08Button Whose Distance Value Stretches Letter Spacing

<main class="stage">
<div class="ls is-demo" id="ls">
  <p class="ls__badge" id="lsBadge">Spacing value 0em</p>
  <div class="ls__scene" id="lsScene">
    <button class="ls__btn" id="lsBtn" type="button">
      <span class="ls__label" id="lsLabel">Learn more</span>
      <span class="ls__underline" id="lsUnderline" aria-hidden="true"></span>
    </button>
  </div>
</div>
</main>

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

// 핵심: 거리 값(0~1)을 letter-spacing 값(em)으로 바로 매핑한다. 커서가 멀 때는
// 글자가 좁게 뭉쳐 있다가 가까워질수록 라벨 글자 사이가 넓게 벌어진다.

$duration: $dur-loop;
$easing: linear;
$color: $subject-blue-deep;

@include stage($stage-paper);

.ls {
  display: grid;
  gap: $sp-4;
  justify-items: center;
}

.ls__badge {
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(23, 20, 26, .06);
  color: $subject-ink;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.ls__scene {
  padding: $sp-10;
}

.ls__btn {
  position: relative;
  border: 0;
  cursor: pointer;
  padding: $sp-4 $sp-8;
  border-radius: $r-control;
  background: $color;
  color: $subject-cream;
  font-size: 15px;
  font-weight: 700;
  will-change: filter;
  --label-w: 0px;
  --label-o: 0;
  --underline-o: 0;
}

.ls__btn::before {
  content: '';
  position: absolute;
  left: 50%;
  top: 50%;
  width: calc(var(--label-w) + #{$sp-4});
  height: 28px;
  transform: translate(-50%, -50%);
  border-radius: $r-pill;
  background: rgba($subject-cream, .22);
  opacity: var(--label-o);
  pointer-events: none;
  z-index: 0;
}

.ls__label {
  position: relative;
  z-index: 1;
  display: inline-block;
  will-change: letter-spacing;
}

.ls__underline {
  position: absolute;
  left: 50%;
  bottom: $sp-2;
  transform: translateX(-50%);
  height: 2px;
  width: 0;
  background: $subject-cream;
  opacity: var(--underline-o);
  border-radius: $r-pill;
  pointer-events: none;
  z-index: 1;
}

.ls__btn:focus-visible {
  outline: 2px solid $subject-cream;
  outline-offset: 3px;
}

@media (max-width: 340px) {
  .ls__scene { padding: $sp-6; }
  .ls__btn { padding: $sp-3 $sp-6; font-size: 13px; }
}

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

.ls {
  display: grid;
  gap: 16px;
  justify-items: center;
}

.ls__badge {
  margin: 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.06);
  color: #17141a;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.ls__scene {
  padding: 40px;
}

.ls__btn {
  position: relative;
  border: 0;
  cursor: pointer;
  padding: 16px 32px;
  border-radius: 12px;
  background: #0a4fe8;
  color: #fff7e6;
  font-size: 15px;
  font-weight: 700;
  will-change: filter;
  --label-w: 0px;
  --label-o: 0;
  --underline-o: 0;
}

.ls__btn::before {
  content: "";
  position: absolute;
  left: 50%;
  top: 50%;
  width: calc(var(--label-w) + 16px);
  height: 28px;
  transform: translate(-50%, -50%);
  border-radius: 999px;
  background: rgba(255, 247, 230, 0.22);
  opacity: var(--label-o);
  pointer-events: none;
  z-index: 0;
}

.ls__label {
  position: relative;
  z-index: 1;
  display: inline-block;
  will-change: letter-spacing;
}

.ls__underline {
  position: absolute;
  left: 50%;
  bottom: 8px;
  transform: translateX(-50%);
  height: 2px;
  width: 0;
  background: #fff7e6;
  opacity: var(--underline-o);
  border-radius: 999px;
  pointer-events: none;
  z-index: 1;
}

.ls__btn:focus-visible {
  outline: 2px solid #fff7e6;
  outline-offset: 3px;
}

@media (max-width: 340px) {
  .ls__scene {
    padding: 24px;
  }
  .ls__btn {
    padding: 12px 24px;
    font-size: 13px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .ls__label {
    transition: none;
  }
}

