01Elastic pull — a refresh trigger whose resistance grows as the dragged distance value increases

<main class="stage">
<div class="pred is-demo" id="pred">
  <div class="pred__phone" id="predPhone">
    <div class="pred__zone">
      <span class="pred__ghost" id="predGhost"></span>
      <span class="pred__dot" id="predDot"></span>
    </div>
    <div class="pred__list" id="predList">
      <div class="pred__row"></div>
      <div class="pred__row"></div>
      <div class="pred__row"></div>
    </div>
  </div>
  <span class="pred__hint" id="predHint">Pull down to try</span>
</div>
</main>

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

// 계산 겹(저항 곡선 resist())은 JS 에 있다 — 여기는 정적 배치만.
@include stage($stage-paper);

.pred {
  position: relative;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-3;
}

.pred__phone {
  position: relative;
  width: 220px;
  height: 168px;
  overflow: hidden;
  border-radius: $r-card;
  background: $subject-cream;
  touch-action: none;
  cursor: grab;
}

.pred__zone {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 48px;
  display: grid;
  place-items: center;
}

.pred__ghost {
  position: absolute;
  top: 6px;
  width: 4px;
  height: 4px;
  border-radius: $r-pill;
  background: rgba($subject-ink, .28);
  opacity: 0;
}

.pred__dot {
  position: relative;
  width: 20px;
  height: 20px;
  border-radius: $r-pill;
  background: $subject-blue-deep;
  opacity: 0;
}

.pred__list {
  position: relative;
  display: flex;
  flex-direction: column;
  gap: $sp-2;
  padding: $sp-3 $sp-4;
  padding-top: 52px;
}

.pred__row {
  height: 20px;
  border-radius: $r-sm;
  background: $stage-paper;
  box-shadow: 0 1px 0 rgba($subject-ink, .1);
}

.pred__hint {
  color: rgba($subject-ink, .72);
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .pred__phone { width: 176px; height: 132px; }
  .pred__zone { height: 40px; }
  .pred__list { padding: $sp-2 $sp-3; padding-top: 44px; gap: $sp-1; }
  .pred__row { height: 16px; }
}

@media (prefers-reduced-motion: reduce) {
  .pred__dot, .pred__list, .pred__ghost { 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);
}

.pred {
  position: relative;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
}

.pred__phone {
  position: relative;
  width: 220px;
  height: 168px;
  overflow: hidden;
  border-radius: 16px;
  background: #fff7e6;
  touch-action: none;
  cursor: grab;
}

.pred__zone {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 48px;
  display: grid;
  place-items: center;
}

.pred__ghost {
  position: absolute;
  top: 6px;
  width: 4px;
  height: 4px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.28);
  opacity: 0;
}

.pred__dot {
  position: relative;
  width: 20px;
  height: 20px;
  border-radius: 999px;
  background: #0a4fe8;
  opacity: 0;
}

.pred__list {
  position: relative;
  display: flex;
  flex-direction: column;
  gap: 8px;
  padding: 12px 16px;
  padding-top: 52px;
}

.pred__row {
  height: 20px;
  border-radius: 8px;
  background: #ffffff;
  box-shadow: 0 1px 0 rgba(23, 20, 26, 0.1);
}

.pred__hint {
  color: rgba(23, 20, 26, 0.72);
  font-size: 12px;
  text-align: center;
}

@media (max-width: 340px) {
  .pred__phone {
    width: 176px;
    height: 132px;
  }
  .pred__zone {
    height: 40px;
  }
  .pred__list {
    padding: 8px 12px;
    padding-top: 44px;
    gap: 4px;
  }
  .pred__row {
    height: 16px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .pred__dot, .pred__list, .pred__ghost {
    transition: none !important;
  }
}

(function () {
  var phone = document.getElementById('predPhone');
  var ghost = document.getElementById('predGhost');
  var dot = document.getElementById('predDot');
  var list = document.getElementById('predList');
  var hint = document.getElementById('predHint');
  var root = document.getElementById('pred');

