09Drag-and-release inertia toggle

<main class="stage">
<div class="dt is-demo" id="dt">
  <section class="dt__panel">
    <p class="dt__label">Unlock</p>
    <div class="dt__track" id="dtTrack">
      <span class="dt__fill" aria-hidden="true"></span>
      <button class="dt__knob" id="dtKnob" type="button" role="switch" aria-checked="false" aria-label="Unlock switch">
        <span class="dt__grip" aria-hidden="true"></span>
      </button>
    </div>
    <p class="dt__hint">Drag it across</p>
    <p class="dt__state" role="status"><span class="dt__wordOff">Locked</span><span class="dt__wordOn">Open</span></p>
  </section>
</div>
</main>

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

// 끌었다 놓는 관성 토글 — 끄는 동안은 손끝을 그대로 따라오고(transition 없음), 놓는 순간에만 관성이 붙는다.
// 그 두 구간을 가르는 것이 .is-dragging 하나다: 끌 때는 transition 을 끄고, 놓으면 $easing 이 남은 거리를 달린다.
// 포인터는 setPointerCapture 로 노브에 묶어 트랙 밖으로 손이 나가도 끊기지 않게 한다.
// 클릭·Space 로도 같은 동작이 되고(버튼), role="switch" + aria-checked 로 상태를 읽어 준다.

$duration: $dur-base;
$easing: $ease-pop;
$color: $subject-blue-deep;

@include stage($stage-ink);

.dt { display: grid; place-items: center; width: 100%; }

.dt__panel {
  box-sizing: border-box; width: clamp(196px, 62vw, 232px);
  display: flex; flex-direction: column; align-items: center; gap: $sp-2;
  padding: $sp-4; border-radius: $r-card;
  background: rgba($subject-cream, .07);
  border: 1px solid rgba($subject-cream, .18);
  box-shadow: $shadow-raised;
}

.dt__label {
  margin: 0; font-weight: 700; font-size: clamp(11px, 2.8vw, 13px); line-height: 1;
  letter-spacing: .14em; color: $subject-cream;
}

// ── 트랙 ─────────────────────────────────────────────────────
.dt__track {
  position: relative; box-sizing: border-box;
  width: 168px; height: 48px; border-radius: $r-pill;
  background: rgba($stage-ink, .8);
  box-shadow: inset 0 3px 6px rgba($stage-ink, .9), inset 0 -1px 0 rgba($subject-cream, .18);
}

// 노브가 지나온 만큼 차오르는 자국
.dt__fill {
  position: absolute; left: 4px; top: 4px; width: 160px; height: 40px;
  border-radius: $r-pill; background: $color;
  transform: scaleX(0); transform-origin: left center; // 왼쪽 변에서 자라는 채움 막대
  transition: transform $duration $easing;
}

.dt__knob {
  position: absolute; left: 4px; top: 4px; z-index: 2;
  box-sizing: border-box; width: 44px; height: 40px;
  margin: 0; padding: 0; border: 0; cursor: grab;
  border-radius: $r-pill; touch-action: none;
  background: linear-gradient(180deg, $subject-cream, rgba($subject-cream, .72) 62%, rgba($stage-ink, .45));
  box-shadow: 0 4px 8px rgba($stage-ink, .7), inset 0 1px 0 rgba($stage-paper, .9);
  transform: translateX(0);
  transition: transform $duration $easing;
}
.dt__knob:focus-visible { outline: 2px solid $stage-yellow; outline-offset: 2px; }

// 끄는 동안에는 손끝과 노브가 어긋나면 안 되므로 전환을 끈다
.dt.is-dragging .dt__knob { transition: none; cursor: grabbing; }
.dt.is-dragging .dt__fill { transition: none; }

.dt__grip {
  position: absolute; left: calc(50% - 7px); top: calc(50% - 7px); width: 14px; height: 14px;
  background:
    linear-gradient(90deg, rgba($subject-ink, .38) 2px, transparent 2px) 0 0 / 6px 100% repeat-x;
}

.dt__hint {
  margin: 0; font-weight: 500; font-size: clamp(10px, 2.4vw, 11px); line-height: 1;
  color: rgba($subject-cream, .78);
}

