09Badge count inertia — a number on the icon that eases toward a changed value frame by frame

<main class="stage">
<div class="cbci is-demo" id="cbci">
  <button class="cbci__fab" id="cbciAdd" type="button" aria-label="Add">
    <svg viewBox="0 0 256 256" width="26" height="26" aria-hidden="true"><g fill="currentColor"><path d="M222.14 58.87A8 8 0 0 0 216 56H62.68L56.16 22.35A8 8 0 0 0 48 16H16a8 8 0 0 0 0 16h25.6l32 165.4A24 24 0 1 0 105.14 224a23.94 23.94 0 0 0 15.5-8h34.72a23.94 23.94 0 1 0 22.32-32h-51a8 8 0 0 1-7.86-6.43L114.68 160H188a24 24 0 0 0 23.53-19.24l16.29-75.63A8 8 0 0 0 222.14 58.87Z"/></g></svg>
    <span class="cbci__badge" id="cbciBadge">0</span>
  </button>
</div>
</main>

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

// 계산 겹: 표시 숫자가 목표 숫자를 매 프레임 lerp() 로 따라잡는다(tick(), index.html) —
// 값이 바뀔 때마다 즉시 세지 않고 차이를 매 프레임 조금씩 줄이는 관성 계산이다.

$duration: $dur-quick;
$easing: $ease-pop;
$color: $error;

@include stage($stage-ink);

.stage { position: relative; }

.cbci__fab {
  position: relative;
  width: 64px;
  height: 64px;
  border: 0;
  border-radius: $r-pill;
  background: $subject-cream;
  color: $subject-ink;
  display: grid;
  place-items: center;
  cursor: pointer;
  box-shadow: $shadow-raised;
}

.cbci__badge {
  position: absolute;
  top: -6px;
  right: -6px;
  min-width: 22px;
  height: 22px;
  padding: 0 $sp-1;
  border-radius: $r-pill;
  background: $color;
  color: $subject-cream;
  font-family: inherit;
  font-weight: 700;
  font-size: 12px;
  display: grid;
  place-items: center;
  transform-origin: center;
}

@media (max-width: 340px) {
  .cbci__fab { width: 52px; height: 52px; }
  .cbci__badge { min-width: 18px; height: 18px; font-size: 10px; }
}

@media (prefers-reduced-motion: reduce) {
  .cbci__badge { 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: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.stage {
  position: relative;
}

.cbci__fab {
  position: relative;
  width: 64px;
  height: 64px;
  border: 0;
  border-radius: 999px;
  background: #fff7e6;
  color: #17141a;
  display: grid;
  place-items: center;
  cursor: pointer;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

.cbci__badge {
  position: absolute;
  top: -6px;
  right: -6px;
  min-width: 22px;
  height: 22px;
  padding: 0 4px;
  border-radius: 999px;
  background: #c2361a;
  color: #fff7e6;
  font-family: inherit;
  font-weight: 700;
  font-size: 12px;
  display: grid;
  place-items: center;
  transform-origin: center;
}

@media (max-width: 340px) {
  .cbci__fab {
    width: 52px;
    height: 52px;
  }
  .cbci__badge {
    min-width: 18px;
    height: 18px;
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .cbci__badge {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('cbci');
  var addBtn = document.getElementById('cbciAdd');
  var badge = document.getElementById('cbciBadge');

  // 계산 겹: 표시 숫자(displayed)가 목표 숫자(target)를 매 프레임 향해 관성 lerp() 로
  // 따라잡는다 — 값이 바뀔 때마다 다시 세지 않고, 두 값의 차이를 매 프레임 조금씩 줄인다.
  function lerp(a, b, t) { return a + (b - a) * t; }

  var target = 0;
  var displayed = 0;
  var raf = null;

  function tick() {
    displayed = lerp(displayed, target, 0.16);
    var shown = Math.round(displayed);
    badge.textContent = String(shown);
    var pulse = 1 + Math.min(0.3, Math.abs(target - displayed) / (target + 1) * 0.6);
    badge.style.transform = 'scale(' + pulse.toFixed(3) + ')';
    if (Math.abs(target - displayed) > 0.03) {
      raf = requestAnimationFrame(tick);
    } else {
      displayed = target;
      badge.textContent = String(target);
      badge.style.transform = 'scale(1)';
      raf = null;
    }
  }
  function kick() {
    if (!raf) { raf = requestAnimationFrame(tick); }
  }

