06Momentum-Fling Drag Where Release Speed Pushes the Remaining Distance

<main class="stage">
<div class="fl is-demo" id="fl">
  <div class="fl__box" id="flBox">
    <button type="button" class="fl__ball" id="flBall" aria-label="A ball that bounces off walls and gradually slows when thrown. Press Enter to grab, arrow keys to add force in a direction"></button>
  </div>
  <p class="fl__caption" id="flCaption">Release it fast and it flies off at that speed, bouncing off the walls</p>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;
@include stage($stage-orange);

.fl {
  position: relative;
  width: min(92vw, calc(160vh - 12.8vw));
  max-width: 400px;
  margin: 0 auto;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-4;
}

.fl__box {
  position: relative;
  width: 100%;
  height: 170px;
  max-width: 320px;
  border-radius: $r-card;
  border: 2px dashed rgba($subject-cream, .3);
  display: flex;
  align-items: center;
  justify-content: center;
  overflow: hidden;
}

.fl__ball {
  appearance: none;
  border: 0;
  width: 46px;
  height: 46px;
  border-radius: $r-pill;
  background: $subject-orange-deep;
  box-shadow: $shadow-raised;
  cursor: grab;
  touch-action: none;
  will-change: transform;
}

.fl.is-bounce .fl__ball {
  background: $subject-cream;
}

.fl__ball.is-grabbed,
.fl__ball:focus-visible {
  outline: 2px solid $subject-cream;
  outline-offset: 3px;
}

.fl.is-dragging .fl__ball {
  cursor: grabbing;
}

.fl__caption {
  margin: 0;
  color: $subject-cream;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  text-align: center;
}

@media (max-width: 340px) {
  .fl__box { height: 130px; }
  .fl__ball { width: 36px; height: 36px; }
  .fl__caption { font-size: 10px; }
}

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

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.fl {
  position: relative;
  width: min(92vw, 160vh - 12.8vw);
  max-width: 400px;
  margin: 0 auto;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 16px;
}

.fl__box {
  position: relative;
  width: 100%;
  height: 170px;
  max-width: 320px;
  border-radius: 16px;
  border: 2px dashed rgba(255, 247, 230, 0.3);
  display: flex;
  align-items: center;
  justify-content: center;
  overflow: hidden;
}

.fl__ball {
  appearance: none;
  border: 0;
  width: 46px;
  height: 46px;
  border-radius: 999px;
  background: #c93000;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  cursor: grab;
  touch-action: none;
  will-change: transform;
}

.fl.is-bounce .fl__ball {
  background: #fff7e6;
}

.fl__ball.is-grabbed,
.fl__ball:focus-visible {
  outline: 2px solid #fff7e6;
  outline-offset: 3px;
}

.fl.is-dragging .fl__ball {
  cursor: grabbing;
}

.fl__caption {
  margin: 0;
  color: #fff7e6;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  text-align: center;
}

@media (max-width: 340px) {
  .fl__box {
    height: 130px;
  }
  .fl__ball {
    width: 36px;
    height: 36px;
  }
  .fl__caption {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .fl__ball {
    transition: none;
  }
}

(function () {
  // 기법: 드래그 중 마지막 몇 프레임의 이동량으로 놓는 순간의 속도(vx,vy)를 구하고,
  // 놓으면 그 속도로 계속 이동시키며 매 프레임 FLING_DECAY_RATE 로 감쇠, 경계에
  // 부딪히면 해당 축 속도의 부호를 반전(반사)시킨다.
  var root = document.getElementById('fl');
  var box = document.getElementById('flBox');
  var ball = document.getElementById('flBall');
  var caption = document.getElementById('flCaption');

  var FLING_DECAY_RATE = 0.92;
  var KEY_IMPULSE_PX_S = 260;

  var pos = { x: 0, y: 0 };
  var vel = { x: 0, y: 0 };
  var pointerStart = null;
  var startPos = null;
  var lastSample = null;
  var dragging = false;
  var grabbed = false;
  var raf = 0, lastTs = null;

  function applyBall() {
    ball.style.transform = 'translate(' + pos.x.toFixed(1) + 'px,' + pos.y.toFixed(1) + 'px)';
  }

  function bounds() {
    var br = box.getBoundingClientRect();
    var brc = ball.getBoundingClientRect();
    var halfW = brc.width / 2, halfH = brc.height / 2;
    return { minX: br.left + halfW, maxX: br.right - halfW, minY: br.top + halfH, maxY: br.bottom - halfH };
  }