.dt__state { position: relative; margin: 0; width: 96px; height: 18px; }
.dt__wordOff, .dt__wordOn {
  position: absolute; inset: 0; display: flex; align-items: center; justify-content: center;
  font-weight: 800; font-size: clamp(12px, 3vw, 14px); line-height: 1;
  color: $subject-cream;
}
.dt__wordOn { display: none; color: $stage-yellow; }

// ── 열림 ─────────────────────────────────────────────────────
.dt.is-open .dt__knob { transform: translateX(116px); }
.dt.is-open .dt__fill { transform: scaleX(1); }
.dt.is-open .dt__wordOff { display: none; }
.dt.is-open .dt__wordOn { display: flex; }

// ── 미리보기 루프(2초 한 바퀴) ────────────────────────────────
// 끄는 손을 흉내 낸다: 천천히 끌려가다(등속) 절반을 넘긴 뒤 놓으면 나머지를 관성으로 달린다.
.dt.is-demo .dt__knob {
  animation-name: dtDrag; animation-duration: $dur-loop;
  animation-timing-function: $easing; animation-iteration-count: infinite;
}
@keyframes dtDrag {
  0%   { transform: translateX(0); }
  8%   { transform: translateX(0); }
  28%  { transform: translateX(64px); }
  40%  { transform: translateX(122px); }
  46%  { transform: translateX(116px); }
  72%  { transform: translateX(116px); }
  84%  { transform: translateX(-4px); }
  88%  { transform: translateX(0); }
  100% { transform: translateX(0); }
}
.dt.is-demo .dt__fill {
  animation-name: dtFill; animation-duration: $dur-loop;
  animation-timing-function: $easing; animation-iteration-count: infinite;
}
@keyframes dtFill {
  0%   { transform: scaleX(0); }
  8%   { transform: scaleX(0); }
  28%  { transform: scaleX(.42); }
  40%  { transform: scaleX(1); }
  72%  { transform: scaleX(1); }
  84%  { transform: scaleX(0); }
  100% { transform: scaleX(0); }
}
.dt.is-demo .dt__wordOn { display: flex; opacity: 0; }
.dt.is-demo .dt__wordOff { opacity: 1; }
.dt.is-demo .dt__wordOn {
  animation-name: dtWordOn; animation-duration: $dur-loop;
  animation-timing-function: steps(1, end); animation-iteration-count: infinite;
}
@keyframes dtWordOn {
  0%   { opacity: 0; }
  42%  { opacity: 1; }
  82%  { opacity: 0; }
  100% { opacity: 0; }
}
.dt.is-demo .dt__wordOff {
  animation-name: dtWordOff; animation-duration: $dur-loop;
  animation-timing-function: steps(1, end); animation-iteration-count: infinite;
}
@keyframes dtWordOff {
  0%   { opacity: 1; }
  42%  { opacity: 0; }
  82%  { opacity: 1; }
  100% { opacity: 1; }
}

@media (max-width: 340px) {
  .dt__panel { gap: $sp-1; padding: $sp-3; width: clamp(168px, 86vw, 196px); }
  .dt__track { width: 144px; height: 40px; }
  .dt__fill { width: 136px; height: 32px; }
  .dt__knob { width: 40px; height: 32px; }
  .dt.is-open .dt__knob { transform: translateX(96px); }
  .dt__state { width: 84px; height: 16px; }
}

@media (prefers-reduced-motion: reduce) {
  .dt .dt__knob { transition: none !important; }
  .dt .dt__fill { transition: none !important; }
  .dt.is-demo .dt__knob { animation: none !important; transform: translateX(0) !important; }
  .dt.is-demo .dt__fill { animation: none !important; transform: scaleX(0) !important; }
  .dt.is-demo .dt__wordOn { animation: none !important; opacity: 0 !important; }
  .dt.is-demo .dt__wordOff { animation: none !important; opacity: 1 !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);
}

.dt {
  display: grid;
  place-items: center;
  width: 100%;
}

.dt__panel {
  box-sizing: border-box;
  width: clamp(196px, 62vw, 232px);
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
  padding: 16px;
  border-radius: 16px;
  background: rgba(255, 247, 230, 0.07);
  border: 1px solid rgba(255, 247, 230, 0.18);
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.dt__label {
  margin: 0;
  font-weight: 700;
  font-size: clamp(11px, 2.8vw, 13px);
  line-height: 1;
  letter-spacing: 0.14em;
  color: #fff7e6;
}

