다 — 여기는 정적 배치만.
@include stage($stage-paper);

.stage { position: relative; }

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

.bma__field {
  position: relative;
  width: 240px;
  height: 64px;
  border-radius: $r-card;
  background: rgba($subject-ink, .06);
  border: 1px solid rgba($subject-ink, .12);
}

.bma__anchor {
  position: absolute;
  top: 50%;
  width: 6px;
  height: 6px;
  margin-left: -3px;
  margin-top: -3px;
  border-radius: $r-pill;
  background: $subject-mint;
}

.bma__btn {
  position: absolute;
  top: 50%;
  left: 0;
  width: 40px;
  height: 40px;
  margin-top: -20px;
  margin-left: -20px;
  border: 0;
  border-radius: $r-pill;
  background: $subject-blue-deep;
  color: $subject-cream;
  font-size: 14px;
  cursor: grab;
  touch-action: none;
  box-shadow: $shadow-raised;
}

.bma__hint {
  color: rgba($subject-ink, .72);
  font-size: 12px;
  text-align: center;
  max-width: 240px;
}

@media (max-width: 340px) {
  .bma__field { width: 190px; }
  .bma__btn { width: 34px; height: 34px; margin-top: -17px; margin-left: -17px; font-size: 12px; }
}

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

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

.bma__field {
  position: relative;
  width: 240px;
  height: 64px;
  border-radius: 16px;
  background: rgba(23, 20, 26, 0.06);
  border: 1px solid rgba(23, 20, 26, 0.12);
}

.bma__anchor {
  position: absolute;
  top: 50%;
  width: 6px;
  height: 6px;
  margin-left: -3px;
  margin-top: -3px;
  border-radius: 999px;
  background: #4cd4a6;
}

.bma__btn {
  position: absolute;
  top: 50%;
  left: 0;
  width: 40px;
  height: 40px;
  margin-top: -20px;
  margin-left: -20px;
  border: 0;
  border-radius: 999px;
  background: #0a4fe8;
  color: #fff7e6;
  font-size: 14px;
  cursor: grab;
  touch-action: none;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.bma__hint {
  color: rgba(23, 20, 26, 0.72);
  font-size: 12px;
  text-align: center;
  max-width: 240px;
}

@media (max-width: 340px) {
  .bma__field {
    width: 190px;
  }
  .bma__btn {
    width: 34px;
    height: 34px;
    margin-top: -17px;
    margin-left: -17px;
    font-size: 12px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bma__btn {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('bma');
  var field = document.getElementById('bmaField');
  var btn = document.getElementById('bmaBtn');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 놓인 위치 값(x)에서 가장 가까운 고정 지점 값(ANCHORS 중 하나)을 골라 target 으로
  // 삼고, 스프링(K·C)이 목표를 살짝 넘겼다가 되돌아오는 튕김으로 그 지점에 맞춘다.
  var ANCHORS = [30, 110, 190], K = 340, C = 10, BTN_R = 20;
  var x = ANCHORS[0], vel = 0, target = ANCHORS[0], dragging = false, startX = 0, startBtnX = 0, raf = null, last = 0;

  function nearestAnchor(px) {
    var best = ANCHORS[0], bestD = Infinity;
    for (var i = 0; i < ANCHORS.length; i++) {
      var d = Math.abs(ANCHORS[i] - px);
      if (d < bestD) { bestD = d; best = ANCHORS[i]; }
    }
    return best;
  }
  function paint() { btn.style.transform = 'translateX(' + x.toFixed(1) + 'px)'; }
  function tick(now) {
    var dt = Math.min((now - last) / 1000, 0.032);
    last = now;
    var accel = -K * (x - target) - C * vel;
    vel += accel * dt;
    x += vel * dt;
    paint();
    if (Math.abs(x - target) < 0.05 && Math.abs(vel) < 0.4) { x = target; vel = 0; paint(); raf = null; return; }
    raf = requestAnimationFrame(tick);
  }
  function startSpring() { if (raf === null) { last = performance.now(); raf = requestAnimationFrame(tick); } }

