06End-of-list reveal — a message appearing once the remaining-item value reaches zero

<main class="stage">
<div class="emr is-demo" id="emr">
  <div class="emr__view" id="emrView">
    <ul class="emr__track" id="emrTrack">
      <li class="emr__sentinel" id="emrSentinel" aria-hidden="true"></li>
    </ul>
    <div class="emr__end" id="emrEnd" role="status" hidden>
      <span class="emr__end-check" id="emrCheck" aria-hidden="true"></span>
      <span class="emr__end-text">Loaded up to the last notification</span>
    </div>
  </div>
  <p class="emr__hint" id="emrHint">Reaching the end fetches the next batch</p>
</div>
</main>

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

// 끝 도달 안내 — 남은 항목 값이 0으로 확정된 순간에만 revealEnd() 의 rAF 감쇠 계산이
// 실행된다. 체크는 BD16 규칙대로 같은 꼭짓점(left/top 동일)에서 출발하는 막대 두 개.

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

@include stage($stage-ink);

.emr {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-3;
  width: min(100%, 280px);
}

.emr__view {
  position: relative;
  box-sizing: border-box;
  width: 100%;
  height: 192px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: $r-card;
  background: rgba(255, 255, 255, .06);
  padding: $sp-3;
  scrollbar-width: thin;
}

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

.emr__row {
  box-sizing: border-box;
  padding: $sp-2 $sp-3;
  border-radius: $r-sm;
  background: rgba(255, 255, 255, .08);
  color: $stage-paper;
  font-weight: 600;
  font-size: 12px;
}

.emr__sentinel {
  height: 6px;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .18);
  transition: background $duration $easing;
}

.emr__sentinel--loading {
  background: $color;
}

.emr__end {
  position: sticky;
  bottom: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: $sp-2;
  box-sizing: border-box;
  margin-top: $sp-2;
  padding: $sp-3;
  border-radius: $r-sm;
  background: rgba(76, 212, 166, .16);
}

.emr__end-text {
  font-weight: 700;
  font-size: 11px;
  color: $color;
}

// 체크 막대 두 개 — 같은 꼭짓점(4px 8px)에서 출발해 rotate 로 벌어진다 (BD16)
.emr__end-check {
  position: relative;
  display: inline-block;
  width: 12px;
  height: 12px;
}

.emr__end-check::before,
.emr__end-check::after {
  content: "";
  position: absolute;
  left: 4px;
  top: 8px;
  height: 2px;
  border-radius: $r-pill;
  background: $color;
  transform-origin: left center;
}

.emr__end-check::before {
  width: 5px;
  transform: rotate(45deg);
}

.emr__end-check::after {
  width: 9px;
  transform: rotate(-45deg);
}

.emr__hint {
  margin: 0;
  font-size: 11px;
  color: rgba(255, 255, 255, .55);
  text-align: center;
}

@media (max-width: 340px) {
  .emr { gap: $sp-1; }
  .emr__view { height: 90px; }
  .emr__row { font-size: 11px; }
}

@media (prefers-reduced-motion: reduce) {
  .emr__sentinel { 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: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.emr {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
  width: min(100%, 280px);
}

.emr__view {
  position: relative;
  box-sizing: border-box;
  width: 100%;
  height: 192px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.06);
  padding: 12px;
  scrollbar-width: thin;
}

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

.emr__row {
  box-sizing: border-box;
  padding: 8px 12px;
  border-radius: 8px;
  background: rgba(255, 255, 255, 0.08);
  color: #ffffff;
  font-weight: 600;
  font-size: 12px;
}

.emr__sentinel {
  height: 6px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.18);
  transition: background 300ms ease-out;
}

.emr__sentinel--loading {
  background: #4cd4a6;
}

.emr__end {
  position: sticky;
  bottom: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  box-sizing: border-box;
  margin-top: 8px;
  padding: 12px;
  border-radius: 8px;
  background: rgba(76, 212, 166, 0.16);
}

.emr__end-text {
  font-weight: 700;
  font-size: 11px;
  color: #4cd4a6;
}

.emr__end-check {
  position: relative;
  display: inline-block;
  width: 12px;
  height: 12px;
}

.emr__end-check::before,
.emr__end-check::after {
  content: "";
  position: absolute;
  left: 4px;
  top: 8px;
  height: 2px;
  border-radius: 999px;
  background: #4cd4a6;
  transform-origin: left center;
}

.emr__end-check::before {
  width: 5px;
  transform: rotate(45deg);
}

.emr__end-check::after {
  width: 9px;
  transform: rotate(-45deg);
}

.emr__hint {
  margin: 0;
  font-size: 11px;
  color: rgba(255, 255, 255, 0.55);
  text-align: center;
}

