07Thrown-speed decay move — motion continues at the speed value captured at release, with that value shrinking over time until it stops

<main class="stage">
<div class="bft is-demo" id="bft">
  <div class="bft__field" id="bftField">
    <button class="bft__chip" id="bftChip" type="button" aria-label="Floating button">↗</button>
  </div>
  <p class="bft__hint">Flick it lightly and the speed value decays until it stops</p>
</div>
</main>

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

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

.stage { position: relative; }

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

.bft__field {
  position: relative;
  width: 240px;
  height: 168px;
  border-radius: $r-card;
  background: rgba($stage-paper, .08);
  border: 1px solid rgba($stage-paper, .18);
  overflow: hidden;
}

.bft__chip {
  position: absolute;
  left: 0;
  top: 0;
  width: 48px;
  height: 48px;
  border: 0;
  border-radius: $r-pill;
  background: $subject-lilac;
  color: $subject-ink;
  font-size: 20px;
  font-weight: 700;
  cursor: grab;
  touch-action: none;
  box-shadow: $shadow-raised;
}

.bft__hint {
  color: rgba($stage-paper, .72);
  font-size: 12px;
  text-align: center;
  max-width: 240px;
}

@media (max-width: 340px) {
  .bft__field { width: 170px; height: 100px; }
  .bft__chip { width: 36px; height: 36px; font-size: 15px; }
}

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

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

.bft__field {
  position: relative;
  width: 240px;
  height: 168px;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.08);
  border: 1px solid rgba(255, 255, 255, 0.18);
  overflow: hidden;
}

.bft__chip {
  position: absolute;
  left: 0;
  top: 0;
  width: 48px;
  height: 48px;
  border: 0;
  border-radius: 999px;
  background: #b9a0ff;
  color: #17141a;
  font-size: 20px;
  font-weight: 700;
  cursor: grab;
  touch-action: none;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.bft__hint {
  color: rgba(255, 255, 255, 0.72);
  font-size: 12px;
  text-align: center;
  max-width: 240px;
}

@media (max-width: 340px) {
  .bft__field {
    width: 170px;
    height: 100px;
  }
  .bft__chip {
    width: 36px;
    height: 36px;
    font-size: 15px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bft__chip {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('bft');
  var field = document.getElementById('bftField');
  var chip = document.getElementById('bftChip');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 놓는 순간의 속도 값(vx, vy, px/s)을 물려받아 매 프레임 위치에 더하고,
  // 그 속도 값 자체를 FRICTION 비율만큼 지수적으로 줄인다 — vel *= FRICTION**dt.
  // 벽(WALL)에 닿으면 그 축의 속도만 0으로 죽여 멈춘 자리에서 정지한다.
  var FRICTION = 0.05, STOP_VEL = 6, CHIP_R = 24;
  var x = 100, y = 60, vx = 0, vy = 0, last = 0, raf = null;
  var dragging = false, samples = [];

  function bounds() { return { w: field.clientWidth, h: field.clientHeight }; }
  function paint() { chip.style.transform = 'translate(' + (x - CHIP_R).toFixed(1) + 'px,' + (y - CHIP_R).toFixed(1) + 'px)'; }
  function tick(now) {
    var dt = Math.min((now - last) / 1000, 0.032);
    last = now;
    var decay = Math.pow(FRICTION, dt);
    vx *= decay; vy *= decay;
    x += vx * dt; y += vy * dt;
    var b = bounds();
    if (x < CHIP_R) { x = CHIP_R; vx = 0; }
    if (x > b.w - CHIP_R) { x = b.w - CHIP_R; vx = 0; }
    if (y < CHIP_R) { y = CHIP_R; vy = 0; }
    if (y > b.h - CHIP_R) { y = b.h - CHIP_R; vy = 0; }
    paint();
    if (Math.abs(vx) < STOP_VEL && Math.abs(vy) < STOP_VEL) { vx = 0; vy = 0; raf = null; return; }
    raf = requestAnimationFrame(tick);
  }
  function start() { if (raf === null) { last = performance.now(); raf = requestAnimationFrame(tick); } }
  function throwWith(nvx, nvy) {
    vx = nvx; vy = nvy;
    if (reduce) { vx = 0; vy = 0; return; }
    start();
  }

  function onDown(clientX, clientY) {
    dragging = true; samples = [{ t: performance.now(), x: clientX, y: clientY }];
    if (raf !== null) { cancelAnimationFrame(raf); raf = null; }
  }
  function onMove(clientX, clientY) {
    if (!dragging) { return; }
    var r = field.getBoundingClientRect();
    var b = bounds();
    x = Math.max(CHIP_R, Math.min(b.w - CHIP_R, clientX - r.left));
    y = Math.max(CHIP_R, Math.min(b.h - CHIP_R, clientY - r.top));
    paint();
    samples.push({ t: performance.now(), x: clientX, y: clientY });
    if (samples.length > 6) { samples.shift(); }
  }
  function onUp() {
    if (!dragging) { return; }
    dragging = false;
    if (samples.length >= 2) {
      var a = samples[0], b2 = samples[samples.length - 1];
      var dt = Math.max(1, b2.t - a.t) / 1000;
      throwWith((b2.x - a.x) / dt, (b2.y - a.y) / dt);
    }
  }