  function addOne() {
    target += 1;
    if (target > 9) { target = 0; displayed = 0; badge.textContent = '0'; }
    kick();
  }

  addBtn.addEventListener('click', function () {
    root.classList.remove('is-demo');
    stopAutoplay();
    addOne();
  });

  var autoplayTimer = null;
  function stopAutoplay() {
    if (autoplayTimer) { clearInterval(autoplayTimer); autoplayTimer = null; }
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (!reduce) {
    autoplayTimer = setInterval(addOne, 380);
  }
})();

import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./CartDrawerBadgeCountInertia.module.scss";

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

// 계산 겹: 목표 수량으로 즉시 점프하지 않고 매 프레임 lerp() 로 표시 숫자를 목표 쪽으로
// 끌어당긴다(관성) — 거리(distance)가 클수록 배지 pulse 스케일도 함께 커진다.
function lerp(a: number, b: number, t: number): number {
  return a + (b - a) * t;
}

const INERTIA = 0.16;
const MAX_COUNT = 9;
const TICK_MS = 380;

export default function CartDrawerBadgeCountInertia({
  color = "#c2361a",
}: CartDrawerBadgeCountInertiaProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const badgeRef = useRef<HTMLSpanElement | null>(null);
  const fabRef = useRef<HTMLButtonElement | null>(null);
  const targetRef = useRef(0);
  const displayedRef = useRef(0);
  const rafRef = useRef<number | null>(null);
  const timerRef = useRef<ReturnType<typeof setInterval> | null>(null);

  useEffect(() => {
    const badge = badgeRef.current;
    const fab = fabRef.current;
    if (!badge || !fab) return;

    function ensureLoop() {
      if (rafRef.current) return;
      const step = () => {
        const target = targetRef.current ?? 0;
        const dist = target - (displayedRef.current ?? 0);
        displayedRef.current = lerp(displayedRef.current ?? 0, target, INERTIA);
        const shown = Math.round(displayedRef.current ?? 0);
        badge!.textContent = String(shown);
        const pulse = 1 + Math.min(0.4, Math.abs(dist) * 0.05);
        badge!.style.transform = `scale(${pulse.toFixed(3)})`;
        if (Math.abs(dist) > 0.02) {
          rafRef.current = requestAnimationFrame(step);
        } else {
          displayedRef.current = target;
          badge!.textContent = String(target);
          badge!.style.transform = "scale(1)";
          rafRef.current = null;
        }
      };
      rafRef.current = requestAnimationFrame(step);
    }

    function addOne() {
      const t = targetRef.current ?? 0;
      targetRef.current = t >= MAX_COUNT ? 0 : t + 1;
      ensureLoop();
    }

    function stopAutoplay() {
      if (timerRef.current) {
        clearInterval(timerRef.current);
        timerRef.current = null;
      }
    }

    const onClick = () => {
      rootRef.current?.classList.remove(styles.isDemo);
      stopAutoplay();
      addOne();
    };
    fab.addEventListener("click", onClick);

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (!reduce) {
      timerRef.current = setInterval(addOne, TICK_MS);
    }

    return () => {
      stopAutoplay();
      if (rafRef.current) cancelAnimationFrame(rafRef.current);
      fab.removeEventListener("click", onClick);
    };
  }, []);

  return (
    <div ref={rootRef} className={`${styles.stage} ${styles.isDemo}`} style={{ "--color": color } as CSSProperties}>
      <button ref={fabRef} type="button" className={styles.fab} aria-label="담기">
        <svg viewBox="0 0 256 256" width="26" height="26" aria-hidden="true">
          <g fill="currentColor">
            <path opacity=".5" d="M216 48h-40v-8a24 24 0 0 0-24-24h-48a24 24 0 0 0-24 24v8H40a8 8 0 0 0 0 16h8v144a16 16 0 0 0 16 16h128a16 16 0 0 0 16-16V64h8a8 8 0 0 0 0-16ZM96 40a8 8 0 0 1 8-8h48a8 8 0 0 1 8 8v8H96Zm96 168H64V64h128Z" />
          </g>
        </svg>
        <span ref={badgeRef} className={styles.badge}>0</span>
      </button>
    </div>
  );
}