@media (max-width: 340px) {
  .emr {
    gap: 4px;
  }
  .emr__view {
    height: 90px;
  }
  .emr__row {
    font-size: 11px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .emr__sentinel {
    transition: none !important;
  }
}

(function () {
  var view = document.getElementById('emrView');
  var track = document.getElementById('emrTrack');
  var sentinel = document.getElementById('emrSentinel');
  var endBox = document.getElementById('emrEnd');
  var hint = document.getElementById('emrHint');
  var root = document.getElementById('emr');

  var TOTAL_PAGES = 3;
  var PAGE_SIZE = 4;
  var LOAD_DELAY_MS = 560;
  var REVEAL_PX = 34;   // 안내 문구가 아래에서 밀려 올라오는 거리 값

  var page = 0;
  var loading = false;
  var exhausted = false;
  var rowSeq = 0;
  var revealRaf = null;
  var dir = 1;
  var autoplayTimer = null;

  function addRow() {
    var li = document.createElement('li');
    li.className = 'emr__row';
    li.textContent = 'Alert ' + String(rowSeq + 1).padStart(2, '0');
    track.insertBefore(li, sentinel);
    rowSeq += 1;
  }
  for (var i = 0; i < 4; i += 1) { addRow(); }

  // 계산 겹: 안내 문구의 올라오는 정도를 고정 트랜지션이 아니라 rAF 로 매 프레임 계산한다 —
  // 시작 값을 REVEAL_PX 로 두고 목표 0 까지 지수 감쇠로 당긴다. 남은 항목 값이 0이 되기 전엔
  // 이 계산 자체가 실행되지 않는다(한 번 확정되면 다시 계산되지 않는다).
  function revealEnd() {
    endBox.hidden = false;
    var y = REVEAL_PX;
    function tick() {
      y += (0 - y) * 0.18;
      endBox.style.transform = 'translateY(' + y.toFixed(2) + 'px)';
      if (y > 0.4) { revealRaf = requestAnimationFrame(tick); }
      else { endBox.style.transform = 'translateY(0px)'; revealRaf = null; }
    }
    revealRaf = requestAnimationFrame(tick);
  }

  function recompute() {
    if (exhausted) { return; }   // 남은 항목 값이 0 — 끝에 닿아도 다시 계산하지 않는다
    var remaining = sentinel.getBoundingClientRect().top - view.getBoundingClientRect().bottom;
    if (remaining <= 2 && !loading) { startLoad(); }
  }

  function startLoad() {
    loading = true;
    sentinel.classList.add('emr__sentinel--loading');
    setTimeout(function () {
      page += 1;
      for (var n = 0; n < PAGE_SIZE; n += 1) { addRow(); }
      sentinel.classList.remove('emr__sentinel--loading');
      loading = false;
      if (page >= TOTAL_PAGES) {
        exhausted = true;      // 남은 항목 값 0 확정 — 이 계산은 두 번 다시 일어나지 않는다
        sentinel.remove();
        revealEnd();
        hint.textContent = 'The remaining-item value is zero — reaching the edge sends no request';
      }
    }, LOAD_DELAY_MS);
  }

  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 (exhausted && view.scrollTop >= max - 1 && dir !== 0) {
      dir = 0;
      setTimeout(function () {
        while (track.firstChild) { track.removeChild(track.firstChild); }
        rowSeq = 0; page = 0; exhausted = false; dir = 1;
        endBox.hidden = true;
        track.appendChild(sentinel);
        for (var i = 0; i < 4; i += 1) { addRow(); }
        view.scrollTop = 0;
        hint.textContent = 'Reaching the end fetches the next batch';
      }, 700);
      return;
    }
    if (dir === 0) { return; }
    view.scrollTop = Math.min(view.scrollTop + 5, 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 styles from "./EndMessageReveal.module.scss";

// 끝 도달 안내 — 남은 항목 값이 0으로 확정된 순간에만 revealEnd() 의 rAF 감쇠 계산이 실행된다.
// 계산 겹: 시작 값 REVEAL_PX 에서 목표 0 까지 지수 감쇠로 매 프레임 당긴다.
const TOTAL_PAGES = 3;
const PAGE_SIZE = 4;
const LOAD_DELAY_MS = 560;
const REVEAL_PX = 34;

export default function EndMessageReveal() {
  const rootRef = useRef<HTMLDivElement>(null);
  const viewRef = useRef<HTMLDivElement>(null);
  const sentinelRef = useRef<HTMLLIElement>(null);
  const endRef = useRef<HTMLDivElement>(null);
  const [rows, setRows] = useState(4);
  const [exhausted, setExhausted] = useState(false);
  const [hint, setHint] = useState("끝에 닿으면 다음 묶음을 부릅니다");
  const loadingRef = useRef(false);
  const pageRef = useRef(0);
  const rowsRef = useRef(4);
  rowsRef.current = rows;
  const exhaustedRef = useRef(false);
  exhaustedRef.current = exhausted;
  const revealRafRef = useRef<number | null>(null);
  const dirRef = useRef(1);
  const autoplayRef = useRef<number | null>(null);

