09Multi-pair comparison carousel

<main class="stage">
  <div class="track" id="track" aria-label="Before/after comparison carousel">
    <div class="ba is-demo is-active" data-i="1">
      <div class="ba__after"></div>
      <div class="ba__before"></div>
    </div>
    <div class="ba is-demo" data-i="2">
      <div class="ba__after"></div>
      <div class="ba__before"></div>
    </div>
    <div class="ba is-demo" data-i="3">
      <div class="ba__after"></div>
      <div class="ba__before"></div>
    </div>
  </div>
</main>

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

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

@property --reveal {
  syntax: "<percentage>";
  inherits: true;
  initial-value: 50%;
}

@include stage($stage-ink);

.track {
  display: flex;
  gap: $sp-3;
  width: 260px;
  max-width: 100%;
  overflow-x: auto;
  scroll-snap-type: x mandatory;
  border-radius: $r-card;
}
.ba {
  position: relative;
  flex: 0 0 200px;
  aspect-ratio: 4 / 3;
  scroll-snap-align: center;
  border-radius: $r-card;
  overflow: hidden;
  box-shadow: $shadow-press;
  touch-action: pan-x;
  --reveal: 50%;
}
.ba.is-active { box-shadow: $shadow-raised, 0 0 0 $sp-1 $subject-cream; cursor: ew-resize; }
.ba__after,
.ba__before {
  position: absolute;
  inset: 0;
}
.ba__after { background: linear-gradient(155deg, $color 0%, $subject-cream 100%); }
.ba__before {
  background: linear-gradient(155deg, $stage-ink 0%, $color 70%);
  clip-path: inset(0 calc(100% - var(--reveal)) 0 0);
}

.ba.is-demo { animation: reveal-loop $dur-loop $easing infinite; }
.ba.is-demo[data-i="2"] { animation-delay: -0.6s; }
.ba.is-demo[data-i="3"] { animation-delay: -1.2s; }

@keyframes reveal-loop {
  0%, 8%   { --reveal: 20%; }
  50%      { --reveal: 80%; }
  92%, 100% { --reveal: 20%; }
}

@media (prefers-reduced-motion: reduce) {
  .ba.is-demo { animation: none; --reveal: 50%; }
}

@property --reveal {
  syntax: "<percentage>";
  inherits: true;
  initial-value: 50%;
}
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);
}

.track {
  display: flex;
  gap: 12px;
  width: 260px;
  max-width: 100%;
  overflow-x: auto;
  scroll-snap-type: x mandatory;
  border-radius: 16px;
}

.ba {
  position: relative;
  flex: 0 0 200px;
  aspect-ratio: 4/3;
  scroll-snap-align: center;
  border-radius: 16px;
  overflow: hidden;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  touch-action: pan-x;
  --reveal: 50%;
}

.ba.is-active {
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32), 0 0 0 4px #fff7e6;
  cursor: ew-resize;
}

.ba__after,
.ba__before {
  position: absolute;
  inset: 0;
}

.ba__after {
  background: linear-gradient(155deg, #2f6df6 0%, #fff7e6 100%);
}

.ba__before {
  background: linear-gradient(155deg, #17141a 0%, #2f6df6 70%);
  clip-path: inset(0 calc(100% - var(--reveal)) 0 0);
}

.ba.is-demo {
  animation: reveal-loop 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

.ba.is-demo[data-i="2"] {
  animation-delay: -0.6s;
}

.ba.is-demo[data-i="3"] {
  animation-delay: -1.2s;
}

@keyframes reveal-loop {
  0%, 8% {
    --reveal: 20%;
  }
  50% {
    --reveal: 80%;
  }
  92%, 100% {
    --reveal: 20%;
  }
}
@media (prefers-reduced-motion: reduce) {
  .ba.is-demo {
    animation: none;
    --reveal: 50%;
  }
}