(function () {
  var PROX_RADIUS_PX = 170;
  var LS_MAX_EM = 0.22;
  var LERP = 0.17;

  var root = document.getElementById('ls');
  var scene = document.getElementById('lsScene');
  var btn = document.getElementById('lsBtn');
  var label = document.getElementById('lsLabel');
  var underline = document.getElementById('lsUnderline');
  var badge = document.getElementById('lsBadge');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  function clamp(v, lo, hi) { return Math.max(lo, Math.min(hi, v)); }

  var targetP = 0;
  var curP = 0;
  var hasPointer = false;

  function distanceToButton(x, y) {
    var r = btn.getBoundingClientRect();
    var cx = r.left + r.width / 2;
    var cy = r.top + r.height / 2;
    return Math.hypot(x - cx, y - cy);
  }

  // 거리 값을 글자 사이 간격 값으로 바꾼다 — 멀 때는 0, 가까워질수록 벌어진다
  function render(p) {
    var spacing = p * LS_MAX_EM;
    label.style.letterSpacing = spacing.toFixed(3) + 'em';
    btn.style.filter = 'brightness(' + (1 + p * 0.12).toFixed(3) + ')';
    // 계산 레이어: 벌어진 실제 글자 폭(getBoundingClientRect)을 라벨 배경·밑줄 길이에 전파한다.
    var w = label.getBoundingClientRect().width;
    underline.style.width = w.toFixed(1) + 'px';
    btn.style.setProperty('--underline-o', (0.2 + p * 0.6).toFixed(2));
    btn.style.setProperty('--label-w', w.toFixed(1) + 'px');
    btn.style.setProperty('--label-o', (0.12 + p * 0.4).toFixed(2));
    // 계산 레이어: 같은 진행률(p)을 버튼 후광 그림자 반경·불투명도에도 전파한다.
    var spread = (6 + p * 26).toFixed(1);
    btn.style.boxShadow = '0 0 ' + spread + 'px ' + spread + 'px rgba(10,79,232,' + (0.16 + p * 0.42).toFixed(2) + ')';
    badge.textContent = 'Spacing value ' + spacing.toFixed(2) + 'em';
  }

  function syntheticOffset(t) {
    var ang = t * 1.6 + 2.1;
    var rad = 110 * (1 + Math.cos(t * Math.PI));
    return [Math.cos(ang) * rad, Math.sin(ang) * rad];
  }

  var startTime = null;
  function loop(now) {
    if (startTime === null) { startTime = now; }
    var t = (now - startTime) / 1000;
    if (!hasPointer && !reduce) {
      var off = syntheticOffset(t);
      var d = Math.hypot(off[0], off[1]);
      targetP = clamp(1 - d / PROX_RADIUS_PX, 0, 1);
    }
    curP += (targetP - curP) * LERP;
    render(curP);
    requestAnimationFrame(loop);
  }

  scene.addEventListener('pointermove', function (e) {
    hasPointer = true;
    root.classList.remove('is-demo');
    var d = distanceToButton(e.clientX, e.clientY);
    targetP = clamp(1 - d / PROX_RADIUS_PX, 0, 1);
    if (reduce) { curP = targetP; render(curP); }
  });
  scene.addEventListener('pointerleave', function () {
    hasPointer = false;
    targetP = 0;
    if (reduce) { curP = 0; render(0); }
  });
  btn.addEventListener('focus', function () {
    hasPointer = true;
    targetP = 1;
    if (reduce) { curP = 1; render(1); }
  });
  btn.addEventListener('blur', function () {
    hasPointer = false;
    targetP = 0;
    if (reduce) { curP = 0; render(0); }
  });

  if (reduce) {
    render(0);
  } else {
    requestAnimationFrame(loop);
  }
})();

// ProximityLetterSpacingButton — vanilla 판(run/408/demo/08-btn-proximity-letter-spacing-expand/index.html) 의 React 포트.
// 거리 값(0~1)을 글자 사이 간격 값(em)으로 바로 매핑한다. 커서가 멀 때는 글자가 좁게
// 뭉쳐 있다가 가까워질수록 라벨 글자 사이가 넓게 벌어진다.
import { useEffect, useRef, useState } from "react";
import type { PointerEvent as ReactPointerEvent } from "react";
import styles from "./ProximityLetterSpacingButton.module.scss";

type Box<T> = { current: T };

const PROX_RADIUS_PX = 170;
const LS_MAX_EM = 0.22;
const LERP = 0.17;

function clamp(v: number, lo: number, hi: number): number {
  return Math.max(lo, Math.min(hi, v));
}

function syntheticOffset(t: number): [number, number] {
  const ang = t * 1.6 + 2.1;
  const rad = 110 * (1 + Math.cos(t * Math.PI));
  return [Math.cos(ang) * rad, Math.sin(ang) * rad];
}

