Compound Components

import { createContext, useContext, useState, type ReactNode } from 'react';

// ---- Accordion - compound component ----
interface AccordionContextValue {
  openItems: Set<string>;
  toggle: (id: string) => void;
  allowMultiple: boolean;
}

const AccordionContext = createContext<AccordionContextValue | null>(null);

function useAccordionContext() {
  const ctx = useContext(AccordionContext);
  if (!ctx) throw new Error('Accordion components must be used within <Accordion>');
  return ctx;
}

// Root component - owns state
function Accordion({ children, allowMultiple = false }: { children: ReactNode; allowMultiple?: boolean }) {
  const [openItems, setOpenItems] = useState<Set<string>>(new Set());

  function toggle(id: string) {
    setOpenItems(prev => {
      const next = new Set(prev);
      if (next.has(id)) {
        next.delete(id);
      } else {
        if (!allowMultiple) next.clear(); // close others
        next.add(id);
      }
      return next;
    });
  }

  return (
    <AccordionContext.Provider value={{ openItems, toggle, allowMultiple }}>
      <div role="region">{children}</div>
    </AccordionContext.Provider>
  );
}

// Item component - uses parent context
function AccordionItem({ id, children }: { id: string; children: ReactNode }) {
  return <div className="accordion-item">{children}</div>;
}

// Trigger component - reads/updates context
function AccordionTrigger({ id, children }: { id: string; children: ReactNode }) {
  const { openItems, toggle } = useAccordionContext();
  const isOpen = openItems.has(id);

  return (
    <button
      onClick={() => toggle(id)}
      aria-expanded={isOpen}
      aria-controls={`panel-${id}`}
      style={{ width: '100%', display: 'flex', justifyContent: 'space-between', alignItems: 'center', padding: '16px 20px', background: 'transparent', border: 'none', cursor: 'pointer', textAlign: 'left', fontWeight: 600 }}
    >
      {children}
      <span style={{ transform: isOpen ? 'rotate(180deg)' : 'rotate(0)', transition: 'transform 0.2s' }}>▼</span>
    </button>
  );
}

// Panel component - conditionally shown
function AccordionPanel({ id, children }: { id: string; children: ReactNode }) {
  const { openItems } = useAccordionContext();
  const isOpen = openItems.has(id);

  return (
    <div
      id={`panel-${id}`}
      role="region"
      hidden={!isOpen}
      style={{ padding: isOpen ? '0 20px 16px' : 0, overflow: 'hidden', transition: 'padding 0.2s' }}
    >
      {children}
    </div>
  );
}

// Attach sub-components to parent (dot notation API)
Accordion.Item = AccordionItem;
Accordion.Trigger = AccordionTrigger;
Accordion.Panel = AccordionPanel;

// ---- Usage - reads like HTML ----
function FAQ() {
  return (
    <Accordion allowMultiple>
      <Accordion.Item id="q1">
        <Accordion.Trigger id="q1">What is React?</Accordion.Trigger>
        <Accordion.Panel id="q1">React is a JavaScript library for building user interfaces.</Accordion.Panel>
      </Accordion.Item>
      <Accordion.Item id="q2">
        <Accordion.Trigger id="q2">What are hooks?</Accordion.Trigger>
        <Accordion.Panel id="q2">Hooks let you use state and lifecycle features in function components.</Accordion.Panel>
      </Accordion.Item>
    </Accordion>
  );
}