02Skeleton row append — a list computing its preview row count from the viewport height

<main class="stage">
<div class="sra is-demo" id="sra">
  <div class="sra__view" id="sraView">
    <ul class="sra__track" id="sraTrack" aria-busy="false">
      <li class="sra__sentinel" id="sraSentinel" aria-hidden="true"></li>
    </ul>
  </div>
</div>
</main>

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

// 스켈레톤 행 이어붙이기 — 스켈레톤 개수는 skeletonCountFor() 가 실측 여백 높이 값을
// 행 높이 값으로 나눠 매번 다시 정한다. 도착하면 같은 자리(replaceChild)에 실제 내용이 앉는다.

$duration: $dur-base;
$easing: $ease-out;
$color: $subject-mint;

@include stage($stage-paper);

.sra {
  width: min(100%, 280px);
}

.sra__view {
  box-sizing: border-box;
  width: 100%;
  height: 196px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: $r-card;
  border: 1px solid rgba(23, 20, 26, .1);
  background: $stage-paper;
  padding: $sp-3;
  scrollbar-width: thin;
}

.sra__track {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: $sp-2;
}

.sra__row {
  box-sizing: border-box;
  height: 32px;
  display: flex;
  align-items: center;
  padding: 0 $sp-3;
  border-radius: $r-sm;
  background: rgba(76, 212, 166, .14);
  color: $subject-ink;
  font-weight: 600;
  font-size: 12px;
}

.sra__row--skeleton {
  background: linear-gradient(
    100deg,
    rgba(23, 20, 26, .07) 30%,
    rgba(23, 20, 26, .14) 45%,
    rgba(23, 20, 26, .07) 60%
  );
  background-size: 240% 100%;
  animation: sraShimmer 1.1s ease-in-out infinite;
  animation-delay: calc(var(--stagger, 0) * -120ms);
}

.sra__sentinel {
  height: 1px;
}

@keyframes sraShimmer {
  0%   { background-position: 120% 0; }
  100% { background-position: -20% 0; }
}

@media (max-width: 340px) {
  .sra__view { height: 168px; }
  .sra__row { height: 28px; font-size: 11px; }
}

@media (prefers-reduced-motion: reduce) {
  .sra__row--skeleton { 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);
}

.sra {
  width: min(100%, 280px);
}

.sra__view {
  box-sizing: border-box;
  width: 100%;
  height: 196px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: 16px;
  border: 1px solid rgba(23, 20, 26, 0.1);
  background: #ffffff;
  padding: 12px;
  scrollbar-width: thin;
}

.sra__track {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.sra__row {
  box-sizing: border-box;
  height: 32px;
  display: flex;
  align-items: center;
  padding: 0 12px;
  border-radius: 8px;
  background: rgba(76, 212, 166, 0.14);
  color: #17141a;
  font-weight: 600;
  font-size: 12px;
}

.sra__row--skeleton {
  background: linear-gradient(100deg, rgba(23, 20, 26, 0.07) 30%, rgba(23, 20, 26, 0.14) 45%, rgba(23, 20, 26, 0.07) 60%);
  background-size: 240% 100%;
  animation: sraShimmer 1.1s ease-in-out infinite;
  animation-delay: calc(var(--stagger, 0) * -120ms);
}

.sra__sentinel {
  height: 1px;
}

@keyframes sraShimmer {
  0% {
    background-position: 120% 0;
  }
  100% {
    background-position: -20% 0;
  }
}
@media (max-width: 340px) {
  .sra__view {
    height: 168px;
  }
  .sra__row {
    height: 28px;
    font-size: 11px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sra__row--skeleton {
    animation: none !important;
  }
}

(function () {
  var view = document.getElementById('sraView');
  var track = document.getElementById('sraTrack');
  var sentinel = document.getElementById('sraSentinel');
  var root = document.getElementById('sra');

  var ROW_H = 40;   // Row 하나의 높이 값 — 미리보기 Row 개수 계산의 분모
  var TITLES = ['Morning brief', 'Weekly top', 'Suggested channels', 'New comments', 'Reading list', 'Bookmarks'];
  var LOAD_DELAY_MS = 680;
  var MAX_BATCHES = 4;

  var rowSeq = 0;
  var batches = 0;
  var loading = false;
  var dir = 1;
  var autoplayTimer = null;

  function realRow(n) {
    var li = document.createElement('li');
    li.className = 'sra__row';
    li.textContent = TITLES[n % TITLES.length] + ' ' + String(n + 1).padStart(2, '0');
    return li;
  }

  function fillInitial() {
    var need = Math.ceil(view.clientHeight / ROW_H);
    for (var i = 0; i < need; i += 1) {
      track.insertBefore(realRow(rowSeq), sentinel);
      rowSeq += 1;
    }
  }
  fillInitial();

