01Text That Swaps a Fallback Font for the Real One via Lazy Loading

<main class="stage">
<div class="lff is-demo" id="lff">
  <div class="lff__badge" id="lffBadge">Showing fallback font</div>
  <h2 class="lff__head" id="lffHead" aria-label="Lazy-load headline"></h2>
  <p class="lff__hint">Click to replay</p>
</div>
</main>

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

// 대체 서체 → 실제 서체 지연 로딩. 글자마다 인덱스 값으로 transition-delay 를 미리 계산해
// 심어두면(자바스크립트) 클래스 하나만 켜져도 순서대로 스프링 오버슛이 번져 나간다.

$duration: $dur-enter;
$easing: $ease-elastic;
$color: $subject-sky;

@include stage($stage-ink);

.lff {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-4;
  width: min(100%, 300px);
  outline: none;
  cursor: pointer;
}

.lff__badge {
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(110, 200, 255, .16);
  color: $color;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.lff__head {
  margin: 0;
  padding: $sp-4 $sp-4;
  border-radius: $r-card;
  background: rgba(255, 255, 255, .06);
  color: $stage-paper;
  font-size: 22px;
  font-weight: 700;
  font-family: "Times New Roman", Georgia, serif;
  text-align: center;
  transition: font-family 1ms linear;
}

.lff__char {
  display: inline-block;
  transform: scale(.82);
  transform-origin: 50% 100%;
  opacity: .55;
  transition: transform $duration $easing, opacity $dur-base $ease-out;
}

.lff--loaded .lff__head {
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

.lff__char--loaded {
  transform: scale(1);
  opacity: 1;
}

.lff__hint {
  margin: 0;
  color: rgba(255, 255, 255, .5);
  font-size: 11px;
}

@media (max-width: 340px) {
  .lff__head { font-size: 18px; padding: $sp-3 $sp-3; }
  .lff { gap: $sp-2; }
}

@media (prefers-reduced-motion: reduce) {
  .lff__char { transition: none !important; }
  .lff__head { 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);
}

.lff {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 16px;
  width: min(100%, 300px);
  outline: none;
  cursor: pointer;
}

.lff__badge {
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(110, 200, 255, 0.16);
  color: #6ec8ff;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.lff__head {
  margin: 0;
  padding: 16px 16px;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.06);
  color: #ffffff;
  font-size: 22px;
  font-weight: 700;
  font-family: "Times New Roman", Georgia, serif;
  text-align: center;
  transition: font-family 1ms linear;
}

.lff__char {
  display: inline-block;
  transform: scale(0.82);
  transform-origin: 50% 100%;
  opacity: 0.55;
  transition: transform 600ms cubic-bezier(0.34, 1.56, 0.64, 1), opacity 300ms ease-out;
}

.lff--loaded .lff__head {
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
}

.lff__char--loaded {
  transform: scale(1);
  opacity: 1;
}

.lff__hint {
  margin: 0;
  color: rgba(255, 255, 255, 0.5);
  font-size: 11px;
}

@media (max-width: 340px) {
  .lff__head {
    font-size: 18px;
    padding: 12px 12px;
  }
  .lff {
    gap: 8px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .lff__char {
    transition: none !important;
  }
  .lff__head {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('lff');
  var badge = document.getElementById('lffBadge');
  var head = document.getElementById('lffHead');
  var TEXT = 'Lazy-load headline';
  var STAGGER_STEP_MS = 45;
  var FOUT_DELAY_MS = 700;
  var HOLD_MS = 1800;
  var RESET_DELAY_MS = 900;
  var timer = null;

