07Drag slider that decides the rest by release speed

<main class="stage">
<section class="dg is-demo" id="dg" role="region" aria-roledescription="carousel" aria-label="Photo slider you page by dragging" tabindex="0">
  <header class="dg__bar">
    <span class="dg__title">Drag to page</span>
    <span class="dg__speed" id="speed">0.00 px/ms</span>
  </header>
  <div class="dg__frame" id="frame">
    <div class="dg__track" id="track">
      <article class="dg__slide" role="group" aria-roledescription="slide" aria-label="Slide 1 of 4">
        <span class="dg__ph dg__ph--a" role="img" aria-label="Containers at a dawn harbour"></span>
        <span class="dg__cap" aria-hidden="true">Containers at a dawn harbour</span>
      </article>
      <article class="dg__slide" role="group" aria-roledescription="slide" aria-label="Slide 2 of 4">
        <span class="dg__ph dg__ph--e" role="img" aria-label="Pool ripples at midday"></span>
        <span class="dg__cap" aria-hidden="true">Pool ripples at midday</span>
      </article>
      <article class="dg__slide" role="group" aria-roledescription="slide" aria-label="Slide 3 of 4">
        <span class="dg__ph dg__ph--c" role="img" aria-label="A city alley after the rain"></span>
        <span class="dg__cap" aria-hidden="true">A city alley after the rain</span>
      </article>
      <article class="dg__slide" role="group" aria-roledescription="slide" aria-label="Slide 4 of 4">
        <span class="dg__ph dg__ph--d" role="img" aria-label="A snow-covered birch forest"></span>
        <span class="dg__cap" aria-hidden="true">A snow-covered birch forest</span>
      </article>
    </div>
  </div>
  <nav class="dg__dots" aria-label="Choose a photo">
    <button class="dg__dot" type="button" aria-current="true" aria-label="Show photo 1"></button>
    <button class="dg__dot" type="button" aria-current="false" aria-label="Show photo 2"></button>
    <button class="dg__dot" type="button" aria-current="false" aria-label="Show photo 3"></button>
    <button class="dg__dot" type="button" aria-current="false" aria-label="Show photo 4"></button>
  </nav>
</section>
</main>

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

// 손을 뗀 속도로 남은 거리를 정하는 드래그 슬라이더.
// 트랙은 슬라이드를 한 줄로 이어 붙이고 translate3d 로만 움직인다 — 끄는 동안에는 JS 가
// transition 을 꺼서 손가락 위치가 곧 트랙 위치가 되고, 놓는 순간 다시 켜서 남은 거리를 잇는다.
// touch-action: pan-y 를 트랙에 걸어야 가로로 끌 때도 페이지 세로 스크롤이 살아 있다.
// 사진 자리는 FLUX 이미지가 들어올 자리다 (run/082/_이미지후보.07.json) — 지금은 토큰 그라디언트 합성.

$duration: $dur-enter;
$easing: $ease-spring;
$color: $subject-blue-deep;

@include stage($stage-orange);

// 장면 하나 = 빛 + 아래로 깔리는 그늘 + 결 + 바탕 네 겹. 색만 바꿔 장면을 가른다
@mixin scene($top, $mid, $low, $angle, $sunx, $suny) {
  background:
    radial-gradient(40% 32% at $sunx $suny, rgba(255, 255, 255, .68), rgba(255, 255, 255, 0) 72%),
    repeating-linear-gradient($angle, rgba(255, 255, 255, .07) 0 5px, rgba(255, 255, 255, 0) 5px 13px),
    linear-gradient(180deg, rgba(255, 255, 255, 0) 44%, rgba($stage-ink, .46) 100%),
    linear-gradient($angle, $top 0%, $mid 54%, $low 100%);
}

.dg {
  box-sizing: border-box;
  display: flex; flex-direction: column; gap: $sp-2;
  width: min(100%, 320px);
}
.dg:focus-visible { outline: 2px solid $subject-ink; outline-offset: 4px; border-radius: $r-card; }