  function flingTick(ts) {
    if (lastTs === null) lastTs = ts;
    var dt = Math.min((ts - lastTs) / 1000, 0.032);
    lastTs = ts;
    pos.x += vel.x * dt;
    pos.y += vel.y * dt;
    var b = bounds();
    var origin = ballOrigin();
    var cx = origin.x + pos.x, cy = origin.y + pos.y;
    if (cx < b.minX) { pos.x += (b.minX - cx); vel.x = Math.abs(vel.x); root.classList.add('is-bounce'); }
    else if (cx > b.maxX) { pos.x -= (cx - b.maxX); vel.x = -Math.abs(vel.x); root.classList.add('is-bounce'); }
    if (cy < b.minY) { pos.y += (b.minY - cy); vel.y = Math.abs(vel.y); root.classList.add('is-bounce'); }
    else if (cy > b.maxY) { pos.y -= (cy - b.maxY); vel.y = -Math.abs(vel.y); root.classList.add('is-bounce'); }
    vel.x *= Math.pow(FLING_DECAY_RATE, dt * 60);
    vel.y *= Math.pow(FLING_DECAY_RATE, dt * 60);
    applyBall();
    var speed = Math.hypot(vel.x, vel.y);
    if (speed > 4) { raf = requestAnimationFrame(flingTick); return; }
    vel.x = 0; vel.y = 0;
    root.classList.remove('is-bounce');
    lastTs = null;
  }

  var originCache = null;
  function ballOrigin() {
    if (originCache) return originCache;
    var r = ball.getBoundingClientRect();
    originCache = { x: r.left + r.width / 2 - pos.x, y: r.top + r.height / 2 - pos.y };
    return originCache;
  }

  function beginDrag(clientX, clientY) {
    cancelAnimationFrame(raf);
    dragging = true;
    vel.x = 0; vel.y = 0;
    pointerStart = { x: clientX, y: clientY };
    startPos = { x: pos.x, y: pos.y };
    lastSample = { x: clientX, y: clientY, t: performance.now() };
    root.classList.add('is-dragging');
  }

  function moveDrag(clientX, clientY) {
    pos.x = startPos.x + (clientX - pointerStart.x);
    pos.y = startPos.y + (clientY - pointerStart.y);
    applyBall();
    var now = performance.now();
    var dt = Math.max((now - lastSample.t) / 1000, 0.001);
    vel.x = (clientX - lastSample.x) / dt;
    vel.y = (clientY - lastSample.y) / dt;
    lastSample = { x: clientX, y: clientY, t: now };
  }

  function endDrag() {
    dragging = false;
    root.classList.remove('is-dragging');
    var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
    if (reduce) { vel.x = 0; vel.y = 0; }
    else { lastTs = null; raf = requestAnimationFrame(flingTick); }
  }

  ball.addEventListener('pointerdown', function (e) {
    root.classList.remove('is-demo');
    ball.setPointerCapture(e.pointerId);
    beginDrag(e.clientX, e.clientY);
    caption.textContent = 'Measuring the speed. Release it and it flies off as-is';
    e.preventDefault();
  });
  ball.addEventListener('pointermove', function (e) {
    if (!dragging) return;
    moveDrag(e.clientX, e.clientY);
    e.preventDefault();
  });
  ball.addEventListener('pointerup', function (e) {
    if (!dragging) return;
    endDrag();
    caption.textContent = 'Release it fast and it flies off at that speed, bouncing off the walls';
    e.preventDefault();
  });

  ball.addEventListener('keydown', function (e) {
    root.classList.remove('is-demo');
    if (e.key === 'Enter' || e.key === ' ') {
      e.preventDefault();
      grabbed = !grabbed;
      ball.classList.toggle('is-grabbed', grabbed);
      caption.textContent = grabbed ? 'Grabbed. Add force in a direction with arrow keys' : 'Release it fast and it flies off at that speed, bouncing off the walls';
      return;
    }
    if (!grabbed) return;
    var ix = 0, iy = 0;
    if (e.key === 'ArrowLeft') ix = -KEY_IMPULSE_PX_S;
    else if (e.key === 'ArrowRight') ix = KEY_IMPULSE_PX_S;
    else if (e.key === 'ArrowUp') iy = -KEY_IMPULSE_PX_S;
    else if (e.key === 'ArrowDown') iy = KEY_IMPULSE_PX_S;
    else if (e.key === 'Escape') {
      e.preventDefault();
      grabbed = false;
      ball.classList.remove('is-grabbed');
      caption.textContent = 'Release it fast and it flies off at that speed, bouncing off the walls';
      return;
    } else { return; }
    e.preventDefault();
    vel.x += ix; vel.y += iy;
    grabbed = false;
    ball.classList.remove('is-grabbed');
    lastTs = null;
    cancelAnimationFrame(raf);
    raf = requestAnimationFrame(flingTick);
  });

  applyBall();

