02追従値の遅延補間 — ポインター位置の値をボタン位置の値が一定の比率ずつ追いかけて縮める方式

<main class="stage">
<div class="bil is-demo" id="bil">
  <div class="bil__field" id="bilField" role="application" aria-label="フローティングボタンのドラッグ実験場">
    <button class="bil__fab" id="bilFab" type="button" aria-label="フローティングアクションボタン">+</button>
  </div>
  <p class="bil__hint">押して動かすと値が後を追いかけます</p>
</div>
</main>

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

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

.stage { position: relative; }

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

.bil__field {
  position: relative;
  width: 220px;
  height: 160px;
  border-radius: $r-card;
  background: rgba($stage-paper, .08);
  border: 1px solid rgba($stage-paper, .18);
}

.bil__fab {
  position: absolute;
  left: 0;
  top: 0;
  width: 52px;
  height: 52px;
  border: 0;
  border-radius: $r-pill;
  background: $subject-mint;
  color: $subject-ink;
  font-size: 24px;
  font-weight: 700;
  line-height: 1;
  cursor: grab;
  touch-action: none;
  box-shadow: $shadow-raised;
}

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

@media (max-width: 340px) {
  .bil__field { width: 160px; height: 96px; }
  .bil__fab { width: 40px; height: 40px; font-size: 18px; }
}

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

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

.bil__field {
  position: relative;
  width: 220px;
  height: 160px;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.08);
  border: 1px solid rgba(255, 255, 255, 0.18);
}

.bil__fab {
  position: absolute;
  left: 0;
  top: 0;
  width: 52px;
  height: 52px;
  border: 0;
  border-radius: 999px;
  background: #4cd4a6;
  color: #17141a;
  font-size: 24px;
  font-weight: 700;
  line-height: 1;
  cursor: grab;
  touch-action: none;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

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

@media (max-width: 340px) {
  .bil__field {
    width: 160px;
    height: 96px;
  }
  .bil__fab {
    width: 40px;
    height: 40px;
    font-size: 18px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bil__fab {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('bil');
  var field = document.getElementById('bilField');
  var fab = document.getElementById('bilFab');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 포인터 위치 값(targetX/Y)과 버튼 위치 값(x/y)의 차이를 매 프레임 LERP_RATIO
  // 만큼만 좁힌다 — x += (targetX-x)*LERP_RATIO. 비율이 1에 가까울수록 즉시 따라붙고,
  // 낮을수록 지연(트레일)이 길어진다.
  var LERP_RATIO = 0.14;
  var FAB_R = 26;
  var x = 0, y = 0, targetX = 0, targetY = 0, raf = null;

  function bounds() { return { w: field.clientWidth, h: field.clientHeight }; }
  function clampPos(px, py) {
    var b = bounds();
    return [Math.max(FAB_R, Math.min(b.w - FAB_R, px)), Math.max(FAB_R, Math.min(b.h - FAB_R, py))];
  }
  function paint() { fab.style.transform = 'translate(' + (x - FAB_R).toFixed(1) + 'px,' + (y - FAB_R).toFixed(1) + 'px)'; }
  function tick() {
    x += (targetX - x) * LERP_RATIO;
    y += (targetY - y) * LERP_RATIO;
    paint();
    raf = requestAnimationFrame(tick);
  }
  function start() { if (raf === null) { raf = requestAnimationFrame(tick); } }
  function setTarget(px, py) {
    var c = clampPos(px, py);
    targetX = c[0]; targetY = c[1];
    if (reduce) { x = targetX; y = targetY; paint(); return; }
    start();
  }

  function wireReal() {
    var dragging = false;
    fab.addEventListener('pointerdown', function (e) {
      root.classList.remove('is-demo'); stopAutoplay(); dragging = true;
      fab.setPointerCapture(e.pointerId);
    });
    fab.addEventListener('pointermove', function (e) {
      if (!dragging) { return; }
      var r = field.getBoundingClientRect();
      setTarget(e.clientX - r.left, e.clientY - r.top);
    });
    fab.addEventListener('pointerup', function () { dragging = false; });
  }
  wireReal();
  var b0 = bounds();
  x = targetX = b0.w / 2; y = targetY = b0.h / 2;
  paint();