.dg__bar { display: flex; align-items: center; gap: $sp-2; }
.dg__title {
  flex: 1 1 auto; color: $subject-ink;
  font-weight: 700; font-size: 13px; line-height: 1.3;
}
.dg__speed {
  flex: 0 0 auto; padding: $sp-1 $sp-2; border-radius: $r-pill;
  background: $stage-ink; color: $subject-cream;
  font-family: ui-monospace, SFMono-Regular, Consolas, monospace;
  font-weight: 700; font-size: 11px; line-height: 1.3;
}

.dg__frame {
  overflow: hidden; border-radius: $r-card;
  box-shadow: $shadow-raised;
}
.dg__track {
  display: flex;
  touch-action: pan-y;          // 가로는 우리가 받고 세로 스크롤은 브라우저에 남긴다
  cursor: grab;
  transform: translate3d(0, 0, 0);   // JS 가 처음 놓는 값과 같은 항등 변환 — 첫 칠에 헛 transition 이 안 생긴다
  transition: transform $duration $easing;
  will-change: transform;
}
.dg__track:active { cursor: grabbing; }

.dg__slide { position: relative; flex: 0 0 100%; }
.dg__ph {
  display: block; width: 100%; aspect-ratio: 16 / 9;
  -webkit-user-drag: none; user-select: none;
}
.dg__ph--a { @include scene($subject-blue-deep, $subject-sky, $stage-ink, 196deg, 74%, 22%); }
// 무대가 주황이라 주황 계열 장면(사막 노을)은 07 에 쓰지 않는다 — 사진과 무대가 붙어 보인다
.dg__ph--e { @include scene($subject-mint, $subject-sky, $chart-1, 160deg, 66%, 34%); }
.dg__ph--c { @include scene($stage-ink, $subject-blue-light, $tick-panel, 208deg, 58%, 18%); }
.dg__ph--d { @include scene($subject-sky, $subject-cream, $chart-3, 184deg, 44%, 30%); }

.dg__cap {
  position: absolute; left: $sp-2; bottom: $sp-2;
  max-width: calc(100% - #{$sp-4});
  padding: $sp-1 $sp-2; border-radius: $r-sm;
  background: $color; color: $subject-cream;
  font-weight: 700; font-size: 11px; line-height: 1.3;
  white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
}

.dg__dots { display: flex; justify-content: center; gap: $sp-2; }
.dg__dot {
  flex: 0 0 auto; width: 8px; height: 8px; padding: 0;
  border: 0; border-radius: 50%; background: rgba($subject-ink, .42);
  cursor: pointer; transform-origin: 50% 50%;
  transition: transform $dur-quick $easing, background $dur-quick $easing;
}
.dg__dot[aria-current="true"] { background: $subject-ink; transform: scale(1.5); }
.dg__dot:focus-visible { outline: 2px solid $subject-ink; outline-offset: 3px; }

// 좁은 화면 — 프레임을 좁혀 16:9 사진 높이를 무대 안에 넣는다 (320×200 에서 세로 넘침 0)
@media (max-width: 340px) {
  .dg { width: min(100%, 216px); gap: $sp-1; }
  .dg__title { font-size: 11px; }
  .dg__speed { font-size: 10px; padding: 0 $sp-1; }
  .dg__cap { font-size: 10px; left: $sp-1; bottom: $sp-1; }
  .dg__dots { gap: $sp-1; }
}

@media (prefers-reduced-motion: reduce) {
  .dg__track, .dg__dot { 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: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.dg {
  box-sizing: border-box;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: min(100%, 320px);
}

.dg:focus-visible {
  outline: 2px solid #17141a;
  outline-offset: 4px;
  border-radius: 16px;
}

.dg__bar {
  display: flex;
  align-items: center;
  gap: 8px;
}

.dg__title {
  flex: 1 1 auto;
  color: #17141a;
  font-weight: 700;
  font-size: 13px;
  line-height: 1.3;
}