  // 계산 겹: 당긴 원 거리(raw)를 Displaced 거리(displayed)로 바꾸는 저항 곡선.
  // displayed = D_MAX * raw / (raw + K) — raw 가 커질수록 늘어나는 폭이 점점 좁아지는 포화 곡선(Michaelis-Menten).
  var D_MAX = 56, K = 80, RAW_MAX = 200;

  function resist(raw) { return D_MAX * raw / (raw + K); }

  function apply(raw) {
    var displayed = resist(raw);
    var ratio = displayed / D_MAX;
    ghost.style.transform = 'translateY(' + Math.min(raw, D_MAX + 24).toFixed(1) + 'px)';
    ghost.style.opacity = String(Math.min(1, raw / 30));
    dot.style.transform = 'translateY(' + displayed.toFixed(1) + 'px) scale(' + (0.6 + 0.4 * ratio).toFixed(2) + ')';
    dot.style.opacity = String(ratio);
    list.style.transform = 'translateY(' + displayed.toFixed(1) + 'px)';
    hint.textContent = raw > 4 ? ('Pull ' + Math.round(raw) + 'px · Displaced ' + Math.round(displayed) + 'px') : 'Pull down to try';
  }

  var raw = 0, target = 0, raf = null;
  function settle() {
    raw += (target - raw) * 0.28;
    if (Math.abs(target - raw) < 0.3) { raw = target; }
    apply(raw);
    if (raw !== target) { raf = requestAnimationFrame(settle); } else { raf = null; }
  }
  function goTo(v) { target = v; if (raf === null) { raf = requestAnimationFrame(settle); } }

  var dragging = false, startY = 0, startRaw = 0;
  function onDown(y) { dragging = true; startY = y; startRaw = raw; if (raf) { cancelAnimationFrame(raf); raf = null; } }
  function onMove(y) {
    if (!dragging) { return; }
    raw = Math.max(0, Math.min(RAW_MAX, startRaw + (y - startY)));
    apply(raw);
  }
  function onUp() { if (!dragging) { return; } dragging = false; goTo(0); }

  function wireReal() {
    phone.addEventListener('pointerdown', function (e) { root.classList.remove('is-demo'); stopAutoplay(); onDown(e.clientY); });
    window.addEventListener('pointermove', function (e) { onMove(e.clientY); });
    window.addEventListener('pointerup', onUp);
  }
  phone.style.touchAction = 'none';
  wireReal();
  apply(0);

  // 자동 루프: raw 를 0→RAW_MAX 로 단계적으로 올렸다가 놓아 같은 resist()·settle() 경로를 탄다.
  var autoRaw = 0, autoTimer = null, autoDir = 1;
  function autoTick() {
    autoRaw += autoDir * 40;
    if (autoRaw >= RAW_MAX) { autoRaw = RAW_MAX; autoDir = -1; goTo(0); return; }
    if (autoRaw <= 0 && autoDir < 0) { autoRaw = 0; autoDir = 1; }
    if (autoDir > 0) { raw = autoRaw; apply(raw); }
  }
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  if (!reduce) { autoTimer = setInterval(autoTick, 180); }
})();

import { useEffect, useRef } from "react";
import styles from "./PullRefreshElasticDrag.module.scss";

export type PullRefreshElasticDragProps = {
  color?: string;
};

export default function PullRefreshElasticDrag({
  color = "#0a4fe8",
}: PullRefreshElasticDragProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    const phone = root.querySelector<HTMLDivElement>(`.${styles.phone}`);
    const ghost = root.querySelector<HTMLSpanElement>(`.${styles.ghost}`);
    const dot = root.querySelector<HTMLSpanElement>(`.${styles.dot}`);
    const list = root.querySelector<HTMLDivElement>(`.${styles.list}`);
    const hint = root.querySelector<HTMLSpanElement>(`.${styles.hint}`);
    if (!phone || !ghost || !dot || !list || !hint) return;