(function () {
  var track = document.getElementById("track");
  var panes = Array.prototype.slice.call(track.querySelectorAll(".ba"));

  panes.forEach(function (pane) {
    var dragging = false;
    function set(pct) {
      pct = Math.max(0, Math.min(100, pct));
      pane.style.setProperty("--reveal", pct + "%");
    }
    function fromX(x) {
      var r = pane.getBoundingClientRect();
      return ((x - r.left) / r.width) * 100;
    }
    pane.addEventListener("pointerdown", function (e) {
      if (!pane.classList.contains("is-active")) return;
      pane.classList.remove("is-demo");
      dragging = true;
      pane.setPointerCapture(e.pointerId);
      set(fromX(e.clientX));
    });
    pane.addEventListener("pointermove", function (e) { if (dragging) set(fromX(e.clientX)); });
    pane.addEventListener("pointerup", function () { dragging = false; });
  });

  if ("IntersectionObserver" in window) {
    var io = new IntersectionObserver(function (entries) {
      entries.forEach(function (en) {
        en.target.classList.toggle("is-active", en.intersectionRatio > 0.6);
      });
    }, { root: track, threshold: [0.6] });
    panes.forEach(function (p) { io.observe(p); });
  }
})();

import { useRef, useState, useEffect, useCallback } from "react";
import type { CSSProperties, PointerEvent } from "react";
import styles from "./BeforeAfterMultiCarousel.module.scss";

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

type Vars = CSSProperties & { "--reveal"?: string };

/** 전후 슬라이더 여러 쌍을 scroll-snap 캐러셀에 나란히 두고, IntersectionObserver 로 화면 중앙에 온 것만 드래그를 켠다. */
export default function BeforeAfterMultiCarousel({
  duration = "2s",
  easing = "cubic-bezier(.2,.8,.2,1)",
  color = "#2f6df6",
  pairs = 3,
}: BeforeAfterMultiCarouselProps) {
  const trackRef = useRef<HTMLDivElement | null>(null);
  const paneRefs = useRef<(HTMLDivElement | null)[]>([]);
  const [reveals, setReveals] = useState<number[]>(() => Array(pairs).fill(50));
  const [demoIdx, setDemoIdx] = useState<Set<number>>(() => new Set(Array.from({ length: pairs }, (_, i) => i)));
  const [activeIdx, setActiveIdx] = useState(0);
  const dragging = useRef<number | null>(null);

  useEffect(() => {
    const track = trackRef.current;
    if (!track || !("IntersectionObserver" in window)) return;
    const io = new IntersectionObserver(
      (entries) => {
        entries.forEach((en) => {
          if (en.intersectionRatio > 0.6) {
            const i = (paneRefs.current || []).indexOf(en.target as HTMLDivElement);
            if (i >= 0) setActiveIdx(i);
          }
        });
      },
      { root: track, threshold: [0.6] }
    );
    (paneRefs.current || []).forEach((p) => p && io.observe(p));
    return () => io.disconnect();
  }, [pairs]);

  const setFromX = useCallback((i: number, x: number) => {
    const el = (paneRefs.current || [])[i];
    if (!el) return;
    const r = el.getBoundingClientRect();
    const pct = Math.max(0, Math.min(100, ((x - r.left) / r.width) * 100));
    setReveals((arr) => arr.map((v, idx) => (idx === i ? pct : v)));
  }, []);

  const onDown = useCallback((i: number) => (e: PointerEvent<HTMLDivElement>) => {
    if (activeIdx !== i) return;
    setDemoIdx((s) => { const n = new Set(s); n.delete(i); return n; });
    dragging.current = i;
    (e.currentTarget as HTMLDivElement).setPointerCapture(e.pointerId);
    setFromX(i, e.clientX);
  }, [activeIdx, setFromX]);
  const onMove = useCallback((i: number) => (e: PointerEvent<HTMLDivElement>) => {
    if (dragging.current === i) setFromX(i, e.clientX);
  }, [setFromX]);
  const onUp = useCallback(() => { dragging.current = null; }, []);

  return (
    <div className={styles.track} ref={trackRef}>
      {reveals.map((reveal, i) => {
        const vars: Vars = { "--reveal": `${reveal}%`, ["--duration" as string]: duration, ["--easing" as string]: easing, ["--color" as string]: color };
        const isDemo = demoIdx.has(i);
        const isActive = activeIdx === i;
        const cls = [styles.ba, isDemo ? styles.isDemo : "", isActive ? styles.isActive : ""].filter(Boolean).join(" ");
        return (
          <div
            key={i}
            ref={(el: HTMLDivElement | null) => { if (paneRefs.current) paneRefs.current[i] = el; }}
            className={cls}
            style={vars}
            onPointerDown={onDown(i)}
            onPointerMove={onMove(i)}
            onPointerUp={onUp}
          >
            <div className={styles.after} />
            <div className={styles.before} />
          </div>
        );
      })}
    </div>
  );
}