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (reduce) {
    root.classList.remove('is-demo');
  } else {
    var ballRect0 = ball.getBoundingClientRect();
    var ox0 = ballRect0.left + ballRect0.width / 2;
    var oy0 = ballRect0.top + ballRect0.height / 2;
    var demoStart = null;
    function demoTick(ts) {
      if (demoStart === null) demoStart = ts;
      var t = ts - demoStart;
      if (t < 200) {
        // 대기
      } else if (t < 420) {
        var p = (t - 200) / 220;
        var cx = ox0 - 60 * p;
        var cy = oy0 - 8 * p;
        if (t < 220) beginDrag(ox0, oy0);
        moveDrag(cx, cy);
      } else if (t < 460) {
        endDrag();
      } else {
        if (t > 1900) demoStart = ts - 60;
      }
      requestAnimationFrame(demoTick);
    }
    requestAnimationFrame(demoTick);
  }
})();

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

type Box<T> = { current: T };
type Pt = { x: number; y: number };

export default function DragPhysMomentumFling() {
  const rootRef = useRef<HTMLDivElement>(null);
  const boxRef = useRef<HTMLDivElement>(null);
  const ballRef = useRef<HTMLButtonElement>(null);
  const captionRef = useRef<HTMLParagraphElement>(null);

  useEffect(() => {
    const root = rootRef.current;
    const box = boxRef.current;
    const ball = ballRef.current;
    const caption = captionRef.current;
    if (!root || !box || !ball || !caption) return;

    const FLING_DECAY_RATE = 0.92;
    const KEY_IMPULSE_PX_S = 260;

    const pos: Pt = { x: 0, y: 0 };
    const vel: Pt = { x: 0, y: 0 };
    let pointerStart: Pt | null = null;
    let startPos: Pt = { x: 0, y: 0 };
    let lastSample: { x: number; y: number; t: number } | null = null;
    let dragging = false;
    let grabbed = false;
    let raf = 0, lastTs: number | null = null;
    let originCache: Pt | null = null;

    function applyBall() {
      ball!.style.transform = "translate(" + pos.x.toFixed(1) + "px," + pos.y.toFixed(1) + "px)";
    }

    function bounds() {
      const br = box!.getBoundingClientRect();
      const brc = ball!.getBoundingClientRect();
      const halfW = brc.width / 2, halfH = brc.height / 2;
      return { minX: br.left + halfW, maxX: br.right - halfW, minY: br.top + halfH, maxY: br.bottom - halfH };
    }

    function ballOrigin() {
      if (originCache) return originCache;
      const r = ball!.getBoundingClientRect();
      originCache = { x: r.left + r.width / 2 - pos.x, y: r.top + r.height / 2 - pos.y };
      return originCache;
    }

    function flingTick(ts: number) {
      if (lastTs === null) lastTs = ts;
      const dt = Math.min((ts - lastTs) / 1000, 0.032);
      lastTs = ts;
      pos.x += vel.x * dt;
      pos.y += vel.y * dt;
      const b = bounds();
      const origin = ballOrigin();
      const cx = origin.x + pos.x, cy = origin.y + pos.y;
      if (cx < b.minX) { pos.x += b.minX - cx; vel.x = Math.abs(vel.x); root!.classList.add(styles.isBounce); }
      else if (cx > b.maxX) { pos.x -= cx - b.maxX; vel.x = -Math.abs(vel.x); root!.classList.add(styles.isBounce); }
      if (cy < b.minY) { pos.y += b.minY - cy; vel.y = Math.abs(vel.y); root!.classList.add(styles.isBounce); }
      else if (cy > b.maxY) { pos.y -= cy - b.maxY; vel.y = -Math.abs(vel.y); root!.classList.add(styles.isBounce); }
      vel.x *= Math.pow(FLING_DECAY_RATE, dt * 60);
      vel.y *= Math.pow(FLING_DECAY_RATE, dt * 60);
      applyBall();
      const speed = Math.hypot(vel.x, vel.y);
      if (speed > 4) { raf = requestAnimationFrame(flingTick); return; }
      vel.x = 0; vel.y = 0;
      root!.classList.remove(styles.isBounce);
      lastTs = null;
    }

    function beginDrag(clientX: number, clientY: number) {
      cancelAnimationFrame(raf);
      dragging = true;
      vel.x = 0; vel.y = 0;
      pointerStart = { x: clientX, y: clientY };
      startPos = { x: pos.x, y: pos.y };
      lastSample = { x: clientX, y: clientY, t: performance.now() };
      root!.classList.add(styles.isDragging);
    }

    function moveDrag(clientX: number, clientY: number) {
      pos.x = startPos.x + (clientX - pointerStart!.x);
      pos.y = startPos.y + (clientY - pointerStart!.y);
      applyBall();
      const now = performance.now();
      const dt = Math.max((now - lastSample!.t) / 1000, 0.001);
      vel.x = (clientX - lastSample!.x) / dt;
      vel.y = (clientY - lastSample!.y) / dt;
      lastSample = { x: clientX, y: clientY, t: now };
    }