    // 계산 겹: 당긴 원 거리(raw)를 밀림 거리(displayed)로 바꾸는 저항 곡선.
    // displayed = D_MAX * raw / (raw + K) — raw 가 커질수록 늘어나는 폭이 점점 좁아지는 포화 곡선(Michaelis-Menten).
    const D_MAX = 56, K = 80, RAW_MAX = 200;
    function resist(raw: number) { return D_MAX * raw / (raw + K); }

    function apply(raw: number) {
      const displayed = resist(raw);
      const ratio = displayed / D_MAX;
      ghost!.style.transform = "translateY(" + Math.min(raw, D_MAX + 24).toFixed(1) + "px)";
      ghost!.style.opacity = String(Math.min(1, raw / 30));
      dot!.style.transform = "translateY(" + displayed.toFixed(1) + "px) scale(" + (0.6 + 0.4 * ratio).toFixed(2) + ")";
      dot!.style.opacity = String(ratio);
      list!.style.transform = "translateY(" + displayed.toFixed(1) + "px)";
      hint!.textContent = raw > 4 ? ("당김 " + Math.round(raw) + "px · 밀림 " + Math.round(displayed) + "px") : "아래로 당겨보세요";
    }

    let raw = 0, target = 0, raf: number | null = null;
    function settle() {
      raw += (target - raw) * 0.28;
      if (Math.abs(target - raw) < 0.3) { raw = target; }
      apply(raw);
      raf = raw !== target ? requestAnimationFrame(settle) : null;
    }
    function goTo(v: number) { target = v; if (raf === null) { raf = requestAnimationFrame(settle); } }

    let dragging = false, startY = 0, startRaw = 0;
    function onDown(y: number) { dragging = true; startY = y; startRaw = raw; if (raf !== null) { cancelAnimationFrame(raf); raf = null; } }
    function onMove(y: number) {
      if (!dragging) return;
      raw = Math.max(0, Math.min(RAW_MAX, startRaw + (y - startY)));
      apply(raw);
    }
    function onUp() { if (!dragging) return; dragging = false; goTo(0); }

    function onPointerDown(e: PointerEvent) { stopAutoplay(); onDown(e.clientY); }
    function onPointerMove(e: PointerEvent) { onMove(e.clientY); }
    phone.addEventListener("pointerdown", onPointerDown);
    window.addEventListener("pointermove", onPointerMove);
    window.addEventListener("pointerup", onUp);
    apply(0);

    let autoRaw = 0, autoTimer: number | null = null, autoDir = 1;
    function autoTick() {
      autoRaw += autoDir * 40;
      if (autoRaw >= RAW_MAX) { autoRaw = RAW_MAX; autoDir = -1; goTo(0); return; }
      if (autoRaw <= 0 && autoDir < 0) { autoRaw = 0; autoDir = 1; }
      if (autoDir > 0) { raw = autoRaw; apply(raw); }
    }
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (!reduce) { autoTimer = window.setInterval(autoTick, 180); }

    return () => {
      stopAutoplay();
      if (raf !== null) { cancelAnimationFrame(raf); }
      phone.removeEventListener("pointerdown", onPointerDown);
      window.removeEventListener("pointermove", onPointerMove);
      window.removeEventListener("pointerup", onUp);
    };
  }, []);

  return (
    <div ref={rootRef} className={styles.stage}>
      <div className={styles.pred}>
        <div className={styles.phone}>
          <div className={styles.zone}>
            <span className={styles.ghost} />
            <span className={styles.dot} style={{ background: color }} />
          </div>
          <div className={styles.list}>
            <div className={styles.row} />
            <div className={styles.row} />
            <div className={styles.row} />
          </div>
        </div>
        <span className={styles.hint}>아래로 당겨보세요</span>
      </div>
    </div>
  );
}