07Loader That Recomputes Front-to-Back Order Every Time a Panel Flips

<main class="stage">
<div class="ld" id="ld">
  <p class="ld__hint">Click or press the space bar to pause or restart</p>
  <div class="ld__field" id="field" role="application" aria-label="Flip-panel depth-reorder test panel" tabindex="0">
    <div class="c3__scene">
      <div class="c3__stack" id="stack"></div>
    </div>
    <b class="ld__badge" aria-hidden="true"><span class="ld__badgeNum" id="pct">0%</span><span class="ld__badgeUnit">Progress</span></b>
  </div>
</div>
</main>

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

// 플립 패널 깊이 재정렬 로더 — 패널마다 회전 각도(뒤집힘)와 깊이 순위를 같이 계산한다.
// 진행률 값이 패널 하나의 구간을 지날 때마다 그 패널이 0→180도로 뒤집히고, 동시에 모든 패널의
// 깊이 순위가 "지금 뒤집는 패널까지의 거리"로 다시 매겨져 뒤집히는 패널이 항상 맨 앞에 자연스럽게 겹친다.
// 01(문턱 인덱스 스왑)·02(정점 좌표 보간)·03(연속 순환 랭크)·04(고리별 개별 기울기)·05(구면 중심 수렴)
// ·06(면별 독립 문턱 램프) 과 다른 축 — 회전 각도와 거리 기반 깊이 순위 두 값을 함께 계산한다.

$duration: $dur-loop;
$easing: linear;
$color: $subject-cream;

@include stage($stage-orange);

.ld { display: flex; flex-direction: column; gap: $sp-2; width: 100%; max-width: 360px; }

.ld__hint {
  margin: 0;
  font-weight: 700; font-size: clamp(10px, 2.8vw, 12px); line-height: 1.35;
  color: rgba($subject-cream, .82);
  text-align: center;
}

.ld__field {
  position: relative; height: 208px;
  border-radius: $r-card; overflow: hidden;
  background: darken($stage-orange, 6%);
  box-shadow: inset 0 0 0 1px rgba($subject-ink, .16);
  cursor: pointer;
}
.ld__field:focus-visible { outline: 2px solid $subject-blue-light; outline-offset: 2px; }

.c3__scene { position: absolute; inset: 0; perspective: clamp(320px, 70vw, 480px); }

.c3__stack { position: absolute; left: 50%; top: 50%; width: 0; height: 0; transform-style: preserve-3d; }

.c3__panel {
  position: absolute; left: -58px; top: -66px;
  width: 116px; height: 132px;
  transform-style: preserve-3d;
  will-change: transform;
}
.c3__panelFace,
.c3__panelBack {
  position: absolute; inset: 0;
  border-radius: $r-sm;
  backface-visibility: hidden;
  display: flex; align-items: center; justify-content: center;
  font-weight: 700; font-size: 13px;
}
.c3__panelFace { background: $color; color: $subject-ink; }
.c3__panelBack { background: $subject-blue-deep; color: $subject-cream; transform: rotateY(180deg); }

.ld__badge {
  position: absolute; right: $sp-2; top: $sp-2;
  display: flex; align-items: baseline; gap: $sp-1;
  padding: $sp-1 $sp-2; border-radius: $r-pill;
  background: rgba($subject-ink, .78);
  color: $subject-cream; font-weight: 700; font-size: 11px;
}
.ld__badgeNum { font-size: 13px; color: $stage-yellow; }
.ld__badgeUnit { font-weight: 700; }

@media (max-width: 340px) {
  .ld { gap: $sp-1; }
  .ld__field { height: 124px; }
  .ld__badge { font-size: 10px; padding: 0 $sp-1; }
  .ld__badgeNum { font-size: 11px; }
}

@media (prefers-reduced-motion: reduce) {
  .ld.is-still .ld__field { cursor: default !important; }
  .ld.is-still .ld__badge { opacity: .92 !important; }
}

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);
}

.ld {
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 360px;
}

.ld__hint {
  margin: 0;
  font-weight: 700;
  font-size: clamp(10px, 2.8vw, 12px);
  line-height: 1.35;
  color: rgba(255, 247, 230, 0.82);
  text-align: center;
}

.ld__field {
  position: relative;
  height: 208px;
  border-radius: 16px;
  overflow: hidden;
  background: rgb(100%, 20.6603641457%, 0.1568627451%);
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.16);
  cursor: pointer;
}

.ld__field:focus-visible {
  outline: 2px solid #5b8cf8;
  outline-offset: 2px;
}

.c3__scene {
  position: absolute;
  inset: 0;
  perspective: clamp(320px, 70vw, 480px);
}

.c3__stack {
  position: absolute;
  left: 50%;
  top: 50%;
  width: 0;
  height: 0;
  transform-style: preserve-3d;
}

.c3__panel {
  position: absolute;
  left: -58px;
  top: -66px;
  width: 116px;
  height: 132px;
  transform-style: preserve-3d;
  will-change: transform;
}

