Skeleton loader for React

'use client';

import { useLayoutEffect, useRef, useState } from 'react';
import { cn, prefersReducedMotion, useTexts } from '@/lib/kit';

const TEXTS = {
  en: { loading: 'Loading' },
  de: { loading: 'Wird geladen' },
};

/*
 * A slow, slanted sheen that sweeps across and then rests, instead of a pulse.
 * All placeholders start together, so the light reads as one pass over the page.
 * When content arrives, the placeholders fade out over it and the content
 * comes in from a soft blur, one item after another.
 */
const STYLES =
  '.kitfx-sk::after{content:"";position:absolute;inset:0 -50%;transform:translateX(-60%) skewX(-18deg);background:linear-gradient(90deg,transparent 30%,color-mix(in oklab,var(--foreground) 5%,transparent) 44%,color-mix(in oklab,color-mix(in oklab,var(--primary) 30%,var(--foreground)) 9%,transparent) 50%,color-mix(in oklab,var(--foreground) 5%,transparent) 56%,transparent 70%);animation:kitfx-sk 2.4s cubic-bezier(.45,.05,.35,1) infinite}' +
  '@keyframes kitfx-sk{0%{transform:translateX(-60%) skewX(-18deg)}62%,100%{transform:translateX(60%) skewX(-18deg)}}' +
  '.kitfx-sk-ghost{animation:kitfx-sk-out .45s var(--ease-kit) forwards}@keyframes kitfx-sk-out{to{opacity:0;filter:blur(2px)}}' +
  '.kitfx-sk-reveal>:not(.kitfx-sk-ghost){animation:kitfx-sk-in .65s var(--ease-kit) both}' +
  '.kitfx-sk-reveal>:nth-child(2){animation-delay:70ms}.kitfx-sk-reveal>:nth-child(3){animation-delay:140ms}.kitfx-sk-reveal>:nth-child(4){animation-delay:210ms}.kitfx-sk-reveal>:nth-child(n+5){animation-delay:280ms}' +
  '@keyframes kitfx-sk-in{from{opacity:0;filter:blur(6px);transform:translateY(6px)}}' +
  '@media (prefers-reduced-motion:reduce){.kitfx-sk::after{animation:none;opacity:0}.kitfx-sk-ghost{display:none}.kitfx-sk-reveal>*{animation:none}}';

export interface SkeletonProps {
  /** line: text height, rounded. circle: avatars. rect: images and cards. */
  shape?: 'line' | 'circle' | 'rect';
  className?: string;
  style?: React.CSSProperties;
}

/**
 * A placeholder that holds the space of content still loading. A slanted light
 * sweeps over it and rests; with reduced motion it stays still. Decorative: wrap
 * a group in <SkeletonGroup> so screen readers hear "Loading" once.
 */
export function Skeleton({ shape = 'line', className, style }: SkeletonProps) {
  return (
    <>
      {/* React 19 hoists this once into the head, however many skeletons render */}
      <style href="kitfx-sk" precedence="default">
        {STYLES}
      </style>
      <span
        aria-hidden
        style={style}
        className={cn(
          'kitfx-sk relative block overflow-hidden bg-[color-mix(in_oklab,var(--foreground)_7%,transparent)]',
          shape === 'line' && 'h-3 rounded-full',
          shape === 'circle' && 'aspect-square rounded-full',
          shape === 'rect' && 'rounded-lg',
          className,
        )}
      />
    </>
  );
}

/** Several lines of text, the last one shorter, like a paragraph. */
export function SkeletonText({ lines = 3, className }: { lines?: number; className?: string }) {
  const widths = ['100%', '94%', '97%', '88%', '92%'];
  return (
    <span className={cn('block space-y-2.5', className)} aria-hidden>
      {Array.from({ length: lines }, (_, i) => (
        <Skeleton key={i} style={{ width: i === lines - 1 && lines > 1 ? '62%' : widths[i % widths.length] }} />
      ))}
    </span>
  );
}

/**
 * Wraps placeholders: marks the region busy and announces "Loading" once.
 * When `loading` turns false the children are replaced by `content`: the
 * placeholders fade out on top while the content comes in, so nothing jumps.
 */
export function SkeletonGroup({ loading = true, label, content, className, children }: { loading?: boolean; label?: string; content?: React.ReactNode; className?: string; children: React.ReactNode }) {
  const t = useTexts(TEXTS);
  const was = useRef(loading);
  const [swap, setSwap] = useState(false);
  // Before paint, so the content never shows a frame without its entrance
  useLayoutEffect(() => {
    const before = was.current;
    was.current = loading;
    if (!before || loading || prefersReducedMotion()) {
      setSwap(false);
      return;
    }
    setSwap(true);
    const timer = window.setTimeout(() => setSwap(false), 700);
    return () => window.clearTimeout(timer);
  }, [loading]);
  return (
    <div aria-busy={loading} aria-live="polite" className={cn(swap && 'kitfx-sk-reveal relative', className)}>
      {loading ? (
        <>
          <span className="sr-only">{label ?? t.loading}</span>
          {children}
        </>
      ) : (
        <>
          {content}
          {swap && (
            <div aria-hidden className={cn('kitfx-sk-ghost pointer-events-none absolute inset-0 z-10', className)} style={{ margin: 0 }}>
              {children}
            </div>
          )}
        </>
      )}
    </div>
  );
}