.dg__speed {
  flex: 0 0 auto;
  padding: 4px 8px;
  border-radius: 999px;
  background: #17141a;
  color: #fff7e6;
  font-family: ui-monospace, SFMono-Regular, Consolas, monospace;
  font-weight: 700;
  font-size: 11px;
  line-height: 1.3;
}

.dg__frame {
  overflow: hidden;
  border-radius: 16px;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.dg__track {
  display: flex;
  touch-action: pan-y;
  cursor: grab;
  transform: translate3d(0, 0, 0);
  transition: transform 600ms cubic-bezier(0.2, 0.8, 0.2, 1);
  will-change: transform;
}

.dg__track:active {
  cursor: grabbing;
}

.dg__slide {
  position: relative;
  flex: 0 0 100%;
}

.dg__ph {
  display: block;
  width: 100%;
  aspect-ratio: 16/9;
  -webkit-user-drag: none;
  user-select: none;
}

.dg__ph--a {
  background: radial-gradient(40% 32% at 74% 22%, rgba(255, 255, 255, 0.68), rgba(255, 255, 255, 0) 72%), repeating-linear-gradient(196deg, rgba(255, 255, 255, 0.07) 0 5px, rgba(255, 255, 255, 0) 5px 13px), linear-gradient(180deg, rgba(255, 255, 255, 0) 44%, rgba(23, 20, 26, 0.46) 100%), linear-gradient(196deg, #0a4fe8 0%, #6ec8ff 54%, #17141a 100%);
}

.dg__ph--e {
  background: radial-gradient(40% 32% at 66% 34%, rgba(255, 255, 255, 0.68), rgba(255, 255, 255, 0) 72%), repeating-linear-gradient(160deg, rgba(255, 255, 255, 0.07) 0 5px, rgba(255, 255, 255, 0) 5px 13px), linear-gradient(180deg, rgba(255, 255, 255, 0) 44%, rgba(23, 20, 26, 0.46) 100%), linear-gradient(160deg, #4cd4a6 0%, #6ec8ff 54%, #2f6df6 100%);
}

.dg__ph--c {
  background: radial-gradient(40% 32% at 58% 18%, rgba(255, 255, 255, 0.68), rgba(255, 255, 255, 0) 72%), repeating-linear-gradient(208deg, rgba(255, 255, 255, 0.07) 0 5px, rgba(255, 255, 255, 0) 5px 13px), linear-gradient(180deg, rgba(255, 255, 255, 0) 44%, rgba(23, 20, 26, 0.46) 100%), linear-gradient(208deg, #17141a 0%, #5b8cf8 54%, #231f28 100%);
}

.dg__ph--d {
  background: radial-gradient(40% 32% at 44% 30%, rgba(255, 255, 255, 0.68), rgba(255, 255, 255, 0) 72%), repeating-linear-gradient(184deg, rgba(255, 255, 255, 0.07) 0 5px, rgba(255, 255, 255, 0) 5px 13px), linear-gradient(180deg, rgba(255, 255, 255, 0) 44%, rgba(23, 20, 26, 0.46) 100%), linear-gradient(184deg, #6ec8ff 0%, #fff7e6 54%, #2b9d74 100%);
}

.dg__cap {
  position: absolute;
  left: 8px;
  bottom: 8px;
  max-width: calc(100% - 16px);
  padding: 4px 8px;
  border-radius: 8px;
  background: #0a4fe8;
  color: #fff7e6;
  font-weight: 700;
  font-size: 11px;
  line-height: 1.3;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.dg__dots {
  display: flex;
  justify-content: center;
  gap: 8px;
}

.dg__dot {
  flex: 0 0 auto;
  width: 8px;
  height: 8px;
  padding: 0;
  border: 0;
  border-radius: 50%;
  background: rgba(23, 20, 26, 0.42);
  cursor: pointer;
  transform-origin: 50% 50%;
  transition: transform 220ms cubic-bezier(0.2, 0.8, 0.2, 1), background 220ms cubic-bezier(0.2, 0.8, 0.2, 1);
}

