08정렬 순서 값이 바뀔 때 미끄러지는 전환

<main class="stage">
<div class="lr is-demo" id="lr">
  <button type="button" class="lr__sort" id="lrSort" aria-label="가격순 정렬 뒤집기">가격 오름차순</button>
  <ul class="lr__list" id="lrList">
    <li class="lr__item" data-id="a"><span class="lr__name">알파 상품</span><span class="lr__price" data-v="12000">12,000</span></li>
    <li class="lr__item" data-id="b"><span class="lr__name">베타 상품</span><span class="lr__price" data-v="4000">4,000</span></li>
    <li class="lr__item" data-id="c"><span class="lr__name">감마 상품</span><span class="lr__price" data-v="19000">19,000</span></li>
    <li class="lr__item" data-id="d"><span class="lr__name">델타 상품</span><span class="lr__price" data-v="8000">8,000</span></li>
  </ul>
</div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-enter;
$easing: $ease-spring;
$color: $subject-blue-deep;

@include stage($stage-paper);

.lr { width: min(100%, 300px); display: flex; flex-direction: column; align-items: center; gap: $sp-3; }

.lr__sort {
  border: none; border-radius: $r-pill; cursor: pointer; background: $color;
  color: $subject-cream; font: 700 11px/1 -apple-system, sans-serif; padding: $sp-2 $sp-4;
}
.lr__sort:focus-visible { outline: 2px solid $color; outline-offset: 2px; }

.lr__list { list-style: none; margin: 0; padding: 0; width: 100%; display: flex; flex-direction: column; gap: $sp-2; }
.lr__item {
  display: flex; align-items: center; justify-content: space-between; box-sizing: border-box;
  background: $subject-cream; border-radius: $r-control; padding: $sp-2 $sp-3;
}
.lr__name { font: 600 12px/1 -apple-system, sans-serif; color: $subject-ink; }
.lr__price { font: 700 12px/1 "Space Mono", monospace; color: $color; }

// 실사용 원리: 각 li 가 lr-item-a…d 라는 자기만의 이름표 값을 갖고 있으므로, 정렬 뒤 DOM 순서를
// 다시 붙여도(appendChild) 브라우저가 같은 이름표를 가진 옛·새 위치를 짝지어 미끄러뜨린다 —
// 사라졌다 나타나는 대신 카드 하나하나가 자기 새 자리까지 이동해 보인다.
::view-transition-group(*) {
  animation-duration: $duration;
  animation-timing-function: $easing;
}

@media (max-width: 340px) {
  .lr__item { padding: $sp-1 $sp-2; }
  .lr__name, .lr__price { 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: #ffffff;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.lr {
  width: min(100%, 300px);
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
}

.lr__sort {
  border: none;
  border-radius: 999px;
  cursor: pointer;
  background: #0a4fe8;
  color: #fff7e6;
  font: 700 11px/1 -apple-system, sans-serif;
  padding: 8px 16px;
}

.lr__sort:focus-visible {
  outline: 2px solid #0a4fe8;
  outline-offset: 2px;
}

.lr__list {
  list-style: none;
  margin: 0;
  padding: 0;
  width: 100%;
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.lr__item {
  display: flex;
  align-items: center;
  justify-content: space-between;
  box-sizing: border-box;
  background: #fff7e6;
  border-radius: 12px;
  padding: 8px 12px;
}

.lr__name {
  font: 600 12px/1 -apple-system, sans-serif;
  color: #17141a;
}

.lr__price {
  font: 700 12px/1 "Space Mono", monospace;
  color: #0a4fe8;
}

::view-transition-group(*) {
  animation-duration: 600ms;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
}

@media (max-width: 340px) {
  .lr__item {
    padding: 4px 8px;
  }
  .lr__name, .lr__price {
    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('lr');
  var sortBtn = document.getElementById('lrSort');
  var list = document.getElementById('lrList');
  var items = Array.prototype.slice.call(list.children);
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  // 계산 겹: 카드마다 데이터 id 값으로 고유한 이름표 값을 지어 붙인다 — 순서가 바뀌어도
  // 같은 카드는 같은 이름표를 유지하니 브라우저가 "이 카드가 저 자리로 이동했다"를 알 수 있다.
  items.forEach(function (el) {
    el.style.viewTransitionName = 'lr-item-' + el.dataset.id;
  });

  var asc = true;
  var isDemo = true;

