06Background fill

<main class="stage">
  <a class="nav" href="#">Learn more</a>
</main>

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

// ── 변수 3개 ─────────────────────────────
$duration: $dur-loop;
$easing: $ease-out;
$color: $subject-blue;         // 채워지는 색

// ── 데모 무대 ───────────────────────────────
@include stage($stage-ink);

.nav {
  display: inline-block;
  padding: $sp-2 $sp-4;
  border-radius: $r-sm;
  color: $subject-cream;
  text-decoration: none;
  font-weight: 700;
  font-size: clamp(16px, 4.6vw, 22px);
  line-height: 1;
  font-family: inherit;

  // 배경만 왼쪽부터 차오른다. 위치는 left 로 고정해 채워진 가장자리가 밀리지 않는다
  background: linear-gradient(90deg, $color, $color);
  background-repeat: no-repeat;
  background-position: left center;
  background-size: 0% 100%;
  animation: fill-thickness $duration $easing infinite;
}

@keyframes fill {
  0%   { background-size: 0% 100%; }
  25%  { background-size: 100% 100%; }
  75%  { background-size: 100% 100%; }
  100% { background-size: 0% 100%; }
}

// 페인트 물성: 채워지는 동안 수직 두께(펜 압력)가 불규칙 요동친다 — 이중 background-size 값 보간
@keyframes fill-thickness {
  0%   { background-size: 0% 100%; }
  6%   { background-size: 22% 92%; }
  12%  { background-size: 44% 96%; }
  18%  { background-size: 68% 88%; }
  25%  { background-size: 100% 104%; }
  32%  { background-size: 100% 96%; }
  45%  { background-size: 100% 101%; }
  60%  { background-size: 100% 94%; }
  75%  { background-size: 100% 102%; }
  82%  { background-size: 74% 97%; }
  90%  { background-size: 38% 93%; }
  100% { background-size: 0% 100%; }
}

@media (prefers-reduced-motion: reduce) {
  .nav { animation: none; background-size: 100% 100%; }
}

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);
}

.nav {
  display: inline-block;
  padding: 8px 16px;
  border-radius: 8px;
  color: #fff7e6;
  text-decoration: none;
  font-weight: 700;
  font-size: clamp(16px, 4.6vw, 22px);
  line-height: 1;
  font-family: inherit;
  background: linear-gradient(90deg, #2f6df6, #2f6df6);
  background-repeat: no-repeat;
  background-position: left center;
  background-size: 0% 100%;
  animation: fill-thickness 2s ease-out infinite;
}

@keyframes fill {
  0% {
    background-size: 0% 100%;
  }
  25% {
    background-size: 100% 100%;
  }
  75% {
    background-size: 100% 100%;
  }
  100% {
    background-size: 0% 100%;
  }
}
@keyframes fill-thickness {
  0% {
    background-size: 0% 100%;
  }
  6% {
    background-size: 22% 92%;
  }
  12% {
    background-size: 44% 96%;
  }
  18% {
    background-size: 68% 88%;
  }
  25% {
    background-size: 100% 104%;
  }
  32% {
    background-size: 100% 96%;
  }
  45% {
    background-size: 100% 101%;
  }
  60% {
    background-size: 100% 94%;
  }
  75% {
    background-size: 100% 102%;
  }
  82% {
    background-size: 74% 97%;
  }
  90% {
    background-size: 38% 93%;
  }
  100% {
    background-size: 0% 100%;
  }
}
@media (prefers-reduced-motion: reduce) {
  .nav {
    animation: none;
    background-size: 100% 100%;
  }
}

import type { CSSProperties, ReactNode } from "react";
import styles from "./BgFillLink.module.scss";

export type BgFillLinkProps = {
  children: ReactNode;
  href?: string;
  /** 배경이 다 차는 시간 (기본 2000ms) */
  duration?: number;
  easing?: string;
  /** 채워지는 색 */
  color?: string;
};

type Vars = CSSProperties & {
  "--duration"?: string;
  "--easing"?: string;
  "--color"?: string;
};

export default function BgFillLink({
  children,
  href = "#",
  duration = 2000,
  easing = "ease-out",
  color = "#2f6df6",
}: BgFillLinkProps) {
  const vars: Vars = {
    "--duration": `${duration}ms`,
    "--easing": easing,
    "--color": color,
  };
  return (
    <a className={styles.nav} href={href} style={vars}>
      {children}
    </a>
  );
}