  // 계산 겹: 지금 남아 있는 화면 높이 값을 Row 하나의 높이 값으로 나눠 미리 보여줄
  // 스켈레톤 Row 개수를 구한다 — 화면이 클수록, 스크롤 여유가 클수록 더 많이 깜빡인다.
  function skeletonCountFor() {
    var remainingViewport = view.getBoundingClientRect().bottom - sentinel.getBoundingClientRect().top;
    var visible = Math.max(remainingViewport, view.clientHeight);
    return Math.max(2, Math.ceil(visible / ROW_H));
  }

  function startLoad() {
    if (loading || batches >= MAX_BATCHES) { return; }
    loading = true;
    track.setAttribute('aria-busy', 'true');
    var count = skeletonCountFor();
    var skeletons = [];
    for (var i = 0; i < count; i += 1) {
      var li = document.createElement('li');
      li.className = 'sra__row sra__row--skeleton';
      li.style.setProperty('--stagger', String(i));
      track.insertBefore(li, sentinel);
      skeletons.push(li);
    }
    setTimeout(function () {
      skeletons.forEach(function (sk, i) {
        var real = realRow(rowSeq);
        rowSeq += 1;
        track.replaceChild(real, sk);
      });
      loading = false;
      batches += 1;
      track.setAttribute('aria-busy', 'false');
      recompute();
    }, LOAD_DELAY_MS);
  }

  function recompute() {
    var remaining = sentinel.getBoundingClientRect().top - view.getBoundingClientRect().bottom;
    if (remaining <= 24 && !loading) { startLoad(); }
  }

  view.addEventListener('scroll', recompute);

  function userTakesOver() {
    if (!root.classList.contains('is-demo')) { return; }
    root.classList.remove('is-demo');
    stopAutoplay();
  }
  view.addEventListener('wheel', userTakesOver, { passive: true });
  view.addEventListener('touchstart', userTakesOver, { passive: true });

  function autoplayTick() {
    var max = view.scrollHeight - view.clientHeight;
    if (batches >= MAX_BATCHES && view.scrollTop >= max - 1 && dir !== 0) {
      dir = 0;
      setTimeout(function () {
        while (track.firstChild !== sentinel) { track.removeChild(track.firstChild); }
        rowSeq = 0; batches = 0;
        fillInitial();
        view.scrollTop = 0;
        dir = 1;
      }, 600);
      return;
    }
    if (dir === 0) { return; }
    view.scrollTop = Math.min(view.scrollTop + 6, max);
    recompute();
  }

  function stopAutoplay() {
    if (autoplayTimer) { clearInterval(autoplayTimer); autoplayTimer = null; }
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (!reduce) {
    autoplayTimer = setInterval(autoplayTick, 60);
  }
})();

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

// 스켈레톤 행 이어붙이기 — skeletonCountFor() 가 실측 여백 높이 값을 행 높이 값으로 나눠
// 미리 보여줄 스켈레톤 행 개수를 다시 구한다. 도착하면 같은 자리에 실제 내용이 앉는다.
const ROW_H = 40;
const TITLES = ["아침 소식", "주간 인기", "추천 채널", "새 댓글", "읽던 글", "북마크"];
const LOAD_DELAY_MS = 680;
const MAX_BATCHES = 4;

export default function SkeletonRowAppend() {
  const rootRef = useRef<HTMLDivElement>(null);
  const viewRef = useRef<HTMLDivElement>(null);
  const sentinelRef = useRef<HTMLLIElement>(null);
  const [rows, setRows] = useState<string[]>([]);
  const [skeletons, setSkeletons] = useState<number>(0);
  const rowSeqRef = useRef(0);
  const batchesRef = useRef(0);
  const loadingRef = useRef(false);
  const dirRef = useRef(1);
  const autoplayRef = useRef<number | null>(null);

  const realRow = useCallback((n: number) => {
    return TITLES[n % TITLES.length] + " " + String(n + 1).padStart(2, "0");
  }, []);

  const fillInitial = useCallback(() => {
    const view = viewRef.current;
    if (!view) { return; }
    const need = Math.ceil(view.clientHeight / ROW_H);
    const start: string[] = [];
    for (let i = 0; i < need; i += 1) {
      start.push(realRow(rowSeqRef.current ?? 0));
      rowSeqRef.current = (rowSeqRef.current ?? 0) + 1;
    }
    setRows(start);
  }, [realRow]);

