stack/apps/web/src/providers/ThemeProvider.tsx

"use client";

import { createContext, useCallback, useContext, useEffect, useState, type ReactNode } from "react";

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

interface ThemeContextValue {
  theme: Theme;
  resolvedTheme: "light" | "dark";
  setTheme: (theme: Theme) => void;
  toggleTheme: () => void;
}

const ThemeContext = createContext<ThemeContextValue | null>(null);

const STORAGE_KEY = "jarvis-theme";

function getSystemTheme(): "light" | "dark" {
  if (typeof window === "undefined") return "dark";
  return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
}

function getStoredTheme(): Theme {
  if (typeof window === "undefined") return "system";
  const stored = localStorage.getItem(STORAGE_KEY);
  if (stored === "light" || stored === "dark" || stored === "system") {
    return stored;
  }
  return "system";
}

interface ThemeProviderProps {
  children: ReactNode;
  defaultTheme?: Theme;
}

export function ThemeProvider({
  children,
  defaultTheme = "system",
}: ThemeProviderProps): React.JSX.Element {
  const [theme, setThemeState] = useState<Theme>(defaultTheme);
  const [resolvedTheme, setResolvedTheme] = useState<"light" | "dark">("dark");
  const [mounted, setMounted] = useState(false);

  // Initialize theme from storage on mount
  useEffect(() => {
    setMounted(true);
    const storedTheme = getStoredTheme();
    setThemeState(storedTheme);
    setResolvedTheme(storedTheme === "system" ? getSystemTheme() : storedTheme);
  }, []);

  // Apply theme class to html element
  useEffect(() => {
    if (!mounted) return;

    const root = document.documentElement;
    const resolved = theme === "system" ? getSystemTheme() : theme;

    root.classList.remove("light", "dark");
    root.classList.add(resolved);
    setResolvedTheme(resolved);
  }, [theme, mounted]);

  // Listen for system theme changes
  useEffect(() => {
    if (!mounted || theme !== "system") return;

    const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
    const handleChange = (e: MediaQueryListEvent): void => {
      setResolvedTheme(e.matches ? "dark" : "light");
      document.documentElement.classList.remove("light", "dark");
      document.documentElement.classList.add(e.matches ? "dark" : "light");
    };

    mediaQuery.addEventListener("change", handleChange);
    return (): void => {
      mediaQuery.removeEventListener("change", handleChange);
    };
  }, [theme, mounted]);

  const setTheme = useCallback((newTheme: Theme) => {
    setThemeState(newTheme);
    localStorage.setItem(STORAGE_KEY, newTheme);
  }, []);

  const toggleTheme = useCallback(() => {
    setTheme(resolvedTheme === "dark" ? "light" : "dark");
  }, [resolvedTheme, setTheme]);

  // Prevent flash by not rendering until mounted
  if (!mounted) {
    return (
      <ThemeContext.Provider
        value={{
          theme: defaultTheme,
          resolvedTheme: "dark",
          setTheme: (): void => {
            // No-op during SSR
          },
          toggleTheme: (): void => {
            // No-op during SSR
          },
        }}
      >
        {children}
      </ThemeContext.Provider>
    );
  }

  return (
    <ThemeContext.Provider value={{ theme, resolvedTheme, setTheme, toggleTheme }}>
      {children}
    </ThemeContext.Provider>
  );
}

export function useTheme(): ThemeContextValue {
  const context = useContext(ThemeContext);
  if (!context) {
    throw new Error("useTheme must be used within a ThemeProvider");
  }
  return context;
}