.dt__track {
  position: relative;
  box-sizing: border-box;
  width: 168px;
  height: 48px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.8);
  box-shadow: inset 0 3px 6px rgba(23, 20, 26, 0.9), inset 0 -1px 0 rgba(255, 247, 230, 0.18);
}

.dt__fill {
  position: absolute;
  left: 4px;
  top: 4px;
  width: 160px;
  height: 40px;
  border-radius: 999px;
  background: #0a4fe8;
  transform: scaleX(0);
  transform-origin: left center;
  transition: transform 300ms cubic-bezier(0.2, 1.4, 0.4, 1);
}

.dt__knob {
  position: absolute;
  left: 4px;
  top: 4px;
  z-index: 2;
  box-sizing: border-box;
  width: 44px;
  height: 40px;
  margin: 0;
  padding: 0;
  border: 0;
  cursor: grab;
  border-radius: 999px;
  touch-action: none;
  background: linear-gradient(180deg, #fff7e6, rgba(255, 247, 230, 0.72) 62%, rgba(23, 20, 26, 0.45));
  box-shadow: 0 4px 8px rgba(23, 20, 26, 0.7), inset 0 1px 0 rgba(255, 255, 255, 0.9);
  transform: translateX(0);
  transition: transform 300ms cubic-bezier(0.2, 1.4, 0.4, 1);
}

.dt__knob:focus-visible {
  outline: 2px solid #ffd23f;
  outline-offset: 2px;
}

.dt.is-dragging .dt__knob {
  transition: none;
  cursor: grabbing;
}

.dt.is-dragging .dt__fill {
  transition: none;
}

.dt__grip {
  position: absolute;
  left: calc(50% - 7px);
  top: calc(50% - 7px);
  width: 14px;
  height: 14px;
  background: linear-gradient(90deg, rgba(23, 20, 26, 0.38) 2px, transparent 2px) 0 0/6px 100% repeat-x;
}

.dt__hint {
  margin: 0;
  font-weight: 500;
  font-size: clamp(10px, 2.4vw, 11px);
  line-height: 1;
  color: rgba(255, 247, 230, 0.78);
}

.dt__state {
  position: relative;
  margin: 0;
  width: 96px;
  height: 18px;
}

.dt__wordOff, .dt__wordOn {
  position: absolute;
  inset: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: 800;
  font-size: clamp(12px, 3vw, 14px);
  line-height: 1;
  color: #fff7e6;
}

.dt__wordOn {
  display: none;
  color: #ffd23f;
}

.dt.is-open .dt__knob {
  transform: translateX(116px);
}

.dt.is-open .dt__fill {
  transform: scaleX(1);
}

.dt.is-open .dt__wordOff {
  display: none;
}

.dt.is-open .dt__wordOn {
  display: flex;
}

.dt.is-demo .dt__knob {
  animation-name: dtDrag;
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0.2, 1.4, 0.4, 1);
  animation-iteration-count: infinite;
}

@keyframes dtDrag {
  0% {
    transform: translateX(0);
  }
  8% {
    transform: translateX(0);
  }
  28% {
    transform: translateX(64px);
  }
  40% {
    transform: translateX(122px);
  }
  46% {
    transform: translateX(116px);
  }
  72% {
    transform: translateX(116px);
  }
  84% {
    transform: translateX(-4px);
  }
  88% {
    transform: translateX(0);
  }
  100% {
    transform: translateX(0);
  }
}
.dt.is-demo .dt__fill {
  animation-name: dtFill;
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0.2, 1.4, 0.4, 1);
  animation-iteration-count: infinite;
}

@keyframes dtFill {
  0% {
    transform: scaleX(0);
  }
  8% {
    transform: scaleX(0);
  }
  28% {
    transform: scaleX(0.42);
  }
  40% {
    transform: scaleX(1);
  }
  72% {
    transform: scaleX(1);
  }
  84% {
    transform: scaleX(0);
  }
  100% {
    transform: scaleX(0);
  }
}
.dt.is-demo .dt__wordOn {
  display: flex;
  opacity: 0;
}

.dt.is-demo .dt__wordOff {
  opacity: 1;
}

.dt.is-demo .dt__wordOn {
  animation-name: dtWordOn;
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}

