04Drag-inertia knob — a rotating handle that eases to a stop after release

<main class="stage">
<div class="dr is-demo" id="dr">
  <div class="dr__ring" id="drRing">
    <button class="dr__knob" id="drKnob" type="button" aria-label="Drag to turn the volume knob"></button>
  </div>
  <p class="dr__hint" id="drHint">Speed: 0.0 deg/frame</p>
</div>
</main>

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

// 드래그 관성 손잡이 — 계산 겹은 index.html 의 감쇠 함수(decayStep)에 있다: 놓는 순간의 속도가
// 매 프레임 DRAG_DECAY 배씩 줄며 각도에 계속 더해진다. 여기는 손잡이·바늘·눈금만 그린다.

$duration: $dur-instant;
$easing: $ease-out;
$color: $subject-blue;

@include stage($stage-paper);

.dr { display: flex; flex-direction: column; align-items: center; gap: $sp-4; }

.dr__ring {
  position: relative;
  width: 180px;
  height: 180px;
  border-radius: 50%;
  background: repeating-conic-gradient(rgba($subject-ink, .12) 0 2deg, transparent 2deg 30deg);
  touch-action: none;
}

.dr__knob {
  position: absolute;
  inset: 24px;
  appearance: none;
  border: none;
  border-radius: 50%;
  cursor: grab;
  background: radial-gradient(circle at 35% 30%, rgba($subject-cream, .9), $color 70%);
  box-shadow: $shadow-raised;
  transform: rotate(calc(var(--angle, 0) * 1deg));
}
.dr__knob:active { cursor: grabbing; }
.dr__knob:focus-visible { outline: 2px solid $subject-blue-deep; outline-offset: 3px; }
.dr__knob::after {
  content: "";
  position: absolute;
  top: 10px;
  left: calc(50% - 3px);
  width: 6px;
  height: 22px;
  border-radius: $r-pill;
  background: $subject-cream;
}

.dr__hint {
  margin: 0;
  color: $subject-ink;
  font-size: 11px;
  font-weight: 600;
  font-variant-numeric: tabular-nums;
}

@media (max-width: 340px) {
  .dr__ring { width: 132px; height: 132px; }
  .dr__knob { inset: 16px; }
  .dr__knob::after { height: 16px; top: 8px; }
}

@media (prefers-reduced-motion: reduce) {
  .dr__knob { transition: none; }
}

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);
}

.dr {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 16px;
}

.dr__ring {
  position: relative;
  width: 180px;
  height: 180px;
  border-radius: 50%;
  background: repeating-conic-gradient(rgba(23, 20, 26, 0.12) 0 2deg, transparent 2deg 30deg);
  touch-action: none;
}

.dr__knob {
  position: absolute;
  inset: 24px;
  appearance: none;
  border: none;
  border-radius: 50%;
  cursor: grab;
  background: radial-gradient(circle at 35% 30%, rgba(255, 247, 230, 0.9), #2f6df6 70%);
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  transform: rotate(calc(var(--angle, 0) * 1deg));
}

.dr__knob:active {
  cursor: grabbing;
}

.dr__knob:focus-visible {
  outline: 2px solid #0a4fe8;
  outline-offset: 3px;
}

.dr__knob::after {
  content: "";
  position: absolute;
  top: 10px;
  left: calc(50% - 3px);
  width: 6px;
  height: 22px;
  border-radius: 999px;
  background: #fff7e6;
}

.dr__hint {
  margin: 0;
  color: #17141a;
  font-size: 11px;
  font-weight: 600;
  font-variant-numeric: tabular-nums;
}

@media (max-width: 340px) {
  .dr__ring {
    width: 132px;
    height: 132px;
  }
  .dr__knob {
    inset: 16px;
  }
  .dr__knob::after {
    height: 16px;
    top: 8px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .dr__knob {
    transition: none;
  }
}

(function () {
  var DRAG_DECAY = 0.92;         // 손을 뗀 뒤 매 프레임 속도에 곱하는 감쇠율 — 1에 가까울수록 오래 돈다
  var MIN_VELOCITY = 0.15;       // 이 아래로 떨어지면 정지로 본다(deg/frame)
  var LOOP_MS = 2000;

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var root = document.getElementById('dr');
  var ring = document.getElementById('drRing');
  var knob = document.getElementById('drKnob');
  var hint = document.getElementById('drHint');
  var isDemo = true;

  var angle = 0;
  var velocity = 0;      // deg/frame — 드래그 중엔 포인터 이동에서, 놓은 뒤엔 감쇠식에서 갱신
  var dragging = false;
  var lastAngle = 0;

  function human() { if (isDemo) { root.classList.remove('is-demo'); isDemo = false; } }

  function angleAt(clientX, clientY) {
    var r = ring.getBoundingClientRect();
    var cx = r.left + r.width / 2, cy = r.top + r.height / 2;
    return Math.atan2(clientY - cy, clientX - cx) * 180 / Math.PI;
  }
  function apply() {
    knob.style.setProperty('--angle', angle);
    hint.textContent = 'Speed: ' + Math.abs(velocity).toFixed(1) + ' deg/frame';
  }

  knob.addEventListener('pointerdown', function (e) {
    human();
    dragging = true;
    velocity = 0;
    lastAngle = angleAt(e.clientX, e.clientY);
    knob.setPointerCapture(e.pointerId);
  });
  knob.addEventListener('pointermove', function (e) {
    if (!dragging) return;
    var a = angleAt(e.clientX, e.clientY);
    var delta = a - lastAngle;
    if (delta > 180) delta -= 360; else if (delta < -180) delta += 360;
    angle += delta;
    velocity = reduce ? 0 : delta; // 이동량 자체를 프레임당 속도로 기록
    lastAngle = a;
    apply();
  });
  knob.addEventListener('pointerup', function () { dragging = false; });
  knob.addEventListener('pointercancel', function () { dragging = false; });