  // 계산 겹: 글자 인덱스 값으로 전환 지연 시간을 계산해 순서대로 스프링 오버슛이 번진다.
  var chars = [];
  for (var i = 0; i < TEXT.length; i += 1) {
    var span = document.createElement('span');
    span.className = 'lff__char';
    span.textContent = TEXT[i] === ' ' ? ' ' : TEXT[i];
    span.style.transitionDelay = (i * STAGGER_STEP_MS) + 'ms';
    head.appendChild(span);
    chars.push(span);
  }

  function resetState() {
    root.classList.remove('lff--loaded');
    for (var i = 0; i < chars.length; i += 1) { chars[i].classList.remove('lff__char--loaded'); }
    badge.textContent = 'Showing fallback font';
  }

  function swapToLoaded() {
    root.classList.add('lff--loaded');
    for (var i = 0; i < chars.length; i += 1) { chars[i].classList.add('lff__char--loaded'); }
    badge.textContent = 'Real font loaded';
  }

  function playOnce(loop) {
    resetState();
    timer = setTimeout(function () {
      swapToLoaded();
      if (loop) {
        timer = setTimeout(function () { playOnce(true); }, HOLD_MS + RESET_DELAY_MS);
      }
    }, FOUT_DELAY_MS);
  }

  function replay() {
    if (timer) { clearTimeout(timer); }
    root.classList.remove('is-demo');
    playOnce(false);
  }
  root.addEventListener('click', replay);
  root.addEventListener('keydown', function (e) {
    if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); replay(); }
  });
  root.tabIndex = 0;

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (reduce) {
    root.classList.remove('is-demo');
    swapToLoaded();
  } else {
    playOnce(true);
  }
})();

import { useEffect, useRef, useState, useCallback } from "react";
import styles from "./FontSwapFout.module.scss";

// 대체 서체 → 실제 서체 지연 로딩. 글자 인덱스 값으로 transition-delay 를 계산해
// 순서대로 스프링 오버슛이 번지게 한다.
const TEXT = "지연 로딩 헤드라인";
const STAGGER_STEP_MS = 45;
const FOUT_DELAY_MS = 700;
const HOLD_MS = 1800;
const RESET_DELAY_MS = 900;

export default function FontSwapFout() {
  const rootRef = useRef<HTMLDivElement>(null);
  const [demo, setDemo] = useState(true);
  const [loaded, setLoaded] = useState(false);
  const [badge, setBadge] = useState("대체 서체 표시 중");
  const timerRef = useRef<number | null>(null);

  const playOnce = useCallback((loop: boolean) => {
    setLoaded(false);
    setBadge("대체 서체 표시 중");
    timerRef.current = window.setTimeout(() => {
      setLoaded(true);
      setBadge("실제 서체 로드 완료");
      if (loop) {
        timerRef.current = window.setTimeout(() => { playOnce(true); }, HOLD_MS + RESET_DELAY_MS);
      }
    }, FOUT_DELAY_MS);
  }, []);

  const replay = useCallback(() => {
    if (timerRef.current !== null) { window.clearTimeout(timerRef.current); }
    setDemo(false);
    playOnce(false);
  }, [playOnce]);

  useEffect(() => {
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (reduce) {
      setDemo(false);
      setLoaded(true);
      setBadge("실제 서체 로드 완료");
    } else {
      playOnce(true);
    }
    const root = rootRef.current;
    function onKeyDown(e: KeyboardEvent) {
      if (e.key === "Enter" || e.key === " ") { e.preventDefault(); replay(); }
    }
    root?.addEventListener("keydown", onKeyDown);
    return () => {
      if (timerRef.current !== null) { window.clearTimeout(timerRef.current); }
      root?.removeEventListener("keydown", onKeyDown);
    };
  }, [playOnce, replay]);

  const chars = TEXT.split("").map((ch, i) => (
    <span
      key={i}
      className={`${styles["lff__char"]} ${loaded ? styles["lff__char--loaded"] : ""}`}
      style={{ transitionDelay: `${i * STAGGER_STEP_MS}ms` }}
    >
      {ch === " " ? " " : ch}
    </span>
  ));

  return (
    <main className={styles.stage}>
      <div
        ref={rootRef}
        className={`${styles.lff} ${demo ? styles["is-demo"] : ""} ${loaded ? styles["lff--loaded"] : ""}`}
        tabIndex={0}
        onClick={replay}
      >
        <div className={styles["lff__badge"]}>{badge}</div>
        <h2 className={styles["lff__head"]} aria-label="지연 로딩 헤드라인">{chars}</h2>
        <p className={styles["lff__hint"]}>클릭하면 다시 재생</p>
      </div>
    </main>
  );
}