02Background Chosen by a Screen-width Value via Lazy Loading

<main class="stage">
<div class="lbs is-demo" id="lbs">
  <div class="lbs__badge" id="lbsBadge">Width 180px · low-res</div>
  <div class="lbs__frame" id="lbsFrame">
    <div class="lbs__placeholder" id="lbsPlaceholder"></div>
    <div class="lbs__bg lbs__bg--low" id="lbsBgLow"></div>
    <div class="lbs__bg lbs__bg--high" id="lbsBgHigh"></div>
  </div>
  <p class="lbs__hint">Click to change the width</p>
</div>
</main>

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

// 화면 너비 값 → 해상도 등급. ResizeObserver 가 프레임 폭을 재고, 기준값보다 좁으면
// 저해상도, 넓으면 고해상도 배경을 고른 뒤 진행률(경과/전체 시간)로 블러·불투명도를 계산한다.

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

@include stage($stage-paper);

.lbs {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-3;
  outline: none;
  cursor: pointer;
}

.lbs__badge {
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(76, 212, 166, .16);
  color: darken($color, 18%);
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.lbs__frame {
  position: relative;
  box-sizing: border-box;
  width: 180px;
  height: 160px;
  border-radius: $r-card;
  overflow: hidden;
  transition: width $duration $easing;
  background: $subject-cream;
  box-shadow: $shadow-raised;
}

.lbs__placeholder {
  position: absolute;
  inset: 0;
  background: linear-gradient(135deg, rgba(23, 20, 26, .08), rgba(23, 20, 26, .16));
  transition: opacity $dur-quick $ease-out;
}

.lbs__placeholder--hidden {
  opacity: 0;
}

.lbs__bg {
  position: absolute;
  inset: 0;
  opacity: 0;
}

.lbs__bg--low {
  background: linear-gradient(160deg, $subject-mint, $subject-sky);
}

.lbs__bg--high {
  background:
    radial-gradient(circle at 30% 30%, rgba(255, 255, 255, .5), transparent 55%),
    radial-gradient(circle at 75% 65%, rgba(255, 255, 255, .35), transparent 45%),
    linear-gradient(160deg, $subject-mint, $subject-lilac);
}

.lbs__hint {
  margin: 0;
  color: rgba(23, 20, 26, .5);
  font-size: 11px;
}

@media (max-width: 340px) {
  .lbs { gap: $sp-2; }
  .lbs__frame { width: 140px !important; height: 92px; }
}

@media (prefers-reduced-motion: reduce) {
  .lbs__frame { transition: none !important; }
  .lbs__placeholder { 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: #ffffff;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.lbs {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
  outline: none;
  cursor: pointer;
}

.lbs__badge {
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(76, 212, 166, 0.16);
  color: rgb(14.9030206677%, 62.0381558029%, 46.0953895072%);
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.lbs__frame {
  position: relative;
  box-sizing: border-box;
  width: 180px;
  height: 160px;
  border-radius: 16px;
  overflow: hidden;
  transition: width 300ms ease-out;
  background: #fff7e6;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.lbs__placeholder {
  position: absolute;
  inset: 0;
  background: linear-gradient(135deg, rgba(23, 20, 26, 0.08), rgba(23, 20, 26, 0.16));
  transition: opacity 220ms ease-out;
}

.lbs__placeholder--hidden {
  opacity: 0;
}

.lbs__bg {
  position: absolute;
  inset: 0;
  opacity: 0;
}

.lbs__bg--low {
  background: linear-gradient(160deg, #4cd4a6, #6ec8ff);
}

.lbs__bg--high {
  background: radial-gradient(circle at 30% 30%, rgba(255, 255, 255, 0.5), transparent 55%), radial-gradient(circle at 75% 65%, rgba(255, 255, 255, 0.35), transparent 45%), linear-gradient(160deg, #4cd4a6, #b9a0ff);
}

.lbs__hint {
  margin: 0;
  color: rgba(23, 20, 26, 0.5);
  font-size: 11px;
}

@media (max-width: 340px) {
  .lbs {
    gap: 8px;
  }
  .lbs__frame {
    width: 140px !important;
    height: 92px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .lbs__frame {
    transition: none !important;
  }
  .lbs__placeholder {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('lbs');
  var frame = document.getElementById('lbsFrame');
  var badge = document.getElementById('lbsBadge');
  var placeholder = document.getElementById('lbsPlaceholder');
  var bgLow = document.getElementById('lbsBgLow');
  var bgHigh = document.getElementById('lbsBgHigh');

  var BREAKPOINT_PX = 240;
  var WIDTH_NARROW = 180;
  var WIDTH_WIDE = 300;
  var LOAD_DELAY_MS = 480;
  var CROSSFADE_MS = 480;
  var CYCLE_MS = 2200;

  var lastTier = null;
  var raf = null;
  var loadTimer = null;

