06移動方向の値を読んで反転するスライド遷移

<main class="stage">
<div class="ds is-demo" id="ds">
  <div class="ds__dots" id="dsDots">
    <span class="ds__dot is-active" data-i="0"></span>
    <span class="ds__dot" data-i="1"></span>
    <span class="ds__dot" data-i="2"></span>
    <span class="ds__dot" data-i="3"></span>
  </div>
  <div class="ds__panel" id="dsPanel">
    <p class="ds__num" id="dsNum">1</p>
    <p class="ds__t" id="dsT">住所入力</p>
  </div>
  <div class="ds__nav">
    <button type="button" class="ds__navBtn" id="dsPrev" aria-label="前のステップ">前へ</button>
    <button type="button" class="ds__navBtn" id="dsNext" aria-label="次のステップ">次へ</button>
  </div>
</div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-base;
$easing: $ease-out;
$color: $subject-cream;
$slide-travel-pct: 100%;

@include stage($stage-yellow);

:root { --dir: 1; }

.ds { width: min(100%, 280px); display: flex; flex-direction: column; align-items: center; gap: $sp-4; }

.ds__dots { display: flex; gap: $sp-2; }
.ds__dot { width: 8px; height: 8px; border-radius: 50%; background: rgba($subject-ink, .28); }
.ds__dot.is-active { background: $subject-ink; }

.ds__panel {
  view-transition-name: ds-step;
  position: relative; width: 100%; box-sizing: border-box; overflow: hidden;
  background: $subject-ink; border-radius: $r-card; padding: $sp-5 $sp-4;
  display: flex; flex-direction: column; align-items: center; gap: $sp-2; text-align: center;
}
.ds__num { margin: 0; font: 700 30px/1 "Space Mono", monospace; color: $color; }
.ds__t { margin: 0; font: 600 13px/1.3 -apple-system, "Segoe UI", sans-serif; color: rgba($color, .88); }

.ds__nav { display: flex; gap: $sp-3; }
.ds__navBtn {
  border: none; border-radius: $r-pill; cursor: pointer; background: rgba($subject-ink, .1);
  color: $subject-ink; font: 700 11px/1 -apple-system, sans-serif; padding: $sp-1 $sp-4;
}
.ds__navBtn:focus-visible { outline: 2px solid $subject-ink; outline-offset: 2px; }

// 실사용 원리: --dir 값(+1/-1)을 전환 시작 직전에 문서 루트에 심어 두면 view-transition 의사요소
// 트리도 그 커스텀 프로퍼티를 물려받는다 — 그래서 같은 키프레임 한 쌍으로 앞으로 갈 땐 오른쪽에서
// 뒤로 갈 땐 왼쪽에서 들어오는 반대 방향 슬라이드를 만들 수 있다.
::view-transition-old(ds-step) {
  animation: ds-out $duration $easing both;
}
::view-transition-new(ds-step) {
  animation: ds-in $duration $easing both;
}
@keyframes ds-out {
  to { transform: translateX(calc(var(--dir) * -#{$slide-travel-pct})); opacity: 0; }
}
@keyframes ds-in {
  from { transform: translateX(calc(var(--dir) * #{$slide-travel-pct})); opacity: 0; }
  to { transform: translateX(0); opacity: 1; }
}

@media (max-width: 340px) {
  .ds__panel { padding: $sp-4 $sp-3; }
  .ds__num { font-size: 24px; }
  .ds__t { font-size: 11px; }
}

@media (prefers-reduced-motion: reduce) {
  ::view-transition-group(*),
  ::view-transition-old(*),
  ::view-transition-new(*) { animation: none !important; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

:root {
  --dir: 1;
}

.ds {
  width: min(100%, 280px);
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 16px;
}

.ds__dots {
  display: flex;
  gap: 8px;
}

.ds__dot {
  width: 8px;
  height: 8px;
  border-radius: 50%;
  background: rgba(23, 20, 26, 0.28);
}

.ds__dot.is-active {
  background: #17141a;
}

.ds__panel {
  view-transition-name: ds-step;
  position: relative;
  width: 100%;
  box-sizing: border-box;
  overflow: hidden;
  background: #17141a;
  border-radius: 16px;
  padding: 20px 16px;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
  text-align: center;
}

