units-ui/components/providers/ThemeProvider.tsx

'use client';

import { createContext, useContext, useEffect, useState } from 'react';

type Theme = 'dark' | 'light' | 'system';

interface ThemeProviderProps {
  children: React.ReactNode;
  defaultTheme?: Theme;
  storageKey?: string;
}

interface ThemeProviderState {
  theme: Theme;
  setTheme: (theme: Theme) => void;
}

const ThemeProviderContext = createContext<ThemeProviderState | undefined>(
  undefined
);

export function ThemeProvider({
  children,
  defaultTheme = 'system',
  storageKey = 'units-ui-theme',
}: ThemeProviderProps) {
  const [theme, setTheme] = useState<Theme>(defaultTheme);

  useEffect(() => {
    // Load theme from localStorage
    const stored = localStorage.getItem(storageKey) as Theme | null;
    if (stored) {
      setTheme(stored);
    }
  }, [storageKey]);

  useEffect(() => {
    const root = window.document.documentElement;

    root.classList.remove('light', 'dark');

    if (theme === 'system') {
      const systemTheme = window.matchMedia('(prefers-color-scheme: dark)')
        .matches
        ? 'dark'
        : 'light';

      root.classList.add(systemTheme);
      return;
    }

    root.classList.add(theme);
  }, [theme]);

  const value = {
    theme,
    setTheme: (theme: Theme) => {
      localStorage.setItem(storageKey, theme);
      setTheme(theme);
    },
  };

  return (
    <ThemeProviderContext.Provider value={value}>
      {children}
    </ThemeProviderContext.Provider>
  );
}

export const useTheme = () => {
  const context = useContext(ThemeProviderContext);

  if (context === undefined)
    throw new Error('useTheme must be used within a ThemeProvider');

  return context;
};