07Button Whose Text Flips as the Fill Brightens

<main class="stage">
<div class="bc is-demo" id="bcRoot">
  <button class="bc__btn" id="bcBtn" type="button">Readable label</button>
  <p class="bc__meta"><span class="bc__tag">L</span> <b id="bcLum">0.01</b> <span class="bc__tag">RATIO</span> <b id="bcRatio">17.0</b><span>:1</span></p>
</div>
</main>

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

// 핵심: 바탕이 어두운 색에서 밝은 색으로 옮겨 가는 동안 글자색을 매 프레임 상대 밝기로 정한다.
// 글자 두 색은 순수한 흰색과 검정이다 — 둘이 같은 대비를 내는 밝기(0.179)에서 4.58:1 이라 어느 중간 색도 4.5:1 을 넘는다.

$text-pure-dark: #000;

@include stage($stage-paper);

.bc {
  --c-from: #{$subject-ink};
  --c-to: #{$subject-lilac};
  --t-light: #{$stage-paper};
  --t-dark: #{$text-pure-dark};
  width: min(100%, 320px);
  display: grid;
  gap: $sp-3;
  justify-items: center;
}

.bc__btn {
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: $r-control;
  background: $subject-ink;
  color: $stage-paper;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}
.bc__btn:focus-visible { outline: 3px solid $subject-ink; outline-offset: 3px; }

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  flex-wrap: wrap;
  justify-content: center;
  gap: $sp-1;
  font-size: 12px;
  line-height: 16px;
  color: rgba($subject-ink, .78);
}
.bc__tag { margin: 0 $sp-1 0 $sp-2; font-weight: 700; letter-spacing: .08em; }
.bc__tag:first-child { margin-left: 0; }
.bc__meta b { min-width: 4ch; text-align: right; font-variant-numeric: tabular-nums; color: $subject-ink; }

@media (prefers-reduced-motion: reduce) {
  .bc__btn { 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);
}

.bc {
  --c-from: #17141a;
  --c-to: #b9a0ff;
  --t-light: #ffffff;
  --t-dark: #000;
  width: min(100%, 320px);
  display: grid;
  gap: 12px;
  justify-items: center;
}

.bc__btn {
  box-sizing: border-box;
  width: 100%;
  height: 72px;
  border: 0;
  border-radius: 12px;
  background: #17141a;
  color: #ffffff;
  font-family: inherit;
  font-size: 20px;
  font-weight: 700;
  line-height: 24px;
  cursor: pointer;
}

.bc__btn:focus-visible {
  outline: 3px solid #17141a;
  outline-offset: 3px;
}

.bc__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  flex-wrap: wrap;
  justify-content: center;
  gap: 4px;
  font-size: 12px;
  line-height: 16px;
  color: rgba(23, 20, 26, 0.78);
}

.bc__tag {
  margin: 0 4px 0 8px;
  font-weight: 700;
  letter-spacing: 0.08em;
}

.bc__tag:first-child {
  margin-left: 0;
}

.bc__meta b {
  min-width: 4ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
  color: #17141a;
}

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

(function () {
  // 기법: 바탕색이 어두운 색에서 밝은 색으로 옮겨 가는 동안 매 프레임 상대 밝기 L 을 직접 계산한다.
  // L 이 FLIP_THRESHOLD 를 넘는 순간 글자를 흰색에서 검정으로 뒤집는다.
  // 흰색과 검정이 같은 대비(4.58:1)를 내는 밝기가 0.179 라서, 이 기준이면 어느 중간 색에서도 4.5:1 아래로 떨어지지 않는다.
  var FLIP_THRESHOLD = 0.179;
  var FILL_RATE = 4;
  var START_MS = 200;
  var ON_MS = 1000;
  var CYCLE_MS = 2000;