  // 자동 루프: 목표 값을 좌우로 오가게 옮겨 같은 lerp 경로를 반복 재현한다.
  var autoTimer = null, phase = 0;
  function autoTick() {
    var b = bounds();
    phase += 1;
    var px = b.w / 2 + Math.sin(phase * 0.9) * (b.w / 2 - FAB_R - 6);
    var py = b.h / 2 + Math.cos(phase * 0.6) * (b.h / 2 - FAB_R - 6) * 0.5;
    setTarget(px, py);
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 260); autoTick(); }
})();

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

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

export default function ButtonPhysicsInertiaLerpDrag({
  color = "#4cd4a6",
}: ButtonPhysicsInertiaLerpDragProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

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

    // 계산 겹: 포인터 위치 값(targetX/Y)과 버튼 위치 값(x/y)의 차이를 매 프레임 LERP_RATIO
    // 만큼만 좁힌다 — x += (targetX-x)*LERP_RATIO. 비율이 1에 가까울수록 즉시 따라붙고,
    // 낮을수록 지연(트레일)이 길어진다.
    const LERP_RATIO = 0.14;
    const FAB_R = 26;
    let x = 0, y = 0, targetX = 0, targetY = 0, raf: number | null = null;

    function bounds() { return { w: field!.clientWidth, h: field!.clientHeight }; }
    function clampPos(px: number, py: number): [number, number] {
      const b = bounds();
      return [Math.max(FAB_R, Math.min(b.w - FAB_R, px)), Math.max(FAB_R, Math.min(b.h - FAB_R, py))];
    }
    function paint() { fab!.style.transform = "translate(" + (x - FAB_R).toFixed(1) + "px," + (y - FAB_R).toFixed(1) + "px)"; }
    function tick() {
      x += (targetX - x) * LERP_RATIO;
      y += (targetY - y) * LERP_RATIO;
      paint();
      raf = requestAnimationFrame(tick);
    }
    function start() { if (raf === null) { raf = requestAnimationFrame(tick); } }
    function setTarget(px: number, py: number) {
      const c = clampPos(px, py);
      targetX = c[0]; targetY = c[1];
      if (reduce) { x = targetX; y = targetY; paint(); return; }
      start();
    }

    let dragging = false;
    function onPointerDown(e: PointerEvent) {
      stopAutoplay(); dragging = true;
      fab!.setPointerCapture(e.pointerId);
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      const r = field!.getBoundingClientRect();
      setTarget(e.clientX - r.left, e.clientY - r.top);
    }
    function onPointerUp() { dragging = false; }
    fab.addEventListener("pointerdown", onPointerDown);
    fab.addEventListener("pointermove", onPointerMove);
    fab.addEventListener("pointerup", onPointerUp);
    const b0 = bounds();
    x = targetX = b0.w / 2; y = targetY = b0.h / 2;
    paint();

    let autoTimer: number | null = null, phase = 0;
    function autoTick() {
      const b = bounds();
      phase += 1;
      const px = b.w / 2 + Math.sin(phase * 0.9) * (b.w / 2 - FAB_R - 6);
      const py = b.h / 2 + Math.cos(phase * 0.6) * (b.h / 2 - FAB_R - 6) * 0.5;
      setTarget(px, py);
    }
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    if (!reduce) { autoTimer = window.setInterval(autoTick, 260); autoTick(); }

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

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.bil}>
        <div className={styles.field} role="application" aria-label="플로팅 버튼 드래그 실험판">
          <button className={styles.fab} type="button" aria-label="플로팅 액션 버튼" style={{ background: color }}>+</button>
        </div>
        <p className={styles.hint}>눌러서 옮기면 값이 뒤따라옵니다</p>
      </div>
    </div>
  );
}