.dg__dot[aria-current=true] {
  background: #17141a;
  transform: scale(1.5);
}

.dg__dot:focus-visible {
  outline: 2px solid #17141a;
  outline-offset: 3px;
}

@media (max-width: 340px) {
  .dg {
    width: min(100%, 216px);
    gap: 4px;
  }
  .dg__title {
    font-size: 11px;
  }
  .dg__speed {
    font-size: 10px;
    padding: 0 4px;
  }
  .dg__cap {
    font-size: 10px;
    left: 4px;
    bottom: 4px;
  }
  .dg__dots {
    gap: 4px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .dg__track, .dg__dot {
    transition: none !important;
  }
}

var root = document.getElementById('dg');
var frame = document.getElementById('frame');
var track = document.getElementById('track');
var speed = document.getElementById('speed');
var slides = [].slice.call(track.children);
var dots = [].slice.call(document.querySelectorAll('.dg__dot'));
var index = 0, dragging = false, startX = 0, lastX = 0, lastT = 0, velocity = 0, width = 0, timer = 0;
function measure() { width = frame.clientWidth; }
// 한 칸 = 프레임 폭. 끄는 동안은 transition 을 꺼서 손가락이 곧 트랙 위치가 된다
function place(offset, animate) {
  track.style.transition = animate ? '' : 'none';
  track.style.transform = 'translate3d(' + (offset - index * width) + 'px, 0, 0)';
}
function show(next, animate) {
  index = Math.max(0, Math.min(slides.length - 1, next));
  place(0, animate);
  dots.forEach(function (dot, i) { dot.setAttribute('aria-current', i === index ? 'true' : 'false'); });
}
function stopDemo() { root.classList.remove('is-demo'); clearInterval(timer); }
function onDown(e) {
  stopDemo(); measure();
  dragging = true; startX = e.clientX; lastX = e.clientX; lastT = e.timeStamp; velocity = 0;
  track.setPointerCapture(e.pointerId);
}
function onMove(e) {
  if (!dragging) { return; }
  var dt = e.timeStamp - lastT;
  if (dt > 0) { velocity = (e.clientX - lastX) / dt; }
  lastX = e.clientX; lastT = e.timeStamp;
  speed.textContent = Math.abs(velocity).toFixed(2) + ' px/ms';
  place(e.clientX - startX, false);
}
// 놓는 순간의 속도(px/ms)가 먼저고, 느리게 끌었을 땐 이동 거리(28%)가 두 번째 기준이다
function onUp(e) {
  if (!dragging) { return; }
  dragging = false;
  if (track.hasPointerCapture(e.pointerId)) { track.releasePointerCapture(e.pointerId); }
  var moved = e.clientX - startX;
  var carry = Math.abs(velocity) > 0.4 || Math.abs(moved) > width * 0.28;
  show(index + (carry && moved < 0 ? 1 : 0) - (carry && moved > 0 ? 1 : 0), true);
}
track.addEventListener('pointerdown', onDown);
track.addEventListener('pointermove', onMove);
track.addEventListener('pointerup', onUp); track.addEventListener('pointercancel', onUp);
dots.forEach(function (dot, i) { dot.addEventListener('click', function () { stopDemo(); measure(); show(i, true); }); });
root.addEventListener('keydown', function (e) {
  if (e.key !== 'ArrowRight' && e.key !== 'ArrowLeft') { return; }
  stopDemo(); measure(); show(index + (e.key === 'ArrowRight' ? 1 : -1), true);
});
window.addEventListener('resize', function () { measure(); place(0, false); });
measure(); show(0, false);
if (root.classList.contains('is-demo')) {
  timer = setInterval(function () {
    measure();
    var last = index === slides.length - 1;
    speed.textContent = (last ? 0.52 : 0.84).toFixed(2) + ' px/ms';
    place(last ? 64 : -64, true);
    setTimeout(function () { show(last ? 0 : index + 1, true); }, 180);
  }, 760);
}