.ds__num {
  margin: 0;
  font: 700 30px/1 "Space Mono", monospace;
  color: #fff7e6;
}

.ds__t {
  margin: 0;
  font: 600 13px/1.3 -apple-system, "Segoe UI", sans-serif;
  color: rgba(255, 247, 230, 0.88);
}

.ds__nav {
  display: flex;
  gap: 12px;
}

.ds__navBtn {
  border: none;
  border-radius: 999px;
  cursor: pointer;
  background: rgba(23, 20, 26, 0.1);
  color: #17141a;
  font: 700 11px/1 -apple-system, sans-serif;
  padding: 4px 16px;
}

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

::view-transition-old(ds-step) {
  animation: ds-out 300ms ease-out both;
}

::view-transition-new(ds-step) {
  animation: ds-in 300ms ease-out both;
}

@keyframes ds-out {
  to {
    transform: translateX(calc(var(--dir) * -100%));
    opacity: 0;
  }
}
@keyframes ds-in {
  from {
    transform: translateX(calc(var(--dir) * 100%));
    opacity: 0;
  }
  to {
    transform: translateX(0);
    opacity: 1;
  }
}
@media (max-width: 340px) {
  .ds__panel {
    padding: 16px 12px;
  }
  .ds__num {
    font-size: 24px;
  }
  .ds__t {
    font-size: 11px;
  }
}
@media (prefers-reduced-motion: reduce) {
  ::view-transition-group(*),
  ::view-transition-old(*),
  ::view-transition-new(*) {
    animation: none !important;
  }
}

(function () {
  var SUPPORTS_VT = 'startViewTransition' in document;

  var root = document.getElementById('ds');
  var panel = document.getElementById('dsPanel');
  var numEl = document.getElementById('dsNum');
  var tEl = document.getElementById('dsT');
  var dots = Array.prototype.slice.call(document.querySelectorAll('.ds__dot'));
  var prevBtn = document.getElementById('dsPrev');
  var nextBtn = document.getElementById('dsNext');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var STEPS = ['住所入力', '決済の確認', '受け取り方法', '完了'];
  var idx = 0;
  var isDemo = true;

  function human() {
    if (isDemo) { root.classList.remove('is-demo'); isDemo = false; }
  }

  function render(newIdx) {
    numEl.textContent = String(newIdx + 1);
    tEl.textContent = STEPS[newIdx];
    dots.forEach(function (d, i) { d.classList.toggle('is-active', i === newIdx); });
    idx = newIdx;
  }

  function change(fn) {
    if (SUPPORTS_VT) { document.startViewTransition(fn); }
    else { fn(); }
  }

  function goto(newIdx) {
    if (newIdx === idx || newIdx < 0 || newIdx > 3) return;
    // 계산 겹: 次へ 인덱스 - 현재 인덱스의 부호만 취해 이동 방향 값을 얻는다 (+1 앞으로 · -1 戻る)
    var dir = Math.sign(newIdx - idx) || 1;
    document.documentElement.style.setProperty('--dir', String(dir));
    change(function () { render(newIdx); });
  }

  nextBtn.addEventListener('click', function () { human(); goto(idx + 1 > 3 ? 0 : idx + 1); });
  prevBtn.addEventListener('click', function () { human(); goto(idx - 1 < 0 ? 3 : idx - 1); });
  root.addEventListener('keydown', function (e) {
    if (e.key === 'ArrowRight') { human(); goto(idx + 1 > 3 ? 0 : idx + 1); }
    else if (e.key === 'ArrowLeft') { human(); goto(idx - 1 < 0 ? 3 : idx - 1); }
  });

  var seq = [1, 2, 3, 2, 1, 0];
  var si = 0;
  var HOLD_MS = 900;
  function loop() {
    if (!isDemo) return;
    goto(seq[si % seq.length]);
    si++;
    window.setTimeout(loop, HOLD_MS);
  }

  if (!reduce) {
    window.setTimeout(loop, HOLD_MS);
  } else {
    root.classList.remove('is-demo');
  }
})();