.c3__panelFace,
.c3__panelBack {
  position: absolute;
  inset: 0;
  border-radius: 8px;
  backface-visibility: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: 700;
  font-size: 13px;
}

.c3__panelFace {
  background: #fff7e6;
  color: #17141a;
}

.c3__panelBack {
  background: #0a4fe8;
  color: #fff7e6;
  transform: rotateY(180deg);
}

.ld__badge {
  position: absolute;
  right: 8px;
  top: 8px;
  display: flex;
  align-items: baseline;
  gap: 4px;
  padding: 4px 8px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.78);
  color: #fff7e6;
  font-weight: 700;
  font-size: 11px;
}

.ld__badgeNum {
  font-size: 13px;
  color: #ffd23f;
}

.ld__badgeUnit {
  font-weight: 700;
}

@media (max-width: 340px) {
  .ld {
    gap: 4px;
  }
  .ld__field {
    height: 124px;
  }
  .ld__badge {
    font-size: 10px;
    padding: 0 4px;
  }
  .ld__badgeNum {
    font-size: 11px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .ld.is-still .ld__field {
    cursor: default !important;
  }
  .ld.is-still .ld__badge {
    opacity: 0.92 !important;
  }
}

(function () {
'use strict';
var root = document.getElementById('ld');
var field = document.getElementById('field');
var pctEl = document.getElementById('pct');
var stack = document.getElementById('stack');

var PANEL_COUNT = 5;
var STEP = 1 / PANEL_COUNT;
var RANK_ZGAP = 22;
var RANK_YOFF = 4;

var panels = [];
for (var i = 0; i < PANEL_COUNT; i++) {
  var panel = document.createElement('div');
  panel.className = 'c3__panel';
  var face = document.createElement('div');
  face.className = 'c3__panelFace';
  face.textContent = String(i + 1);
  var back = document.createElement('div');
  back.className = 'c3__panelBack';
  back.textContent = String(i + 1);
  panel.appendChild(face);
  panel.appendChild(back);
  stack.appendChild(panel);
  panels.push(panel);
}

var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
var LOOP = 2;
var REDUCE_P = 0.7;
var t0 = 0, raf = 0, paused = false, lastP = 0;

// 지금 뒤집는 패널 인덱스 — 진행률을 패널 구간(STEP)으로 나눈 정수부.
function currentIndex(p) {
  var idx = Math.floor(p * PANEL_COUNT);
  if (idx > PANEL_COUNT - 1) idx = PANEL_COUNT - 1;
  if (idx < 0) idx = 0;
  return idx;
}
// 패널 i 의 회전 각도 — 자기 구간 안에서만 0→180도로 선형 보간, 구간 전은 0, 구간 후는 180 고정.
function flipAngle(i, p) {
  var start = i * STEP, end = start + STEP;
  if (p < start) return 0;
  if (p >= end) return 180;
  var local = (p - start) / STEP;
  return local * 180;
}
// 패널 i 의 깊이 순위 — "지금 뒤집는 패널까지의 거리". 거리가 멀수록 뒤로 밀린다.
function depthRank(i, p) {
  var cur = currentIndex(p);
  return Math.abs(cur - i);
}

function render(p) {
  lastP = p;
  for (var i = 0; i < PANEL_COUNT; i++) {
    var angle = flipAngle(i, p);
    var rank = depthRank(i, p);
    panels[i].style.transform = 'translateZ(' + (-rank * RANK_ZGAP) + 'px) translateY(' + (rank * RANK_YOFF) + 'px) rotateY(' + angle + 'deg)';
    panels[i].style.zIndex = String(PANEL_COUNT - rank);
  }
  pctEl.textContent = Math.round(p * 100) + '%';
}

function frame(now) {
  if (!paused) { var t = (now - t0) / 1000; render((t % LOOP) / LOOP); }
  raf = requestAnimationFrame(frame);
}
function togglePause() {
  paused = !paused;
  root.classList.toggle('is-paused', paused);
  render(reduce ? REDUCE_P : lastP);
}
function restart() {
  t0 = performance.now();
  render(reduce ? REDUCE_P : 0);
}

field.addEventListener('click', function () { togglePause(); });
field.addEventListener('keydown', function (e) {
  if (e.key === ' ' || e.key === 'Enter') { e.preventDefault(); togglePause(); }
  else if (e.key === 'r' || e.key === 'R') { e.preventDefault(); restart(); }
});
window.addEventListener('resize', function () {
  if (paused || reduce) render(reduce ? REDUCE_P : lastP);
});

t0 = performance.now();
if (reduce) {
  root.classList.add('is-still');
  render(REDUCE_P);
} else {
  raf = requestAnimationFrame(frame);
}
})();