    function endDrag() {
      dragging = false;
      root!.classList.remove(styles.isDragging);
      const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
      if (reduce) { vel.x = 0; vel.y = 0; } else { lastTs = null; raf = requestAnimationFrame(flingTick); }
    }

    function onPointerDown(e: PointerEvent) {
      root!.classList.remove("is-demo");
      ball!.setPointerCapture(e.pointerId);
      beginDrag(e.clientX, e.clientY);
      caption!.textContent = "속도를 재고 있어요. 놓으면 그대로 날아가요";
      e.preventDefault();
    }
    function onPointerMove(e: PointerEvent) {
      if (!dragging) return;
      moveDrag(e.clientX, e.clientY);
      e.preventDefault();
    }
    function onPointerUp(e: PointerEvent) {
      if (!dragging) return;
      endDrag();
      caption!.textContent = "빠르게 놓으면 그 속도로 날아가 벽에 튕겨요";
      e.preventDefault();
    }
    ball.addEventListener("pointerdown", onPointerDown);
    ball.addEventListener("pointermove", onPointerMove);
    ball.addEventListener("pointerup", onPointerUp);

    function onKeyDown(e: KeyboardEvent) {
      root!.classList.remove("is-demo");
      if (e.key === "Enter" || e.key === " ") {
        e.preventDefault();
        grabbed = !grabbed;
        ball!.classList.toggle(styles.isGrabbed, grabbed);
        caption!.textContent = grabbed ? "잡았어요. 화살표로 방향에 힘을 주세요" : "빠르게 놓으면 그 속도로 날아가 벽에 튕겨요";
        return;
      }
      if (!grabbed) return;
      let ix = 0, iy = 0;
      if (e.key === "ArrowLeft") ix = -KEY_IMPULSE_PX_S;
      else if (e.key === "ArrowRight") ix = KEY_IMPULSE_PX_S;
      else if (e.key === "ArrowUp") iy = -KEY_IMPULSE_PX_S;
      else if (e.key === "ArrowDown") iy = KEY_IMPULSE_PX_S;
      else if (e.key === "Escape") {
        e.preventDefault();
        grabbed = false;
        ball!.classList.remove(styles.isGrabbed);
        caption!.textContent = "빠르게 놓으면 그 속도로 날아가 벽에 튕겨요";
        return;
      } else { return; }
      e.preventDefault();
      vel.x += ix; vel.y += iy;
      grabbed = false;
      ball!.classList.remove(styles.isGrabbed);
      lastTs = null;
      cancelAnimationFrame(raf);
      raf = requestAnimationFrame(flingTick);
    }
    ball.addEventListener("keydown", onKeyDown);

    applyBall();

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demoRaf = 0;
    if (reduce) {
      root.classList.remove("is-demo");
    } else {
      const ballRect0 = ball.getBoundingClientRect();
      const ox0 = ballRect0.left + ballRect0.width / 2;
      const oy0 = ballRect0.top + ballRect0.height / 2;
      let demoStart: number | null = null;
      function demoTick(ts: number) {
        if (demoStart === null) demoStart = ts;
        const t = ts - demoStart;
        if (t < 200) {
          // 대기
        } else if (t < 420) {
          const p = (t - 200) / 220;
          const cx = ox0 - 60 * p;
          const cy = oy0 - 8 * p;
          if (t < 220) beginDrag(ox0, oy0);
          moveDrag(cx, cy);
        } else if (t < 460) {
          endDrag();
        } else {
          if (t > 1900) demoStart = ts - 60;
        }
        demoRaf = requestAnimationFrame(demoTick);
      }
      demoRaf = requestAnimationFrame(demoTick);
    }

    return () => {
      ball.removeEventListener("pointerdown", onPointerDown);
      ball.removeEventListener("pointermove", onPointerMove);
      ball.removeEventListener("pointerup", onPointerUp);
      ball.removeEventListener("keydown", onKeyDown);
      cancelAnimationFrame(raf);
      cancelAnimationFrame(demoRaf);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={`${styles.fl} is-demo`} ref={rootRef}>
        <div className={styles.box} ref={boxRef}>
          <button
            type="button"
            className={styles.ball}
            ref={ballRef}
            aria-label="던지면 벽에 튕기며 서서히 멈추는 공. Enter 로 잡고 화살표로 방향에 힘을 줍니다"
          ></button>
        </div>
        <p className={styles.caption} ref={captionRef}>빠르게 놓으면 그 속도로 날아가 벽에 튕겨요</p>
      </div>
    </main>
  );
}