// DirectionAwareSlide — vanilla 판(run/401/demo/06-view-transition-direction-aware-slide/index.html) 의 React 포트.
// 계산 겹: 다음 인덱스 - 현재 인덱스의 부호만 취해 이동 방향 값을 얻는다 (+1 앞으로 · -1 뒤로).
import { useEffect, useRef } from "react";
import styles from "./DirectionAwareSlide.module.scss";

type Box<T> = { current: T };

const STEPS = ["주소 입력", "결제 값 확인", "수령 방법", "완료"];
const SEQ = [1, 2, 3, 2, 1, 0];
const HOLD_MS = 900;

export default function DirectionAwareSlide() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const numRef = useRef(null) as unknown as Box<HTMLElement>;
  const tRef = useRef(null) as unknown as Box<HTMLElement>;
  const dotRefs = useRef([null, null, null, null]) as unknown as Box<(HTMLSpanElement | null)[]>;
  const prevBtnRef = useRef(null) as unknown as Box<HTMLButtonElement>;
  const nextBtnRef = useRef(null) as unknown as Box<HTMLButtonElement>;
  const timerRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    const SUPPORTS_VT = "startViewTransition" in document;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let isDemo = true;
    let idx = 0;

    function human() {
      if (isDemo) { rootRef.current?.classList.remove(styles.isDemo); isDemo = false; }
    }

    function render(newIdx: number) {
      if (numRef.current) numRef.current.textContent = String(newIdx + 1);
      if (tRef.current) tRef.current.textContent = STEPS[newIdx];
      dotRefs.current.forEach((d, i) => d?.classList.toggle(styles.isActive, i === newIdx));
      idx = newIdx;
    }

    function change(fn: () => void) {
      if (SUPPORTS_VT) { document.startViewTransition(fn); }
      else { fn(); }
    }

    function goto(newIdx: number) {
      if (newIdx === idx || newIdx < 0 || newIdx > 3) return;
      const dir = Math.sign(newIdx - idx) || 1;
      document.documentElement.style.setProperty("--dir", String(dir));
      change(() => render(newIdx));
    }

    function onNext() { human(); goto(idx + 1 > 3 ? 0 : idx + 1); }
    function onPrev() { human(); goto(idx - 1 < 0 ? 3 : idx - 1); }
    function onKeydown(e: KeyboardEvent) {
      if (e.key === "ArrowRight") { human(); goto(idx + 1 > 3 ? 0 : idx + 1); }
      else if (e.key === "ArrowLeft") { human(); goto(idx - 1 < 0 ? 3 : idx - 1); }
    }

    nextBtnRef.current?.addEventListener("click", onNext);
    prevBtnRef.current?.addEventListener("click", onPrev);
    rootRef.current?.addEventListener("keydown", onKeydown);

    let si = 0;
    function loop() {
      if (!isDemo) return;
      goto(SEQ[si % SEQ.length]);
      si++;
      timerRef.current = window.setTimeout(loop, HOLD_MS);
    }

    if (!reduce) {
      timerRef.current = window.setTimeout(loop, HOLD_MS);
    } else {
      rootRef.current?.classList.remove(styles.isDemo);
    }

    return () => {
      window.clearTimeout(timerRef.current);
      nextBtnRef.current?.removeEventListener("click", onNext);
      prevBtnRef.current?.removeEventListener("click", onPrev);
      rootRef.current?.removeEventListener("keydown", onKeydown);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.ds + " " + styles.isDemo} ref={rootRef}>
        <div className={styles.dots}>
          <span className={styles.dot + " " + styles.isActive} ref={(el: HTMLSpanElement | null) => { dotRefs.current[0] = el; }} />
          <span className={styles.dot} ref={(el: HTMLSpanElement | null) => { dotRefs.current[1] = el; }} />
          <span className={styles.dot} ref={(el: HTMLSpanElement | null) => { dotRefs.current[2] = el; }} />
          <span className={styles.dot} ref={(el: HTMLSpanElement | null) => { dotRefs.current[3] = el; }} />
        </div>
        <div className={styles.panel}>
          <p className={styles.num} ref={numRef}>1</p>
          <p className={styles.t} ref={tRef}>주소 입력</p>
        </div>
        <div className={styles.nav}>
          <button type="button" className={styles.navBtn} ref={prevBtnRef} aria-label="이전 단계">이전</button>
          <button type="button" className={styles.navBtn} ref={nextBtnRef} aria-label="다음 단계">다음</button>
        </div>
      </div>
    </main>
  );
}