05Split-flap board

<main class="stage">
  <div class="sf-board is-demo" aria-hidden="true">
    <span class="sf-char">S</span>
    <span class="sf-char">A</span>
    <span class="sf-char">L</span>
    <span class="sf-char">E</span>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-loop;
$easing: $ease-out;
$color: $subject-cream;

@include stage($stage-ink);

.sf-board {
  display: flex;
  gap: $sp-2;
  perspective: 600px;
}

.sf-char {
  display: inline-block;
  width: clamp(32px, 9vw, 48px);
  padding: $sp-2 0;
  border-radius: $r-sm;
  background: rgba(255, 255, 255, .08);
  color: $color;
  font: 800 clamp(20px, 6vw, 28px)/1 -apple-system, "Segoe UI", sans-serif;
  text-align: center;
  transform-origin: center;
  transform: rotateX(90deg);
  opacity: 0;
  filter: blur(3px);
}

// 데모 루프: 글자판이 위에서 아래로 뒤집히며 나타났다가 짧게 위로 되감겨 다시 뒤집힌다 (steps 로 기계적인 느낌)
// 한 겹: blur→focus — 판이 모서리로 서는 중간 지점(edge-on, rotateX 근처)에서 살짝 흐려졌다가 착지하며
// 초점이 맞는다(실제 플랩보드의 모션 블러를 흉내). steps() 타이밍은 기계적 느낌이라 건드리지 않는다.
@keyframes sf-flip {
  0% { transform: rotateX(90deg); opacity: 0; filter: blur(3px); }
  15% { filter: blur(4px); }
  30% { transform: rotateX(0deg); opacity: 1; filter: blur(0); }
  90% { transform: rotateX(0deg); opacity: 1; filter: blur(0); }
  95% { filter: blur(4px); }
  100% { transform: rotateX(90deg); opacity: 0; filter: blur(3px); }
}

.sf-board.is-demo .sf-char { animation: sf-flip $duration steps(3, end) infinite; }
.sf-board.is-demo .sf-char:nth-child(1) { animation-delay: 0ms; }
.sf-board.is-demo .sf-char:nth-child(2) { animation-delay: 120ms; }
.sf-board.is-demo .sf-char:nth-child(3) { animation-delay: 240ms; }
.sf-board.is-demo .sf-char:nth-child(4) { animation-delay: 360ms; }

@media (prefers-reduced-motion: reduce) {
  .sf-char,
  .sf-board.is-demo .sf-char {
    animation: none !important;
    opacity: 1 !important;
    transform: none !important;
    filter: 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);
}

.sf-board {
  display: flex;
  gap: 8px;
  perspective: 600px;
}

.sf-char {
  display: inline-block;
  width: clamp(32px, 9vw, 48px);
  padding: 8px 0;
  border-radius: 8px;
  background: rgba(255, 255, 255, 0.08);
  color: #fff7e6;
  font: 800 clamp(20px, 6vw, 28px)/1 -apple-system, "Segoe UI", sans-serif;
  text-align: center;
  transform-origin: center;
  transform: rotateX(90deg);
  opacity: 0;
  filter: blur(3px);
}

@keyframes sf-flip {
  0% {
    transform: rotateX(90deg);
    opacity: 0;
    filter: blur(3px);
  }
  15% {
    filter: blur(4px);
  }
  30% {
    transform: rotateX(0deg);
    opacity: 1;
    filter: blur(0);
  }
  90% {
    transform: rotateX(0deg);
    opacity: 1;
    filter: blur(0);
  }
  95% {
    filter: blur(4px);
  }
  100% {
    transform: rotateX(90deg);
    opacity: 0;
    filter: blur(3px);
  }
}
.sf-board.is-demo .sf-char {
  animation: sf-flip 2s steps(3, end) infinite;
}

.sf-board.is-demo .sf-char:nth-child(1) {
  animation-delay: 0ms;
}

.sf-board.is-demo .sf-char:nth-child(2) {
  animation-delay: 120ms;
}

.sf-board.is-demo .sf-char:nth-child(3) {
  animation-delay: 240ms;
}

.sf-board.is-demo .sf-char:nth-child(4) {
  animation-delay: 360ms;
}

@media (prefers-reduced-motion: reduce) {
  .sf-char,
  .sf-board.is-demo .sf-char {
    animation: none !important;
    opacity: 1 !important;
    transform: none !important;
    filter: none !important;
  }
}

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

export interface SplitFlapBoardProps {
  text?: string;
  duration?: string;
  stagger?: string;
}

// 실사용: 뷰포트에 들어오면 글자판이 위에서 아래로 한 번씩 시차를 두고 뒤집히며 자리 잡는다.
export default function SplitFlapBoard({
  text = "SALE",
  duration = "500ms",
  stagger = "120ms",
}: SplitFlapBoardProps) {
  const ref = useRef<HTMLDivElement>(null);
  const [inView, setInView] = useState(false);

  useEffect(() => {
    const el = ref.current;
    if (!el) return;
    const io = new IntersectionObserver((entries) => {
      entries.forEach((e) => { if (e.isIntersecting) { setInView(true); io.unobserve(e.target); } });
    }, { threshold: 0.3 });
    io.observe(el);
    return () => io.disconnect();
  }, []);

  const style = { "--duration": duration } as CSSProperties;

  return (
    <div className={styles.board} style={style} ref={ref}>
      {text.split("").map((ch, i) => (
        <span
          key={i}
          className={`${styles.char} ${inView ? styles.in : ""}`}
          style={{ transitionDelay: inView ? `calc(${stagger} * ${i})` : undefined }}
        >
          {ch}
        </span>
      ))}
    </div>
  );
}