// DragInertiaSlider — vanilla 판(run/082/demo/07-slider-drag-swipe-inertia/index.html) 의 React 포트.
// pointerdown 부터의 이동량이 트랙을 그대로 따라오고, setPointerCapture 로 포인터를 붙잡아 둔 채
// 놓는 순간의 속도(px/ms)와 이동 거리가 다음 장으로 넘길지 제자리로 돌릴지를 정한다.
// 판정 기준(0.4 px/ms · 폭의 28%)과 미리보기 주기(760ms · 180ms)는 vanilla 와 같은 값이다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties, PointerEvent } from "react";
import styles from "./DragInertiaSlider.module.scss";

export type DragInertiaSliderProps = {
  duration?: number;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
// useRef<T> 는 T | null 을 돌려준다 — ambient 타입에 맞춰 상자 타입을 따로 둔다
type Box<T> = { current: T | null };
type DragState = { on: boolean; startX: number; lastX: number; lastT: number; velocity: number; width: number };

const REGION_LABEL = "드래그로 넘기는 사진 슬라이더";
const TITLE = "끌어서 넘기기";
const DOTS_LABEL = "사진 고르기";
const PHOTOS = [
  { tone: "a", alt: "새벽 부두의 컨테이너", slide: "4장 중 1장", dot: "1번째 사진 보기" },
  { tone: "e", alt: "한낮의 수영장 물결", slide: "4장 중 2장", dot: "2번째 사진 보기" },
  { tone: "c", alt: "비 온 뒤 도심 골목", slide: "4장 중 3장", dot: "3번째 사진 보기" },
  { tone: "d", alt: "눈 덮인 자작나무 숲", slide: "4장 중 4장", dot: "4번째 사진 보기" },
];

export default function DragInertiaSlider(props: DragInertiaSliderProps) {
  const duration = props.duration ?? 600;
  const easing = props.easing ?? "cubic-bezier(.2, .8, .2, 1)";
  const color = props.color ?? "#0a4fe8";
  const [index, setIndex] = useState<number>(0);
  const [offset, setOffset] = useState<number>(0);
  const [animate, setAnimate] = useState<boolean>(true);
  const [speed, setSpeed] = useState<string>("0.00");
  const [preview, setPreview] = useState<boolean>(true);
  const [width, setWidth] = useState<number>(0);
  const frameRef = useRef<HTMLDivElement>(null) as Box<HTMLDivElement>;
  const indexRef = useRef<number>(0) as Box<number>;
  const dragRef = useRef<DragState>({ on: false, startX: 0, lastX: 0, lastT: 0, velocity: 0, width: 0 }) as Box<DragState>;

  useEffect(() => {
    indexRef.current = index;
  }, [index]);

  useEffect(() => {
    const measure = () => {
      const frame = frameRef.current;
      if (frame) {
        setWidth(frame.clientWidth);
      }
    };
    measure();
    window.addEventListener("resize", measure);
    return () => {
      window.removeEventListener("resize", measure);
    };
  }, []);

  // 미리보기 — 한 번 끌었다 놓는 손짓을 흉내 낸다. 먼저 64px 당기고 180ms 뒤에 남은 거리를 잇는다
  useEffect(() => {
    if (!preview) {
      return;
    }
    const id = window.setInterval(() => {
      const at = indexRef.current ?? 0;
      const last = at === PHOTOS.length - 1;
      setSpeed(last ? "0.52" : "0.84");
      setAnimate(true);
      setOffset(last ? 64 : -64);
      window.setTimeout(() => {
        setOffset(0);
        setIndex(last ? 0 : at + 1);
      }, 180);
    }, 760);
    return () => {
      window.clearInterval(id);
    };
  }, [preview]);

  const go = (next: number) => {
    setAnimate(true);
    setOffset(0);
    setIndex(Math.max(0, Math.min(PHOTOS.length - 1, next)));
  };

