07startViewTransition + state update

<main class="stage">
  <div class="vt-frame is-demo">
    <div class="vt-layer vt-layer--a">
      <span class="vt-chip">List view</span>
    </div>
    <div class="vt-layer vt-layer--b">
      <span class="vt-chip">Grid view</span>
    </div>
  </div>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-enter;
$easing: $ease-spring;
$color: $subject-blue;

@include stage($stage-orange);

.vt-frame {
  position: relative;
  width: min(260px, 84vw);
  aspect-ratio: 260 / 170;
  border-radius: $r-card;
  overflow: hidden;
  box-shadow: $shadow-raised;
}

.vt-layer {
  position: absolute;
  inset: 0;
  display: grid;
  place-items: center;
  color: $subject-cream;
}

.vt-layer--a { background: linear-gradient(160deg, $subject-ink, rgba(23, 20, 26, .8)); }
.vt-layer--b { background: linear-gradient(160deg, $color, rgba(23, 20, 26, .85)); }

.vt-chip {
  padding: $sp-2 $sp-4;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .16);
  font: 700 13px/1 -apple-system, "Segoe UI", sans-serif;
}

// 실사용 원리: document.startViewTransition() 이 flushSync 로 감싼 상태 갱신을 가로채,
// 바뀌기 전·후 DOM 스냅샷을 브라우저가 자동으로 크로스페이드한다.
.vt-layer--b { opacity: 0; }

// 캡처용 루프: list view ↔ grid view 를 2초 주기로 크로스페이드
@keyframes vt-cross-out { 0%, 20% { opacity: 1; } 80%, 100% { opacity: 0; } }
@keyframes vt-cross-in  { 0%, 20% { opacity: 0; } 80%, 100% { opacity: 1; } }
.vt-frame.is-demo .vt-layer--a { animation: vt-cross-out $dur-loop $easing infinite; }
.vt-frame.is-demo .vt-layer--b { animation: vt-cross-in $dur-loop $easing infinite; }

@media (prefers-reduced-motion: reduce) {
  .vt-frame.is-demo .vt-layer--a,
  .vt-frame.is-demo .vt-layer--b { animation: none !important; }
  .vt-layer--a { opacity: 1; }
  .vt-layer--b { opacity: 0; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.vt-frame {
  position: relative;
  width: min(260px, 84vw);
  aspect-ratio: 260/170;
  border-radius: 16px;
  overflow: hidden;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.vt-layer {
  position: absolute;
  inset: 0;
  display: grid;
  place-items: center;
  color: #fff7e6;
}

.vt-layer--a {
  background: linear-gradient(160deg, #17141a, rgba(23, 20, 26, 0.8));
}

.vt-layer--b {
  background: linear-gradient(160deg, #2f6df6, rgba(23, 20, 26, 0.85));
}

.vt-chip {
  padding: 8px 16px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.16);
  font: 700 13px/1 -apple-system, "Segoe UI", sans-serif;
}

.vt-layer--b {
  opacity: 0;
}

@keyframes vt-cross-out {
  0%, 20% {
    opacity: 1;
  }
  80%, 100% {
    opacity: 0;
  }
}
@keyframes vt-cross-in {
  0%, 20% {
    opacity: 0;
  }
  80%, 100% {
    opacity: 1;
  }
}
.vt-frame.is-demo .vt-layer--a {
  animation: vt-cross-out 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

.vt-frame.is-demo .vt-layer--b {
  animation: vt-cross-in 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

@media (prefers-reduced-motion: reduce) {
  .vt-frame.is-demo .vt-layer--a,
  .vt-frame.is-demo .vt-layer--b {
    animation: none !important;
  }
  .vt-layer--a {
    opacity: 1;
  }
  .vt-layer--b {
    opacity: 0;
  }
}

import { CSSProperties, useState } from "react";
import { flushSync } from "react-dom";
import styles from "./ViewTransitionReactState.module.scss";

type DocWithViewTransition = Document & {
  startViewTransition?: (callback: () => void) => void;
};

export interface ViewTransitionReactStateProps {
  duration?: string;
  easing?: string;
}

export default function ViewTransitionReactState({
  duration = "600ms",
  easing = "cubic-bezier(.2, .8, .2, 1)",
}: ViewTransitionReactStateProps) {
  const [mode, setMode] = useState<"list" | "grid">("list");

  const toggle = () => {
    const doc = document as DocWithViewTransition;
    const next = mode === "list" ? "grid" : "list";
    if (doc.startViewTransition) {
      // startViewTransition needs the "after" DOM already painted by the time its
      // callback returns — flushSync forces that React commit synchronously instead
      // of letting React 18's automatic batching defer it past the snapshot.
      doc.startViewTransition(() => flushSync(() => setMode(next)));
    } else {
      setMode(next);
    }
  };

  const style: CSSProperties & { "--duration"?: string; "--easing"?: string } = {
    "--duration": duration,
    "--easing": easing,
  };

  return (
    <button className={styles.frame} style={style} onClick={toggle}>
      <span className={styles.chip}>{mode === "list" ? "List view" : "Grid view"}</span>
    </button>
  );
}