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

  function render() {
    var sorted = items.slice().sort(function (a, b) {
      var av = Number(a.querySelector('.lr__price').dataset.v);
      var bv = Number(b.querySelector('.lr__price').dataset.v);
      return asc ? av - bv : bv - av;
    });
    sorted.forEach(function (el) { list.appendChild(el); });
    sortBtn.textContent = asc ? '가격 오름차순' : '가격 내림차순';
  }

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

  function toggleSort() {
    asc = !asc;
    change(render);
  }

  sortBtn.addEventListener('click', function () { human(); toggleSort(); });
  root.addEventListener('keydown', function (e) {
    if (e.key === 'Enter' || e.key === ' ') { human(); e.preventDefault(); toggleSort(); }
  });

  var HOLD_MS = 1000;
  function loop() {
    if (!isDemo) return;
    toggleSort();
    window.setTimeout(loop, HOLD_MS);
  }

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

// ListReorder — vanilla 판(run/401/demo/08-view-transition-list-reorder/index.html) 의 React 포트.
// 계산 겹: 카드마다 데이터 id 값으로 고유한 이름표 값을 지어 붙인다 — 순서가 바뀌어도 같은 카드는
// 같은 이름표를 유지하니 브라우저가 "이 카드가 저 자리로 이동했다"를 알 수 있다.
import { useEffect, useRef } from "react";
import styles from "./ListReorder.module.scss";

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

const ITEMS = [
  { id: "a", name: "알파 상품", price: 12000 },
  { id: "b", name: "베타 상품", price: 4000 },
  { id: "c", name: "감마 상품", price: 19000 },
  { id: "d", name: "델타 상품", price: 8000 },
];
const HOLD_MS = 1000;

export default function ListReorder() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const listRef = useRef(null) as unknown as Box<HTMLUListElement>;
  const sortBtnRef = useRef(null) as unknown as Box<HTMLButtonElement>;
  const itemRefs = useRef([null, null, null, null]) as unknown as Box<(HTMLLIElement | null)[]>;
  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 asc = true;

    const items = itemRefs.current.filter((el): el is HTMLLIElement => !!el);
    // 계산 겹: 카드마다 고유한 이름표 값을 지어 붙인다
    items.forEach((el) => {
      el.style.viewTransitionName = "lr-item-" + el.dataset.id;
    });

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

    function render() {
      const list = listRef.current;
      if (!list) return;
      const sorted = items.slice().sort((a, b) => {
        const av = Number(a.dataset.v);
        const bv = Number(b.dataset.v);
        return asc ? av - bv : bv - av;
      });
      sorted.forEach((el) => list.appendChild(el));
      if (sortBtnRef.current) sortBtnRef.current.textContent = asc ? "가격 오름차순" : "가격 내림차순";
    }

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

    function toggleSort() {
      asc = !asc;
      change(render);
    }

    function onClick() { human(); toggleSort(); }
    function onKeydown(e: KeyboardEvent) {
      if (e.key === "Enter" || e.key === " ") { human(); e.preventDefault(); toggleSort(); }
    }

    sortBtnRef.current?.addEventListener("click", onClick);
    rootRef.current?.addEventListener("keydown", onKeydown);

    function loop() {
      if (!isDemo) return;
      toggleSort();
      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);
      sortBtnRef.current?.removeEventListener("click", onClick);
      rootRef.current?.removeEventListener("keydown", onKeydown);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.lr + " " + styles.isDemo} ref={rootRef}>
        <button type="button" className={styles.sort} ref={sortBtnRef} aria-label="가격순 정렬 뒤집기">가격 오름차순</button>
        <ul className={styles.list} ref={listRef}>
          {ITEMS.map((it, i) => (
            <li key={it.id} className={styles.item} data-id={it.id} ref={(el: HTMLLIElement | null) => { itemRefs.current[i] = el; }}>
              <span className={styles.name}>{it.name}</span>
              <span className={styles.price} data-v={it.price}>{it.price.toLocaleString("ko-KR")}</span>
            </li>
          ))}
        </ul>
      </div>
    </main>
  );
}