03List add/remove animation

<main class="stage">
  <ul class="la-list is-demo">
    <li class="la-item">Wash the car</li>
    <li class="la-item">Reply to client email</li>
    <li class="la-item la-item--leave">Buy milk</li>
    <li class="la-item la-item--enter">Book flight tickets</li>
  </ul>
</main>

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

// ── 변수 3개 ──────────────────────────────────────────
$duration: $dur-base;
$easing: $ease-spring;
$color: $subject-ink;

@include stage($stage-orange);

.la-list {
  width: min(240px, 76vw);
  margin: 0;
  padding: 0;
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: $sp-2;
}

.la-item {
  box-sizing: border-box;
  padding: $sp-2 $sp-4;
  border-radius: $r-sm;
  background: $stage-paper;
  color: $color;
  font: 600 12px/1.3 -apple-system, "Segoe UI", sans-serif;
  box-shadow: $shadow-press;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

// 실사용 원리: 배열에서 항목을 지우면 새 <li> 는 @starting-style 로 들어오고,
// 지워지는 항목은 02번 exit 패턴처럼 클래스가 먼저 붙은 뒤 실제로 제거된다.
// 여기서는 둘 다 transform·opacity 로만 움직여 레이아웃 재계산을 피한다.
@keyframes la-leave {
  0%, 40%  { opacity: 1; transform: none; }
  55%      { opacity: 0; transform: translateX(-8px) scale(.97); }
  100%     { opacity: 0; transform: translateX(-8px) scale(.97); }
}
@keyframes la-enter {
  0%, 55%  { opacity: 0; transform: translateX(8px) scale(.97); }
  85%, 100% { opacity: 1; transform: none; }
}
.la-list.is-demo .la-item--leave { animation: la-leave $dur-loop $easing infinite; }
.la-list.is-demo .la-item--enter { animation: la-enter $dur-loop $easing infinite; }

@media (prefers-reduced-motion: reduce) {
  .la-list.is-demo .la-item--leave,
  .la-list.is-demo .la-item--enter { animation: none !important; opacity: 1; transform: none; }
}

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);
}

.la-list {
  width: min(240px, 76vw);
  margin: 0;
  padding: 0;
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.la-item {
  box-sizing: border-box;
  padding: 8px 16px;
  border-radius: 8px;
  background: #ffffff;
  color: #17141a;
  font: 600 12px/1.3 -apple-system, "Segoe UI", sans-serif;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

@keyframes la-leave {
  0%, 40% {
    opacity: 1;
    transform: none;
  }
  55% {
    opacity: 0;
    transform: translateX(-8px) scale(0.97);
  }
  100% {
    opacity: 0;
    transform: translateX(-8px) scale(0.97);
  }
}
@keyframes la-enter {
  0%, 55% {
    opacity: 0;
    transform: translateX(8px) scale(0.97);
  }
  85%, 100% {
    opacity: 1;
    transform: none;
  }
}
.la-list.is-demo .la-item--leave {
  animation: la-leave 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

.la-list.is-demo .la-item--enter {
  animation: la-enter 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

@media (prefers-reduced-motion: reduce) {
  .la-list.is-demo .la-item--leave,
  .la-list.is-demo .la-item--enter {
    animation: none !important;
    opacity: 1;
    transform: none;
  }
}

import { CSSProperties, useState } from "react";
import styles from "./ListAddRemove.module.scss";

interface Task {
  id: number;
  label: string;
  leaving?: boolean;
}

let nextId = 3;

export interface ListAddRemoveProps {
  duration?: string;
  easing?: string;
}

export default function ListAddRemove({
  duration = "300ms",
  easing = "cubic-bezier(.2, .8, .2, 1)",
}: ListAddRemoveProps) {
  const [items, setItems] = useState<Task[]>([
    { id: 1, label: "Wash the car" },
    { id: 2, label: "Reply to client email" },
  ]);

  const add = () => {
    const id = nextId++;
    setItems((list) => [...list, { id, label: "Book flight tickets" }]);
  };

  const startRemove = (id: number) => {
    // Mark it leaving first (drives the CSS exit transition, styles.leaving below).
    // The item keeps rendering — with opacity heading to 0 — until its own
    // onTransitionEnd fires, which is when it actually leaves the array.
    setItems((list) => list.map((it) => (it.id === id ? { ...it, leaving: true } : it)));
  };

  const finishRemove = (id: number) => {
    setItems((list) => list.filter((it) => it.id !== id));
  };

  const style: CSSProperties & { "--duration"?: string; "--easing"?: string } = {
    "--duration": duration,
    "--easing": easing,
  };

  return (
    <div className={styles.wrap} style={style}>
      <ul className={styles.list}>
        {items.map((it) => (
          <li
            key={it.id}
            className={`${styles.item} ${it.leaving ? styles.leaving : styles.entering}`}
            onTransitionEnd={() => it.leaving && finishRemove(it.id)}
          >
            {it.label}
            <button className={styles.remove} onClick={() => startRemove(it.id)}>×</button>
          </li>
        ))}
      </ul>
      <button className={styles.add} onClick={add}>+ Add task</button>
    </div>
  );
}