  const onPointerDown = (e: PointerEvent<HTMLDivElement>) => {
    const frame = frameRef.current;
    const drag = dragRef.current;
    if (!frame || !drag) {
      return;
    }
    setPreview(false);
    setAnimate(false);
    setWidth(frame.clientWidth);
    drag.on = true;
    drag.startX = e.clientX;
    drag.lastX = e.clientX;
    drag.lastT = e.timeStamp;
    drag.velocity = 0;
    drag.width = frame.clientWidth;
    e.currentTarget.setPointerCapture(e.pointerId);
  };

  const onPointerMove = (e: PointerEvent<HTMLDivElement>) => {
    const drag = dragRef.current;
    if (!drag || !drag.on) {
      return;
    }
    const dt = e.timeStamp - drag.lastT;
    if (dt > 0) {
      drag.velocity = (e.clientX - drag.lastX) / dt;
    }
    drag.lastX = e.clientX;
    drag.lastT = e.timeStamp;
    setSpeed(Math.abs(drag.velocity).toFixed(2));
    setOffset(e.clientX - drag.startX);
  };

  // 놓는 순간의 속도가 먼저고, 느리게 끌었을 땐 이동 거리(28%)가 두 번째 기준이다
  const onPointerUp = (e: PointerEvent<HTMLDivElement>) => {
    const drag = dragRef.current;
    if (!drag || !drag.on) {
      return;
    }
    drag.on = false;
    if (e.currentTarget.hasPointerCapture(e.pointerId)) {
      e.currentTarget.releasePointerCapture(e.pointerId);
    }
    const moved = e.clientX - drag.startX;
    const carry = Math.abs(drag.velocity) > 0.4 || Math.abs(moved) > drag.width * 0.28;
    go((indexRef.current ?? 0) + (carry && moved < 0 ? 1 : 0) - (carry && moved > 0 ? 1 : 0));
  };

  const vars: Vars = { "--duration": duration + "ms", "--easing": easing, "--color": color };
  const shift = offset - index * width;

  return (
    <main className={styles.stage}>
      <section
        className={styles.root}
        style={vars}
        role="region"
        aria-roledescription="carousel"
        aria-label={REGION_LABEL}
        tabIndex={0}
        onKeyDown={(e: { key: string }) => {
          if (e.key !== "ArrowRight" && e.key !== "ArrowLeft") {
            return;
          }
          setPreview(false);
          go(index + (e.key === "ArrowRight" ? 1 : -1));
        }}
      >
        <header className={styles.bar}>
          <span className={styles.title}>{TITLE}</span>
          <span className={styles.speed}>{speed} px/ms</span>
        </header>
        <div className={styles.frame} ref={frameRef}>
          <div
            className={styles.track}
            style={{ transform: "translate3d(" + shift + "px, 0, 0)", transition: animate ? "" : "none" }}
            onPointerDown={onPointerDown}
            onPointerMove={onPointerMove}
            onPointerUp={onPointerUp}
            onPointerCancel={onPointerUp}
          >
            {PHOTOS.map((photo) => (
              <article
                className={styles.slide}
                key={photo.tone}
                role="group"
                aria-roledescription="slide"
                aria-label={photo.slide}
              >
                <span className={styles.ph + " " + styles["ph--" + photo.tone]} role="img" aria-label={photo.alt} />
                <span className={styles.cap} aria-hidden="true">{photo.alt}</span>
              </article>
            ))}
          </div>
        </div>
        <nav className={styles.dots} aria-label={DOTS_LABEL}>
          {PHOTOS.map((photo, i) => (
            <button
              className={styles.dot}
              key={photo.tone}
              type="button"
              aria-current={i === index ? "true" : "false"}
              aria-label={photo.dot}
              onClick={() => {
                setPreview(false);
                go(i);
              }}
            />
          ))}
        </nav>
      </section>
    </main>
  );
}