  // 계산 겹: 경과 시간 값을 전체 전환 시간 값으로 나눠 진행률을 구하고,
  // 그 진행률로 블러 반경과 불투명도를 매 프레임 다시 계산한다.
  function crossfadeIn(el, duration) {
    var start = performance.now();
    if (raf) { cancelAnimationFrame(raf); }
    function step(now) {
      var progress = Math.min((now - start) / duration, 1);
      el.style.opacity = String(progress);
      el.style.filter = 'blur(' + (6 * (1 - progress)) + 'px)';
      if (progress < 1) { raf = requestAnimationFrame(step); }
    }
    raf = requestAnimationFrame(step);
  }

  function applyTier(widthPx) {
    var tier = widthPx < BREAKPOINT_PX ? 'low' : 'high';
    badge.textContent = 'Width ' + Math.round(widthPx) + 'px · ' + (tier === 'low' ? 'low-res' : 'high-res');
    if (tier === lastTier) { return; }
    lastTier = tier;
    bgLow.style.opacity = '0';
    bgHigh.style.opacity = '0';
    placeholder.classList.remove('lbs__placeholder--hidden');
    if (loadTimer) { clearTimeout(loadTimer); }
    loadTimer = setTimeout(function () {
      var target = tier === 'low' ? bgLow : bgHigh;
      placeholder.classList.add('lbs__placeholder--hidden');
      crossfadeIn(target, CROSSFADE_MS);
    }, LOAD_DELAY_MS);
  }

  var ro = new ResizeObserver(function (entries) {
    for (var i = 0; i < entries.length; i += 1) {
      applyTier(entries[i].contentRect.width);
    }
  });
  ro.observe(frame);

  function setWide(wide) {
    frame.style.width = (wide ? WIDTH_WIDE : WIDTH_NARROW) + 'px';
  }

  var wide = false;
  function toggle() {
    wide = !wide;
    setWide(wide);
  }
  root.addEventListener('click', toggle);
  root.addEventListener('keydown', function (e) {
    if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); toggle(); }
  });
  root.tabIndex = 0;

  var autoplayTimer = null;
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (reduce) {
    root.classList.remove('is-demo');
    setWide(true);
    lastTier = 'high';
    placeholder.classList.add('lbs__placeholder--hidden');
    bgHigh.style.opacity = '1';
    bgHigh.style.filter = 'blur(0px)';
    badge.textContent = 'Width ' + WIDTH_WIDE + 'px · high-res';
  } else {
    setWide(false);
    autoplayTimer = setInterval(function () {
      if (!root.classList.contains('is-demo')) { clearInterval(autoplayTimer); return; }
      toggle();
    }, CYCLE_MS);
  }

  function userTakesOver() {
    root.classList.remove('is-demo');
  }
  root.addEventListener('click', userTakesOver);
})();

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

// 화면 너비 값 → 해상도 등급. ResizeObserver 가 프레임 폭을 재고, 기준값보다 좁으면
// 저해상도, 넓으면 고해상도 배경을 고른 뒤 진행률로 블러·불투명도를 계산한다.
const BREAKPOINT_PX = 240;
const WIDTH_NARROW = 180;
const WIDTH_WIDE = 300;
const LOAD_DELAY_MS = 480;
const CROSSFADE_MS = 480;
const CYCLE_MS = 2200;

export default function BackgroundImageSwap() {
  const rootRef = useRef<HTMLDivElement>(null);
  const frameRef = useRef<HTMLDivElement>(null);
  const placeholderRef = useRef<HTMLDivElement>(null);
  const bgLowRef = useRef<HTMLDivElement>(null);
  const bgHighRef = useRef<HTMLDivElement>(null);
  const [demo, setDemo] = useState(true);
  const [wideState, setWideState] = useState(false);
  const [badge, setBadge] = useState("폭 180px · 저해상도");
  const lastTierRef = useRef<string | null>(null);
  const rafRef = useRef<number | null>(null);
  const loadTimerRef = useRef<number | null>(null);
  const wideRef = useRef(false);