@keyframes dtWordOn {
  0% {
    opacity: 0;
  }
  42% {
    opacity: 1;
  }
  82% {
    opacity: 0;
  }
  100% {
    opacity: 0;
  }
}
.dt.is-demo .dt__wordOff {
  animation-name: dtWordOff;
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}

@keyframes dtWordOff {
  0% {
    opacity: 1;
  }
  42% {
    opacity: 0;
  }
  82% {
    opacity: 1;
  }
  100% {
    opacity: 1;
  }
}
@media (max-width: 340px) {
  .dt__panel {
    gap: 4px;
    padding: 12px;
    width: clamp(168px, 86vw, 196px);
  }
  .dt__track {
    width: 144px;
    height: 40px;
  }
  .dt__fill {
    width: 136px;
    height: 32px;
  }
  .dt__knob {
    width: 40px;
    height: 32px;
  }
  .dt.is-open .dt__knob {
    transform: translateX(96px);
  }
  .dt__state {
    width: 84px;
    height: 16px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .dt .dt__knob {
    transition: none !important;
  }
  .dt .dt__fill {
    transition: none !important;
  }
  .dt.is-demo .dt__knob {
    animation: none !important;
    transform: translateX(0) !important;
  }
  .dt.is-demo .dt__fill {
    animation: none !important;
    transform: scaleX(0) !important;
  }
  .dt.is-demo .dt__wordOn {
    animation: none !important;
    opacity: 0 !important;
  }
  .dt.is-demo .dt__wordOff {
    animation: none !important;
    opacity: 1 !important;
  }
}

var dtRoot = document.getElementById('dt');
var dtTrack = document.getElementById('dtTrack');
var dtKnob = document.getElementById('dtKnob');
var dragging = false;
var startX = 0;
var startOffset = 0;
var offset = 0;
var suppressClick = false; // 끌기가 일어난 뒤 따라오는 click 은 토글이 아니라 끌기의 끝이다

function travel() {
  // 노브가 갈 수 있는 최대 거리 = 트랙 안폭 - 노브 폭 (레일 여백 4px 씩)
  return dtTrack.clientWidth - dtKnob.offsetWidth - 8;
}
function place(px) {
  offset = Math.max(0, Math.min(travel(), px));
  dtKnob.style.transform = 'translateX(' + offset + 'px)';
}
function settle(open) {
  dtKnob.style.transform = '';
  dtKnob.setAttribute('aria-checked', open ? 'true' : 'false');
  dtRoot.classList.toggle('is-open', open);
  offset = open ? travel() : 0;
}
function onDown(e) {
  dtRoot.classList.remove('is-demo');
  dragging = true;
  startX = e.clientX;
  startOffset = dtRoot.classList.contains('is-open') ? travel() : 0;
  dtRoot.classList.add('is-dragging');
  dtKnob.setPointerCapture(e.pointerId);
}
function onMove(e) {
  if (!dragging) return;
  if (Math.abs(e.clientX - startX) > 6) suppressClick = true;
  place(startOffset + (e.clientX - startX));
}
function onUp() {
  if (!dragging) return;
  dragging = false;
  dtRoot.classList.remove('is-dragging');
  // 절반을 넘겼으면 남은 거리를 관성으로 마저 달리고, 못 넘겼으면 제자리로 돌아온다
  settle(offset > travel() / 2);
}
dtKnob.addEventListener('pointerdown', onDown);
dtKnob.addEventListener('pointermove', onMove);
dtKnob.addEventListener('pointerup', onUp);
dtKnob.addEventListener('pointercancel', onUp);
dtKnob.addEventListener('click', function () {
  dtRoot.classList.remove('is-demo');
  if (suppressClick) { suppressClick = false; return; } // 끌기 직후의 click 은 무시한다
  settle(dtKnob.getAttribute('aria-checked') !== 'true');
});

// InertiaDragToggle — vanilla 판(run/076/demo/09-real-inertia-drag-toggle/index.html) 의 React 포트.
// 끄는 동안은 손끝을 그대로 따라오고(transition 없음), 놓는 순간에만 관성이 붙는다.
// 값·키프레임은 vanilla 와 같다(다시 타이핑하지 않는다). 드래그·클릭 모두 미리보기 루프를 뗀다.
import { useRef, useState } from "react";
import type { PointerEvent as ReactPointerEvent } from "react";
import styles from "./InertiaDragToggle.module.scss";