  var root = document.getElementById('bcRoot');
  var btn = document.getElementById('bcBtn');
  var lumEl = document.getElementById('bcLum');
  var ratioEl = document.getElementById('bcRatio');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  function readRgb(name) {
    var v = getComputedStyle(btn).getPropertyValue(name).trim().replace('#', '');
    if (v.length === 3) v = v.split('').map(function (c) { return c + c; }).join('');
    return [parseInt(v.slice(0, 2), 16), parseInt(v.slice(2, 4), 16), parseInt(v.slice(4, 6), 16)];
  }
  function toHex(rgb) {
    return '#' + rgb.map(function (c) { return ('0' + Math.round(c).toString(16)).slice(-2); }).join('');
  }
  function channel(c) {
    var s = c / 255;
    return s <= 0.04045 ? s / 12.92 : Math.pow((s + 0.055) / 1.055, 2.4);
  }
  function luminance(rgb) {
    return 0.2126 * channel(rgb[0]) + 0.7152 * channel(rgb[1]) + 0.0722 * channel(rgb[2]);
  }
  function ratio(a, b) {
    var hi = Math.max(a, b);
    var lo = Math.min(a, b);
    return (hi + 0.05) / (lo + 0.05);
  }

  var from = readRgb('--c-from');
  var to = readRgb('--c-to');
  var textLight = readRgb('--t-light');
  var textDark = readRgb('--t-dark');
  var lumLight = luminance(textLight);
  var lumDark = luminance(textDark);
  var cur = from.slice();
  var pointerIn = false;
  var kbFocus = false;
  var t0 = null;
  var lastTs = null;

  function demoOn(now) {
    return root.classList.contains('is-demo') && now >= START_MS && (now - START_MS) % CYCLE_MS < ON_MS;
  }
  function goalOf(now) { return demoOn(now) || pointerIn || kbFocus ? to : from; }
  function paint() {
    var l = luminance(cur);
    var dark = l > FLIP_THRESHOLD;
    btn.style.background = toHex(cur);
    btn.style.color = toHex(dark ? textDark : textLight);
    lumEl.textContent = l.toFixed(2);
    ratioEl.textContent = ratio(l, dark ? lumDark : lumLight).toFixed(1);
  }
  function stopDemo() { root.classList.remove('is-demo'); }
  function jump() { cur = (pointerIn || kbFocus ? to : from).slice(); paint(); }

  btn.addEventListener('pointerenter', function () { stopDemo(); pointerIn = true; if (reduce) jump(); });
  btn.addEventListener('pointerleave', function () { pointerIn = false; if (reduce) jump(); });
  btn.addEventListener('focus', function () { kbFocus = btn.matches(':focus-visible'); if (kbFocus) stopDemo(); if (reduce) jump(); });
  btn.addEventListener('blur', function () { kbFocus = false; if (reduce) jump(); });

  function tick(ts) {
    if (t0 === null) t0 = ts;
    var now = ts - t0;
    var dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
    lastTs = ts;
    var g = goalOf(now);
    var k = 1 - Math.exp(-dt * FILL_RATE);
    var gap = 0;
    for (var i = 0; i < 3; i += 1) {
      cur[i] += (g[i] - cur[i]) * k;
      gap += Math.abs(g[i] - cur[i]);
    }
    if (gap < 0.5) cur = g.slice();
    paint();
    requestAnimationFrame(tick);
  }

  if (reduce) {
    stopDemo();
    paint();
  } else {
    requestAnimationFrame(tick);
  }
})();

// TextFlipButton — vanilla 판(07-btn-color-text-contrast-flip) 의 React 포트.
// 바탕색이 어두운 색에서 밝은 색으로 옮겨 가는 동안 상대 밝기 L 을 계산해 FLIP_THRESHOLD 를 넘으면 글자색을 뒤집는다.
import { useEffect, useRef } from "react";
import styles from "./TextFlipButton.module.scss";

type Box<T> = { current: T };
type Rgb = [number, number, number];

const FLIP_THRESHOLD = 0.179;
const FILL_RATE = 4;
const START_MS = 200;
const ON_MS = 1000;
const CYCLE_MS = 2000;

function toHex(rgb: Rgb): string {
  return "#" + rgb.map((c) => ("0" + Math.round(c).toString(16)).slice(-2)).join("");
}
function channel(c: number): number {
  const s = c / 255;
  return s <= 0.04045 ? s / 12.92 : Math.pow((s + 0.055) / 1.055, 2.4);
}
function luminance(rgb: Rgb): number {
  return 0.2126 * channel(rgb[0]) + 0.7152 * channel(rgb[1]) + 0.0722 * channel(rgb[2]);
}
function ratio(a: number, b: number): number {
  return (Math.max(a, b) + 0.05) / (Math.min(a, b) + 0.05);
}