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

  // 자동 루프: 실제 드래그 대신 정해둔 속도 값으로 곧장 던져 같은 감쇠 경로를 반복 재현한다.
  var autoTimer = null;
  function autoTick() {
    x = 30; y = 30;
    throwWith(420, 260);
  }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTick(); autoTimer = setInterval(autoTick, 1700); }
})();

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

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

type Sample = { t: number; x: number; y: number };

export default function ButtonPhysicsFlickThrowDecay({
  color = "#b9a0ff",
}: ButtonPhysicsFlickThrowDecayProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

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

    // 계산 겹: 놓는 순간의 속도 값(vx, vy, px/s)을 물려받아 매 프레임 위치에 더하고,
    // 그 속도 값 자체를 FRICTION 비율만큼 지수적으로 줄인다 — vel *= FRICTION**dt.
    // 벽(WALL)에 닿으면 그 축의 속도만 0으로 죽여 멈춘 자리에서 정지한다.
    const FRICTION = 0.05, STOP_VEL = 6, CHIP_R = 24;
    let x = 100, y = 60, vx = 0, vy = 0, last = 0, raf: number | null = null;
    let dragging = false, samples: Sample[] = [];

    function bounds() { return { w: field!.clientWidth, h: field!.clientHeight }; }
    function paint() { chip!.style.transform = "translate(" + (x - CHIP_R).toFixed(1) + "px," + (y - CHIP_R).toFixed(1) + "px)"; }
    function tick(now: number) {
      const dt = Math.min((now - last) / 1000, 0.032);
      last = now;
      const decay = Math.pow(FRICTION, dt);
      vx *= decay; vy *= decay;
      x += vx * dt; y += vy * dt;
      const b = bounds();
      if (x < CHIP_R) { x = CHIP_R; vx = 0; }
      if (x > b.w - CHIP_R) { x = b.w - CHIP_R; vx = 0; }
      if (y < CHIP_R) { y = CHIP_R; vy = 0; }
      if (y > b.h - CHIP_R) { y = b.h - CHIP_R; vy = 0; }
      paint();
      if (Math.abs(vx) < STOP_VEL && Math.abs(vy) < STOP_VEL) { vx = 0; vy = 0; raf = null; return; }
      raf = requestAnimationFrame(tick);
    }
    function start() { if (raf === null) { last = performance.now(); raf = requestAnimationFrame(tick); } }
    function throwWith(nvx: number, nvy: number) {
      vx = nvx; vy = nvy;
      if (reduce) { vx = 0; vy = 0; return; }
      start();
    }

    function onPointerDown(e: PointerEvent) {
      stopAutoplay();
      dragging = true; samples = [{ t: performance.now(), x: e.clientX, y: e.clientY }];
      if (raf !== null) { cancelAnimationFrame(raf); raf = null; }
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      const r = field!.getBoundingClientRect();
      const b = bounds();
      x = Math.max(CHIP_R, Math.min(b.w - CHIP_R, e.clientX - r.left));
      y = Math.max(CHIP_R, Math.min(b.h - CHIP_R, e.clientY - r.top));
      paint();
      samples.push({ t: performance.now(), x: e.clientX, y: e.clientY });
      if (samples.length > 6) { samples.shift(); }
    }
    function onPointerUp() {
      if (!dragging) return;
      dragging = false;
      if (samples.length >= 2) {
        const a = samples[0], b2 = samples[samples.length - 1];
        const dt = Math.max(1, b2.t - a.t) / 1000;
        throwWith((b2.x - a.x) / dt, (b2.y - a.y) / dt);
      }
    }
    chip.addEventListener("pointerdown", onPointerDown);
    window.addEventListener("pointermove", onPointerMove);
    window.addEventListener("pointerup", onPointerUp);
    paint();

    let autoTimer: number | null = null;
    function autoTick() {
      x = 30; y = 30;
      throwWith(420, 260);
    }
    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);
      chip.removeEventListener("pointerdown", onPointerDown);
      window.removeEventListener("pointermove", onPointerMove);
      window.removeEventListener("pointerup", onPointerUp);
    };
  }, []);

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.bft}>
        <div className={styles.field}>
          <button className={styles.chip} type="button" aria-label="플로팅 버튼" style={{ background: color }}>↗</button>
        </div>
        <p className={styles.hint}>가볍게 튕겨 던지면 속도 값이 줄어들며 멈춥니다</p>
      </div>
    </div>
  );
}