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

type Vars = { [k: string]: any };
type PtrEvt = ReactPointerEvent<HTMLButtonElement>;

const LABEL_TEXT = "잠금 해제";
const SWITCH_LABEL = "잠금 해제 스위치";
const HINT_TEXT = "끌어서 밀어 보세요";
const STATE_OFF = "잠김";
const STATE_ON = "열림";

export default function InertiaDragToggle({
  duration = 300,
  easing = "cubic-bezier(.2, 1.4, .4, 1)", // = $ease-pop
  color = "#0a4fe8",
}: InertiaDragToggleProps) {
  const [open, setOpen] = useState(false);
  const [demo, setDemo] = useState(true);
  const [dragging, setDragging] = useState(false);
  const trackRef = useRef<HTMLDivElement>(null);
  const knobRef = useRef<HTMLButtonElement>(null);
  const drag = useRef({ on: false, startX: 0, startOffset: 0, offset: 0, moved: false });
  const vars: Vars = {
    "--duration": `${duration}ms`,
    "--easing": easing,
    "--color": color,
  };
  const cls = [
    styles.root,
    demo ? styles.isDemo : "",
    open ? styles.isOpen : "",
    dragging ? styles.isDragging : "",
  ].filter(Boolean).join(" ");

  function travel(): number {
    // 노브가 갈 수 있는 최대 거리 = 트랙 안폭 - 노브 폭 (레일 여백 4px 씩)
    const track = trackRef.current;
    const knob = knobRef.current;
    if (!track || !knob) return 0;
    return track.clientWidth - knob.offsetWidth - 8;
  }
  function place(px: number) {
    const knob = knobRef.current;
    const d = drag.current;
    if (!knob || !d) return;
    d.offset = Math.max(0, Math.min(travel(), px));
    knob.style.transform = "translateX(" + d.offset + "px)";
  }
  function settle(next: boolean) {
    const knob = knobRef.current;
    const d = drag.current;
    if (knob) knob.style.transform = "";
    setOpen(next);
    if (d) d.offset = next ? travel() : 0;
  }
  function onDown(e: PtrEvt) {
    const d = drag.current;
    if (!d) return;
    setDemo(false);
    d.on = true;
    d.moved = false;
    d.startX = e.clientX;
    d.startOffset = open ? travel() : 0;
    setDragging(true);
    e.currentTarget.setPointerCapture(e.pointerId);
  }
  function onMove(e: PtrEvt) {
    const d = drag.current;
    if (!d || !d.on) return;
    // 끌기가 일어난 뒤 따라오는 click 은 토글이 아니라 끌기의 끝이다
    if (Math.abs(e.clientX - d.startX) > 6) d.moved = true;
    place(d.startOffset + (e.clientX - d.startX));
  }
  function onUp() {
    const d = drag.current;
    if (!d || !d.on) return;
    d.on = false;
    setDragging(false);
    // 절반을 넘겼으면 남은 거리를 관성으로 마저 달리고, 못 넘겼으면 제자리로 돌아온다
    settle(d.offset > travel() / 2);
  }
  function onKey() {
    const d = drag.current;
    setDemo(false);
    if (d && d.moved) {
      d.moved = false;
      return; // 끌기 직후의 click 은 무시한다
    }
    settle(!open);
  }

  return (
    <main className={styles.stage}>
      <div className={cls} style={vars}>
        <section className={styles.panel}>
          <p className={styles.label}>{LABEL_TEXT}</p>
          <div className={styles.track} ref={trackRef}>
            <span className={styles.fill} aria-hidden="true"></span>
            <button
              className={styles.knob}
              type="button"
              role="switch"
              aria-checked={open}
              aria-label={SWITCH_LABEL}
              ref={knobRef}
              onPointerDown={onDown}
              onPointerMove={onMove}
              onPointerUp={onUp}
              onPointerCancel={onUp}
              onClick={onKey}
            >
              <span className={styles.grip} aria-hidden="true"></span>
            </button>
          </div>
          <p className={styles.hint}>{HINT_TEXT}</p>
          <p className={styles.state} role="status">
            <span className={styles.wordOff}>{STATE_OFF}</span>
            <span className={styles.wordOn}>{STATE_ON}</span>
          </p>
        </section>
      </div>
    </main>
  );
}