export default function TextFlipButton() {
  const rootRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement | null>;
  const btnRef = useRef<HTMLButtonElement | null>(null) as Box<HTMLButtonElement | null>;
  const lumRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;
  const ratioRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;

  useEffect(() => {
    const btn = btnRef.current;
    const lumEl = lumRef.current;
    const ratioEl = ratioRef.current;
    if (!btn || !lumEl || !ratioEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    const readRgb = (name: string): Rgb => {
      let v = getComputedStyle(btn).getPropertyValue(name).trim().replace("#", "");
      if (v.length === 3) v = v.split("").map((c) => c + c).join("");
      return [parseInt(v.slice(0, 2), 16), parseInt(v.slice(2, 4), 16), parseInt(v.slice(4, 6), 16)];
    };
    const from = readRgb("--c-from");
    const to = readRgb("--c-to");
    const textLight = readRgb("--t-light");
    const textDark = readRgb("--t-dark");
    const lumLight = luminance(textLight);
    const lumDark = luminance(textDark);
    let cur: Rgb = [from[0], from[1], from[2]];
    let demo = !reduce;
    let pointerIn = false;
    let kbFocus = false;
    let t0: number | null = null;
    let lastTs: number | null = null;
    let raf = 0;

    const demoOn = (now: number): boolean => demo && now >= START_MS && (now - START_MS) % CYCLE_MS < ON_MS;
    const paint = (): void => {
      const l = luminance(cur);
      const dark = l > FLIP_THRESHOLD;
      btn.style.background = toHex(cur);
      btn.style.color = toHex(dark ? textDark : textLight);
      lumEl.textContent = l.toFixed(2);
      ratioEl.textContent = ratio(l, dark ? lumDark : lumLight).toFixed(1);
    };
    const stopDemo = (): void => { demo = false; };
    const jump = (): void => {
      const g = pointerIn || kbFocus ? to : from;
      cur = [g[0], g[1], g[2]];
      paint();
    };
    const onEnter = (): void => { stopDemo(); pointerIn = true; if (reduce) jump(); };
    const onLeave = (): void => { pointerIn = false; if (reduce) jump(); };
    const onFocus = (): void => { kbFocus = btn.matches(":focus-visible"); if (kbFocus) stopDemo(); if (reduce) jump(); };
    const onBlur = (): void => { kbFocus = false; if (reduce) jump(); };
    btn.addEventListener("pointerenter", onEnter);
    btn.addEventListener("pointerleave", onLeave);
    btn.addEventListener("focus", onFocus);
    btn.addEventListener("blur", onBlur);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      const now = ts - t0;
      const dt = lastTs === null ? 0.016 : Math.min(0.05, (ts - lastTs) / 1000);
      lastTs = ts;
      const g = demoOn(now) || pointerIn || kbFocus ? to : from;
      const k = 1 - Math.exp(-dt * FILL_RATE);
      const next: Rgb = [cur[0] + (g[0] - cur[0]) * k, cur[1] + (g[1] - cur[1]) * k, cur[2] + (g[2] - cur[2]) * k];
      const gap = Math.abs(g[0] - next[0]) + Math.abs(g[1] - next[1]) + Math.abs(g[2] - next[2]);
      cur = gap < 0.5 ? [g[0], g[1], g[2]] : next;
      paint();
      raf = requestAnimationFrame(tick);
    }

    if (reduce) paint();
    else raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      btn.removeEventListener("pointerenter", onEnter);
      btn.removeEventListener("pointerleave", onLeave);
      btn.removeEventListener("focus", onFocus);
      btn.removeEventListener("blur", onBlur);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.bc} ref={rootRef}>
        <button className={styles.bc__btn} type="button" ref={btnRef}>읽히는 글자</button>
        <p className={styles.bc__meta}>
          <span className={styles.bc__tag}>L</span> <b ref={lumRef}>0.01</b> <span className={styles.bc__tag}>RATIO</span> <b ref={ratioRef}>17.0</b><span>:1</span>
        </p>
      </div>
    </main>
  );
}