// FlipPanelDepthSort — vanilla 판(run/399/demo/07-loader-3d-flip-panel-depth-sort/index.html) 의 React 포트.
// 패널마다 회전 각도(자기 구간 안에서 0→180도 선형)와 깊이 순위("지금 뒤집는 패널까지의 거리")를
// 매 프레임 p 하나로부터 순수 함수로 계산한다 — 01~06 과 다른 축.
import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./FlipPanelDepthSort.module.scss";

export type FlipPanelDepthSortProps = {
  duration?: string;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type KeyEvt = { key: string; preventDefault(): void };

const PANEL_COUNT = 5;
const STEP = 1 / PANEL_COUNT;
const RANK_ZGAP = 22;
const RANK_YOFF = 4;
const LOOP = 2;
const REDUCE_P = 0.7;
const FIELD_LABEL = "플립 패널 깊이 재정렬 진행 실험판";

function currentIndex(p: number): number {
  let idx = Math.floor(p * PANEL_COUNT);
  if (idx > PANEL_COUNT - 1) idx = PANEL_COUNT - 1;
  if (idx < 0) idx = 0;
  return idx;
}
function flipAngle(i: number, p: number): number {
  const start = i * STEP, end = start + STEP;
  if (p < start) return 0;
  if (p >= end) return 180;
  const local = (p - start) / STEP;
  return local * 180;
}
function depthRank(i: number, p: number): number {
  const cur = currentIndex(p);
  return Math.abs(cur - i);
}

export default function FlipPanelDepthSort({
  duration = "2s",
  easing = "linear",
  color = "#fff7e6",
}: FlipPanelDepthSortProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const fieldRef = useRef<HTMLDivElement | null>(null);
  const pctRef = useRef<HTMLSpanElement | null>(null);
  const panelsRef = useRef<(HTMLDivElement | null)[]>(new Array(PANEL_COUNT).fill(null)) as Box<(HTMLDivElement | null)[]>;
  const pausedRef = useRef(false) as Box<boolean>;
  const lastPRef = useRef(0) as Box<number>;
  const t0Ref = useRef(0) as Box<number>;
  const rafRef = useRef(0) as Box<number>;
  const vars: Vars = { "--duration": duration, "--easing": easing, "--color": color };

  function reduced(): boolean {
    return window.matchMedia("(prefers-reduced-motion: reduce)").matches;
  }

  function render(p: number): void {
    lastPRef.current = p;
    const panels = panelsRef.current;
    for (let i = 0; i < PANEL_COUNT; i++) {
      const el = panels[i];
      if (!el) continue;
      const angle = flipAngle(i, p);
      const rank = depthRank(i, p);
      el.style.transform = "translateZ(" + (-rank * RANK_ZGAP) + "px) translateY(" + (rank * RANK_YOFF) + "px) rotateY(" + angle + "deg)";
      el.style.zIndex = String(PANEL_COUNT - rank);
    }
    if (pctRef.current) pctRef.current.textContent = Math.round(p * 100) + "%";
  }

  function togglePause(): void {
    pausedRef.current = !pausedRef.current;
    const root = rootRef.current;
    if (root) root.classList.toggle("is-paused", pausedRef.current);
    render(reduced() ? REDUCE_P : lastPRef.current);
  }

  function restart(): void {
    t0Ref.current = performance.now();
    render(reduced() ? REDUCE_P : 0);
  }

  function onKey(e: KeyEvt): void {
    if (e.key === " " || e.key === "Enter") { e.preventDefault(); togglePause(); }
    else if (e.key === "r" || e.key === "R") { e.preventDefault(); restart(); }
  }

  function onClick(): void {
    togglePause();
  }

  function onResize(): void {
    if (pausedRef.current || reduced()) render(reduced() ? REDUCE_P : lastPRef.current);
  }

  function frame(now: number): void {
    if (!pausedRef.current) {
      const t = (now - t0Ref.current) / 1000;
      render((t % LOOP) / LOOP);
    }
    rafRef.current = requestAnimationFrame(frame);
  }

  useEffect(() => {
    t0Ref.current = performance.now();
    window.addEventListener("resize", onResize);
    if (reduced()) {
      const root = rootRef.current;
      if (root) root.classList.add("is-still");
      render(REDUCE_P);
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.root} ref={rootRef} style={vars}>
        <p className={styles.hint}>클릭하거나 스페이스바를 누르면 멈추거나 다시 갑니다</p>
        <div
          className={styles.field}
          ref={fieldRef}
          role="application"
          aria-label={FIELD_LABEL}
          tabIndex={0}
          onClick={onClick}
          onKeyDown={onKey}
        >
          <div className={styles.scene}>
            <div className={styles.stack}>
              {Array.from({ length: PANEL_COUNT }, (_, i) => (
                <div key={i} className={styles.panel} ref={(el: HTMLDivElement | null) => { panelsRef.current[i] = el; }}>
                  <div className={styles.panelFace}>{i + 1}</div>
                  <div className={styles.panelBack}>{i + 1}</div>
                </div>
              ))}
            </div>
          </div>
          <b className={styles.badge} aria-hidden="true"><span className={styles.badgeNum} ref={pctRef}>0%</span><span className={styles.badgeUnit}>진행</span></b>
        </div>
      </div>
    </main>
  );
}