  // 계산 겹: 안내 문구의 올라오는 정도를 고정 트랜지션이 아니라 rAF 로 매 프레임 계산한다 —
  // 남은 항목 값이 0이 되기 전엔 이 계산 자체가 실행되지 않는다.
  const revealEnd = useCallback(() => {
    const endBox = endRef.current;
    if (!endBox) { return; }
    let y = REVEAL_PX;
    const tick = () => {
      y += (0 - y) * 0.18;
      endBox.style.transform = "translateY(" + y.toFixed(2) + "px)";
      if (y > 0.4) {
        revealRafRef.current = window.requestAnimationFrame(tick);
      } else {
        endBox.style.transform = "translateY(0px)";
        revealRafRef.current = null;
      }
    };
    revealRafRef.current = window.requestAnimationFrame(tick);
  }, []);

  const recompute = useCallback(() => {
    if (exhaustedRef.current) { return; }   // 남은 항목 값 0 — 끝에 닿아도 다시 계산하지 않는다
    const view = viewRef.current;
    const sentinel = sentinelRef.current;
    if (!view || !sentinel) { return; }
    const remaining = sentinel.getBoundingClientRect().top - view.getBoundingClientRect().bottom;
    if (remaining <= 2 && !loadingRef.current) {
      loadingRef.current = true;
      sentinel.classList.add(styles["emr__sentinel--loading"]);
      window.setTimeout(() => {
        pageRef.current = (pageRef.current ?? 0) + 1;
        setRows((prev) => prev + PAGE_SIZE);
        sentinel.classList.remove(styles["emr__sentinel--loading"]);
        loadingRef.current = false;
        if ((pageRef.current ?? 0) >= TOTAL_PAGES) {
          setExhausted(true);      // 남은 항목 값 0 확정 — 이 계산은 두 번 다시 일어나지 않는다
          revealEnd();
          setHint("남은 항목 값이 0입니다 — 더 닿아도 요청이 없습니다");
        }
      }, LOAD_DELAY_MS);
    }
  }, [revealEnd]);

  useEffect(() => {
    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 (exhaustedRef.current && v.scrollTop >= max - 1 && dirRef.current !== 0) {
          dirRef.current = 0;
          window.setTimeout(() => {
            setRows(4);
            pageRef.current = 0;
            setExhausted(false);
            dirRef.current = 1;
            v.scrollTop = 0;
            setHint("끝에 닿으면 다음 묶음을 부릅니다");
          }, 700);
          return;
        }
        if (dirRef.current === 0) { return; }
        v.scrollTop = Math.min(v.scrollTop + 5, max);
        recompute();
      }, 60);
    }
    return () => {
      view.removeEventListener("scroll", onScroll);
      view.removeEventListener("wheel", takeover);
      view.removeEventListener("touchstart", takeover);
      if (autoplayRef.current !== null) { window.clearInterval(autoplayRef.current); }
      if (revealRafRef.current !== null) { window.cancelAnimationFrame(revealRafRef.current); }
    };
  }, [recompute]);

  const rowEls = [];
  for (let i = 0; i < rows; i += 1) {
    rowEls.push(
      <li key={i} className={styles["emr__row"]}>알림 {String(i + 1).padStart(2, "0")}</li>
    );
  }

  return (
    <main className={styles.stage}>
      <div ref={rootRef} className={`${styles.emr} ${styles["is-demo"]}`} id="emr">
        <div ref={viewRef} className={styles["emr__view"]}>
          <ul className={styles["emr__track"]}>
            {rowEls}
            {!exhausted && (
              <li ref={sentinelRef} className={styles["emr__sentinel"]} aria-hidden="true" />
            )}
          </ul>
          {exhausted && (
            <div ref={endRef} className={styles["emr__end"]} role="status">
              <span className={styles["emr__end-check"]} aria-hidden="true" />
              <span className={styles["emr__end-text"]}>마지막 알림까지 다 불러왔습니다</span>
            </div>
          )}
        </div>
        <p className={styles["emr__hint"]}>{hint}</p>
      </div>
    </main>
  );
}