  // 계산 겹: 관성 감쇠. 놓은 순간의 속도가 매 프레임 DRAG_DECAY 배로 줄며 각도에 계속 더해진다.
  // reduce 에서는(실사용 포함) 감쇠 구간을 건너뛰고 즉시 멈춘다 — human() 뒤에도 이 분기는 그대로 적용된다.
  function decayStep() {
    if (dragging) return;
    if (reduce) { velocity = 0; return; }
    if (Math.abs(velocity) < MIN_VELOCITY) { velocity = 0; return; }
    velocity *= DRAG_DECAY;
    angle += velocity;
  }

  // 자동 루프: Great 속도로 "튕긴" 상태를 반복해 감쇠 곡선 전체를 계속 보여준다
  var demoVelocity = 26;
  function loop(ts) {
    if (isDemo) {
      if (!reduce) {
        var phase = (ts % LOOP_MS) / LOOP_MS;
        if (phase < 0.02) { velocity = demoVelocity; }
        decayStep();
      } else {
        angle = 0; velocity = 0;
      }
      apply();
    } else {
      decayStep();
      apply();
    }
    requestAnimationFrame(loop);
  }
  apply();
  requestAnimationFrame(loop);
})();

// DragRotateKnob — vanilla 판(run/374/demo/04-arc-drag-rotate/index.html) 의 React 포트.
// 계산 겹: 관성 감쇠(decayStep) — 놓은 순간의 속도가 매 프레임 DRAG_DECAY 배로 줄며 각도에 계속 더해진다.
// reduce 체크는 decayStep 안에 있어 isDemo 분기 밖에서도(실사용에서도) 그대로 적용된다.
import { useEffect, useRef, useState } from "react";
import styles from "./DragRotateKnob.module.scss";

type Box<T> = { current: T };

const DRAG_DECAY = 0.92;
const MIN_VELOCITY = 0.15;
const LOOP_MS = 2000;

export default function DragRotateKnob() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const ringRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const knobRef = useRef(null) as unknown as Box<HTMLButtonElement>;
  const rafRef = useRef(0) as unknown as Box<number>;
  const [speedLabel, setSpeedLabel] = useState("속도: 0.0 도/프레임");

  useEffect(() => {
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const root = rootRef.current;
    const ring = ringRef.current;
    const knob = knobRef.current;
    if (!root || !ring || !knob) return;

    let isDemo = true;
    let angle = 0;
    let velocity = 0;
    let dragging = false;
    let lastAngle = 0;

    function human() { if (isDemo) { root!.classList.remove(styles.isDemo); isDemo = false; } }

    function angleAt(clientX: number, clientY: number) {
      const r = ring!.getBoundingClientRect();
      const cx = r.left + r.width / 2;
      const cy = r.top + r.height / 2;
      return (Math.atan2(clientY - cy, clientX - cx) * 180) / Math.PI;
    }
    function apply() {
      knob!.style.setProperty("--angle", String(angle));
      setSpeedLabel("속도: " + Math.abs(velocity).toFixed(1) + " 도/프레임");
    }

    function onDown(e: PointerEvent) {
      human();
      dragging = true;
      velocity = 0;
      lastAngle = angleAt(e.clientX, e.clientY);
      knob!.setPointerCapture(e.pointerId);
    }
    function onMove(e: PointerEvent) {
      if (!dragging) return;
      const a = angleAt(e.clientX, e.clientY);
      let delta = a - lastAngle;
      if (delta > 180) delta -= 360; else if (delta < -180) delta += 360;
      angle += delta;
      velocity = reduce ? 0 : delta;
      lastAngle = a;
      apply();
    }
    function onUp() { dragging = false; }
    knob.addEventListener("pointerdown", onDown);
    knob.addEventListener("pointermove", onMove);
    knob.addEventListener("pointerup", onUp);
    knob.addEventListener("pointercancel", onUp);

    function decayStep() {
      if (dragging) return;
      if (reduce) { velocity = 0; return; }
      if (Math.abs(velocity) < MIN_VELOCITY) { velocity = 0; return; }
      velocity *= DRAG_DECAY;
      angle += velocity;
    }

    const demoVelocity = 26;
    function loop(ts: number) {
      if (isDemo) {
        if (!reduce) {
          const phase = (ts % LOOP_MS) / LOOP_MS;
          if (phase < 0.02) { velocity = demoVelocity; }
          decayStep();
        } else {
          angle = 0; velocity = 0;
        }
        apply();
      } else {
        decayStep();
        apply();
      }
      rafRef.current = requestAnimationFrame(loop);
    }
    apply();
    rafRef.current = requestAnimationFrame(loop);

    return () => {
      knob.removeEventListener("pointerdown", onDown);
      knob.removeEventListener("pointermove", onMove);
      knob.removeEventListener("pointerup", onUp);
      knob.removeEventListener("pointercancel", onUp);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.dr + " " + styles.isDemo} ref={rootRef}>
        <div className={styles.ring} ref={ringRef}>
          <button className={styles.knob} ref={knobRef} type="button" aria-label="드래그해서 볼륨 손잡이 돌리기" />
        </div>
        <p className={styles.hint}>{speedLabel}</p>
      </div>
    </main>
  );
}