  const crossfadeIn = useCallback((el: HTMLDivElement, duration: number) => {
    const start = performance.now();
    if (rafRef.current !== null) { cancelAnimationFrame(rafRef.current); }
    function step(now: number) {
      const progress = Math.min((now - start) / duration, 1);
      el.style.opacity = String(progress);
      el.style.filter = "blur(" + (6 * (1 - progress)) + "px)";
      if (progress < 1) { rafRef.current = requestAnimationFrame(step); }
    }
    rafRef.current = requestAnimationFrame(step);
  }, []);

  const applyTier = useCallback((widthPx: number) => {
    const tier = widthPx < BREAKPOINT_PX ? "low" : "high";
    setBadge("폭 " + Math.round(widthPx) + "px · " + (tier === "low" ? "저해상도" : "고해상도"));
    if (tier === lastTierRef.current) { return; }
    lastTierRef.current = tier;
    const low = bgLowRef.current;
    const high = bgHighRef.current;
    const placeholder = placeholderRef.current;
    if (!low || !high || !placeholder) { return; }
    low.style.opacity = "0";
    high.style.opacity = "0";
    placeholder.classList.remove(styles["lbs__placeholder--hidden"]);
    if (loadTimerRef.current !== null) { window.clearTimeout(loadTimerRef.current); }
    loadTimerRef.current = window.setTimeout(() => {
      const target = tier === "low" ? low : high;
      placeholder.classList.add(styles["lbs__placeholder--hidden"]);
      crossfadeIn(target, CROSSFADE_MS);
    }, LOAD_DELAY_MS);
  }, [crossfadeIn]);

  const toggle = useCallback(() => {
    wideRef.current = !wideRef.current;
    setWideState(wideRef.current);
    setDemo(false);
  }, []);

  useEffect(() => {
    const frame = frameRef.current;
    if (!frame) { return; }
    const ro = new ResizeObserver((entries) => {
      for (const entry of entries) { applyTier(entry.contentRect.width); }
    });
    ro.observe(frame);

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let autoplayTimer: number | null = null;
    if (reduce) {
      setDemo(false);
      wideRef.current = true;
      setWideState(true);
      lastTierRef.current = "high";
      placeholderRef.current?.classList.add(styles["lbs__placeholder--hidden"]);
      if (bgHighRef.current) {
        bgHighRef.current.style.opacity = "1";
        bgHighRef.current.style.filter = "blur(0px)";
      }
      setBadge("폭 " + WIDTH_WIDE + "px · 고해상도");
    } else {
      autoplayTimer = window.setInterval(() => {
        if (!rootRef.current?.classList.contains(styles["is-demo"])) {
          if (autoplayTimer !== null) { window.clearInterval(autoplayTimer); }
          return;
        }
        wideRef.current = !wideRef.current;
        setWideState(wideRef.current);
      }, CYCLE_MS);
    }
    return () => {
      ro.disconnect();
      if (autoplayTimer !== null) { window.clearInterval(autoplayTimer); }
      if (rafRef.current !== null) { cancelAnimationFrame(rafRef.current); }
      if (loadTimerRef.current !== null) { window.clearTimeout(loadTimerRef.current); }
    };
  }, [applyTier]);

  useEffect(() => {
    const root = rootRef.current;
    function onKeyDown(e: KeyboardEvent) {
      if (e.key === "Enter" || e.key === " ") { e.preventDefault(); toggle(); }
    }
    root?.addEventListener("keydown", onKeyDown);
    return () => root?.removeEventListener("keydown", onKeyDown);
  }, [toggle]);

  return (
    <main className={styles.stage}>
      <div
        ref={rootRef}
        className={`${styles.lbs} ${demo ? styles["is-demo"] : ""}`}
        tabIndex={0}
        onClick={toggle}
      >
        <div className={styles["lbs__badge"]}>{badge}</div>
        <div
          ref={frameRef}
          className={styles["lbs__frame"]}
          style={{ width: (wideState ? WIDTH_WIDE : WIDTH_NARROW) + "px" }}
        >
          <div ref={placeholderRef} className={styles["lbs__placeholder"]} />
          <div ref={bgLowRef} className={`${styles["lbs__bg"]} ${styles["lbs__bg--low"]}`} />
          <div ref={bgHighRef} className={`${styles["lbs__bg"]} ${styles["lbs__bg--high"]}`} />
        </div>
        <p className={styles["lbs__hint"]}>클릭하면 폭이 바뀝니다</p>
      </div>
    </main>
  );
}