export default function ProximityLetterSpacingButton() {
  const sceneRef = useRef<HTMLDivElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);
  const labelRef = useRef<HTMLSpanElement | null>(null);
  const underlineRef = useRef<HTMLSpanElement | null>(null);
  const badgeRef = useRef<HTMLParagraphElement | null>(null);
  const targetPRef = useRef<number>(0) as Box<number>;
  const curPRef = useRef<number>(0) as Box<number>;
  const hasPointerRef = useRef<boolean>(false) as Box<boolean>;
  const frameRef = useRef<number>(0) as Box<number>;
  const t0Ref = useRef<number>(0) as Box<number>;
  const [demo, setDemo] = useState(true);
  const [reduce, setReduce] = useState(false);

  function render(p: number): void {
    const btn = btnRef.current;
    const label = labelRef.current;
    const underline = underlineRef.current;
    const badge = badgeRef.current;
    if (!btn || !label || !underline || !badge) return;
    const spacing = p * LS_MAX_EM;
    label.style.letterSpacing = spacing.toFixed(3) + "em";
    btn.style.filter = "brightness(" + (1 + p * 0.12).toFixed(3) + ")";
    // 계산 레이어: 벌어진 실제 글자 폭(getBoundingClientRect)을 라벨 배경·밑줄 길이·후광 그림자에 전파한다.
    const w = label.getBoundingClientRect().width;
    underline.style.width = w.toFixed(1) + "px";
    btn.style.setProperty("--underline-o", (0.2 + p * 0.6).toFixed(2));
    btn.style.setProperty("--label-w", w.toFixed(1) + "px");
    btn.style.setProperty("--label-o", (0.12 + p * 0.4).toFixed(2));
    const spread = (6 + p * 26).toFixed(1);
    btn.style.boxShadow = "0 0 " + spread + "px " + spread + "px rgba(10,79,232," + (0.16 + p * 0.42).toFixed(2) + ")";
    badge.textContent = "간격 값 " + spacing.toFixed(2) + "em";
  }

  function distanceToButton(x: number, y: number): number {
    const btn = btnRef.current;
    if (!btn) return PROX_RADIUS_PX;
    const r = btn.getBoundingClientRect();
    const cx = r.left + r.width / 2;
    const cy = r.top + r.height / 2;
    return Math.hypot(x - cx, y - cy);
  }

  useEffect(() => {
    setReduce(window.matchMedia("(prefers-reduced-motion: reduce)").matches);
  }, []);

  useEffect(() => {
    if (reduce) {
      render(0);
      return;
    }
    function loop(now: number): void {
      if (t0Ref.current === 0) t0Ref.current = now;
      const t = (now - t0Ref.current) / 1000;
      if (!hasPointerRef.current) {
        const off = syntheticOffset(t);
        const d = Math.hypot(off[0], off[1]);
        targetPRef.current = clamp(1 - d / PROX_RADIUS_PX, 0, 1);
      }
      curPRef.current += (targetPRef.current - curPRef.current) * LERP;
      render(curPRef.current);
      frameRef.current = requestAnimationFrame(loop);
    }
    frameRef.current = requestAnimationFrame(loop);
    return () => cancelAnimationFrame(frameRef.current);
  }, [reduce]);

  function onPointerMove(event: ReactPointerEvent<HTMLDivElement>): void {
    hasPointerRef.current = true;
    setDemo(false);
    const d = distanceToButton(event.clientX, event.clientY);
    targetPRef.current = clamp(1 - d / PROX_RADIUS_PX, 0, 1);
    if (reduce) { curPRef.current = targetPRef.current; render(curPRef.current); }
  }

  function onPointerLeave(): void {
    hasPointerRef.current = false;
    targetPRef.current = 0;
    if (reduce) { curPRef.current = 0; render(0); }
  }

  function onFocus(): void {
    hasPointerRef.current = true;
    targetPRef.current = 1;
    if (reduce) { curPRef.current = 1; render(1); }
  }

  function onBlur(): void {
    hasPointerRef.current = false;
    targetPRef.current = 0;
    if (reduce) { curPRef.current = 0; render(0); }
  }

  const cls = [styles.ls, demo ? styles.isDemo : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <div className={cls}>
        <p className={styles.badge} ref={badgeRef}>간격 값 0em</p>
        <div className={styles.scene} ref={sceneRef} onPointerMove={onPointerMove} onPointerLeave={onPointerLeave}>
          <button className={styles.btn} ref={btnRef} type="button" onFocus={onFocus} onBlur={onBlur}>
            <span className={styles.label} ref={labelRef}>더 알아보기</span>
            <span className={styles.underline} ref={underlineRef} aria-hidden="true" />
          </button>
        </div>
      </div>
    </main>
  );
}