  function onDown(clientX) {
    dragging = true; startX = clientX; startBtnX = x;
    if (raf !== null) { cancelAnimationFrame(raf); raf = null; }
  }
  function onMove(clientX) {
    if (!dragging) { return; }
    var dx = clientX - startX;
    x = Math.max(BTN_R, Math.min(field.clientWidth - BTN_R, startBtnX + dx));
    paint();
  }
  function onUp() {
    if (!dragging) { return; }
    dragging = false;
    target = nearestAnchor(x);
    if (reduce) { x = target; vel = 0; 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 놓인 위치 값을 다음 고정 지점으로 옮겨 같은 스냅 경로를 반복한다.
  var autoTimer = null, idx = 0;
  function autoTick() {
    idx = (idx + 1) % ANCHORS.length;
    x = ANCHORS[idx] + (idx % 2 === 0 ? -18 : 18);
    vel = 0;
    target = nearestAnchor(x);
    startSpring();
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 900); }
})();

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

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

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

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

    // 계산 겹: 놓인 위치 값(x)에서 가장 가까운 고정 지점 값(ANCHORS 중 하나)을 골라 target 으로
    // 삼고, 스프링(K·C)이 목표를 살짝 넘겼다가 되돌아오는 튕김으로 그 지점에 맞춘다.
    const ANCHORS = [30, 110, 190], K = 340, C = 10, BTN_R = 20;
    let x = ANCHORS[0], vel = 0, target = ANCHORS[0], dragging = false, startX = 0, startBtnX = 0, raf: number | null = null, last = 0;

    function nearestAnchor(px: number) {
      let best = ANCHORS[0], bestD = Infinity;
      for (let i = 0; i < ANCHORS.length; i++) {
        const d = Math.abs(ANCHORS[i] - px);
        if (d < bestD) { bestD = d; best = ANCHORS[i]; }
      }
      return best;
    }
    function paint() { btn!.style.transform = "translateX(" + x.toFixed(1) + "px)"; }
    function tick(now: number) {
      const dt = Math.min((now - last) / 1000, 0.032);
      last = now;
      const accel = -K * (x - target) - C * vel;
      vel += accel * dt;
      x += vel * dt;
      paint();
      if (Math.abs(x - target) < 0.05 && Math.abs(vel) < 0.4) { x = target; vel = 0; paint(); raf = null; return; }
      raf = requestAnimationFrame(tick);
    }
    function startSpring() { if (raf === null) { last = performance.now(); raf = requestAnimationFrame(tick); } }

    function onPointerDown(e: PointerEvent) {
      stopAutoplay();
      dragging = true; startX = e.clientX; startBtnX = x;
      if (raf !== null) { cancelAnimationFrame(raf); raf = null; }
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      const dx = e.clientX - startX;
      x = Math.max(BTN_R, Math.min(field!.clientWidth - BTN_R, startBtnX + dx));
      paint();
    }
    function onPointerUp() {
      if (!dragging) return;
      dragging = false;
      target = nearestAnchor(x);
      if (reduce) { x = target; vel = 0; paint(); return; }
      startSpring();
    }
    btn.addEventListener("pointerdown", onPointerDown);
    window.addEventListener("pointermove", onPointerMove);
    window.addEventListener("pointerup", onPointerUp);
    paint();

    let autoTimer: number | null = null, idx = 0;
    function autoTick() {
      idx = (idx + 1) % ANCHORS.length;
      x = ANCHORS[idx] + (idx % 2 === 0 ? -18 : 18);
      vel = 0;
      target = nearestAnchor(x);
      startSpring();
    }
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    if (!reduce) { autoTimer = window.setInterval(autoTick, 900); }

    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.bma}>
        <div className={styles.field}>
          <span className={styles.anchor} style={{ left: "30px" }} />
          <span className={styles.anchor} style={{ left: "110px" }} />
          <span className={styles.anchor} style={{ left: "190px" }} />
          <button className={styles.btn} type="button" aria-label="자리 잡는 토글 버튼" style={{ background: color }}>●</button>
        </div>
        <p className={styles.hint}>놓으면 가장 가까운 고정 지점 값으로 튕기듯 맞춰집니다</p>
      </div>
    </div>
  );
}