04Alternating line slide

<main class="stage">
  <div class="ls-block is-demo">
    <p class="ls-line ls-line--l">DESIGN WITH</p>
    <p class="ls-line ls-line--r">INTENTION</p>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-loop;
$easing: $ease-lerp-settle; // 관성 lerp — 줄마다 목표 위치를 살짝 지나쳤다 정착한다 (2026-09-15 형태 2차 한 겹)
$color: $subject-blue;

@include stage($stage-paper);

.ls-block {
  display: grid;
  gap: $sp-1;
  text-align: center;
}

.ls-line {
  margin: 0;
  font: 800 clamp(20px, 6vw, 30px)/1.2 -apple-system, "Segoe UI", sans-serif;
  color: $color;
  opacity: 0;
  filter: blur(7px);
}

.ls-line--l { transform: translateX(-$sp-10); }
.ls-line--r { transform: translateX($sp-10); }

// 데모 루프: 홀수 줄은 왼쪽에서, 짝수 줄은 오른쪽에서 밀려 들어와 지그재그로 자리를 잡는다
// 한 겹: 각 줄이 blur(7px)→0 으로 초점이 맞으며 들어온다 — $easing(관성 lerp)과 같은 구간에서 수렴
@keyframes ls-in-l {
  0% { opacity: 0; transform: translateX(-$sp-10); filter: blur(7px); }
  25%, 80% { opacity: 1; transform: none; filter: blur(0); }
  100% { opacity: 0; transform: translateX(-$sp-10); filter: blur(7px); }
}
@keyframes ls-in-r {
  0% { opacity: 0; transform: translateX($sp-10); filter: blur(7px); }
  25%, 80% { opacity: 1; transform: none; filter: blur(0); }
  100% { opacity: 0; transform: translateX($sp-10); filter: blur(7px); }
}

.ls-block.is-demo .ls-line--l { animation: ls-in-l $duration $easing infinite; }
.ls-block.is-demo .ls-line--r { animation: ls-in-r $duration $easing infinite; animation-delay: 160ms; }

@media (prefers-reduced-motion: reduce) {
  .ls-line,
  .ls-block.is-demo .ls-line {
    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: #ffffff;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.ls-block {
  display: grid;
  gap: 4px;
  text-align: center;
}

.ls-line {
  margin: 0;
  font: 800 clamp(20px, 6vw, 30px)/1.2 -apple-system, "Segoe UI", sans-serif;
  color: #2f6df6;
  opacity: 0;
  filter: blur(7px);
}

.ls-line--l {
  transform: translateX(-40px);
}

.ls-line--r {
  transform: translateX(40px);
}

@keyframes ls-in-l {
  0% {
    opacity: 0;
    transform: translateX(-40px);
    filter: blur(7px);
  }
  25%, 80% {
    opacity: 1;
    transform: none;
    filter: blur(0);
  }
  100% {
    opacity: 0;
    transform: translateX(-40px);
    filter: blur(7px);
  }
}
@keyframes ls-in-r {
  0% {
    opacity: 0;
    transform: translateX(40px);
    filter: blur(7px);
  }
  25%, 80% {
    opacity: 1;
    transform: none;
    filter: blur(0);
  }
  100% {
    opacity: 0;
    transform: translateX(40px);
    filter: blur(7px);
  }
}
.ls-block.is-demo .ls-line--l {
  animation: ls-in-l 2s linear(0, 0.051, 0.176, 0.341, 0.518, 0.685, 0.831, 0.947, 1.032, 1.087, 1.117, 1.126, 1.121, 1.105, 1.085, 1.062, 1.041, 1.022, 1.008, 0.997, 1) infinite;
}

.ls-block.is-demo .ls-line--r {
  animation: ls-in-r 2s linear(0, 0.051, 0.176, 0.341, 0.518, 0.685, 0.831, 0.947, 1.032, 1.087, 1.117, 1.126, 1.121, 1.105, 1.085, 1.062, 1.041, 1.022, 1.008, 0.997, 1) infinite;
  animation-delay: 160ms;
}

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

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

export interface LineSlideAlternateProps {
  lines?: string[];
  duration?: string;
  easing?: string;
  stagger?: string;
}

// 실사용: 뷰포트에 들어오면 홀수 줄은 왼쪽, 짝수 줄은 오른쪽에서 한 번씩 밀려 들어온다.
export default function LineSlideAlternate({
  lines = ["DESIGN WITH", "INTENTION"],
  duration = "550ms",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  stagger = "120ms",
}: LineSlideAlternateProps) {
  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, "--easing": easing } as CSSProperties;

  return (
    <div className={styles.block} style={style} ref={ref}>
      {lines.map((line, i) => (
        <p
          key={line}
          className={`${styles.line} ${i % 2 === 0 ? styles.fromLeft : styles.fromRight} ${inView ? styles.in : ""}`}
          style={{ transitionDelay: inView ? `calc(${stagger} * ${i})` : undefined }}
        >
          {line}
        </p>
      ))}
    </div>
  );
}