  // 계산 겹: 지금 남아 있는 화면 높이 값을 행 하나의 높이 값으로 나눠 미리 보여줄
  // 스켈레톤 행 개수를 구한다 — 화면이 클수록 더 많이 채운다.
  const skeletonCountFor = useCallback(() => {
    const view = viewRef.current;
    const sentinel = sentinelRef.current;
    if (!view || !sentinel) { return 2; }
    const remainingViewport = view.getBoundingClientRect().bottom - sentinel.getBoundingClientRect().top;
    const visible = Math.max(remainingViewport, view.clientHeight);
    return Math.max(2, Math.ceil(visible / ROW_H));
  }, []);

  const recompute = useCallback(() => {
    const view = viewRef.current;
    const sentinel = sentinelRef.current;
    if (!view || !sentinel) { return; }
    const remaining = sentinel.getBoundingClientRect().top - view.getBoundingClientRect().bottom;
    if (remaining <= 24 && !loadingRef.current && (batchesRef.current ?? 0) < MAX_BATCHES) {
      loadingRef.current = true;
      const count = skeletonCountFor();
      setSkeletons(count);
      window.setTimeout(() => {
        setRows((prev) => {
          const next = [...prev];
          for (let i = 0; i < count; i += 1) {
            next.push(realRow(rowSeqRef.current ?? 0));
            rowSeqRef.current = (rowSeqRef.current ?? 0) + 1;
          }
          return next;
        });
        setSkeletons(0);
        loadingRef.current = false;
        batchesRef.current = (batchesRef.current ?? 0) + 1;
      }, LOAD_DELAY_MS);
    }
  }, [realRow, skeletonCountFor]);

  useEffect(() => {
    fillInitial();
    const view = viewRef.current;
    if (!view) { return; }
    const onScroll = () => recompute();
    view.addEventListener("scroll", onScroll);
    const takeover = () => {
      rootRef.current?.classList.remove(styles["is-demo"]);
      if (autoplayRef.current !== null) { window.clearInterval(autoplayRef.current); }
    };
    view.addEventListener("wheel", takeover, { passive: true });
    view.addEventListener("touchstart", takeover, { passive: true });

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (!reduce) {
      autoplayRef.current = window.setInterval(() => {
        const v = viewRef.current;
        if (!v) { return; }
        const max = v.scrollHeight - v.clientHeight;
        if ((batchesRef.current ?? 0) >= MAX_BATCHES && v.scrollTop >= max - 1 && dirRef.current !== 0) {
          dirRef.current = 0;
          window.setTimeout(() => {
            rowSeqRef.current = 0;
            batchesRef.current = 0;
            fillInitial();
            v.scrollTop = 0;
            dirRef.current = 1;
          }, 600);
          return;
        }
        if (dirRef.current === 0) { return; }
        v.scrollTop = Math.min(v.scrollTop + 6, max);
        recompute();
      }, 60);
    }
    return () => {
      view.removeEventListener("scroll", onScroll);
      view.removeEventListener("wheel", takeover);
      view.removeEventListener("touchstart", takeover);
      if (autoplayRef.current !== null) { window.clearInterval(autoplayRef.current); }
    };
  }, [fillInitial, recompute]);

  const skeletonEls: ReactNode[] = [];
  return (
    <main className={styles.stage}>
      <div ref={rootRef} className={`${styles.sra} ${styles["is-demo"]}`} id="sra">
        <div ref={viewRef} className={styles["sra__view"]}>
          <ul className={styles["sra__track"]} aria-busy={skeletons > 0}>
            {rows.map((text, i) => (
              <li key={"r" + i} className={styles["sra__row"]}>{text}</li>
            ))}
            {Array.from({ length: skeletons }).map((_, i) => (
              <li
                key={"s" + i}
                className={`${styles["sra__row"]} ${styles["sra__row--skeleton"]}`}
                style={{ "--stagger": i } as CSSProperties}
              />
            ))}
            <li ref={sentinelRef} className={styles["sra__sentinel"]} aria-hidden="true" />
          </ul>
        </div>
      </div>
    </main>
  );
}