fusion/packages/dashboard/app/hooks/useTheme.ts

import { useState, useEffect, useCallback, useLayoutEffect, useRef } from "react";
import { COLOR_THEMES, type ThemeMode, type ColorTheme } from "@fusion/core";
import { fetchGlobalSettings, updateGlobalSettings } from "../api";

const THEME_MODE_STORAGE_KEY = "kb-dashboard-theme-mode";
const COLOR_THEME_STORAGE_KEY = "kb-dashboard-color-theme";
const VALID_COLOR_THEMES = [...COLOR_THEMES] satisfies ColorTheme[];

// Check if we're in a browser environment
const isBrowser = typeof window !== "undefined";

// Use useLayoutEffect on client, useEffect on server (no-op)
const useIsomorphicLayoutEffect = isBrowser ? useLayoutEffect : useEffect;

interface UseThemeReturn {
  themeMode: ThemeMode;
  colorTheme: ColorTheme;
  setThemeMode: (mode: ThemeMode) => void;
  setColorTheme: (theme: ColorTheme) => void;
  isSystemDark: boolean;
}

function isValidThemeMode(value: unknown): value is ThemeMode {
  return value === "dark" || value === "light" || value === "system";
}

function readCachedThemeMode(): ThemeMode {
  if (!isBrowser) return "dark";
  try {
    const saved = localStorage.getItem(THEME_MODE_STORAGE_KEY);
    if (isValidThemeMode(saved)) {
      return saved;
    }
  } catch {
    // localStorage not available, use default
  }
  return "dark";
}

function readCachedColorTheme(): ColorTheme {
  if (!isBrowser) return "default";
  try {
    const saved = localStorage.getItem(COLOR_THEME_STORAGE_KEY);
    if (saved && VALID_COLOR_THEMES.includes(saved as ColorTheme)) {
      return saved as ColorTheme;
    }
  } catch {
    // localStorage not available, use default
  }
  return "default";
}

function writeCachedThemeMode(mode: ThemeMode): void {
  if (!isBrowser) return;
  try {
    localStorage.setItem(THEME_MODE_STORAGE_KEY, mode);
  } catch {
    // localStorage not available, skip cache write
  }
}

function writeCachedColorTheme(theme: ColorTheme): void {
  if (!isBrowser) return;
  try {
    localStorage.setItem(COLOR_THEME_STORAGE_KEY, theme);
  } catch {
    // localStorage not available, skip cache write
  }
}

/**
 * Get the effective theme mode (resolves "system" to actual dark/light value)
 */
function getEffectiveThemeMode(mode: ThemeMode, systemIsDark: boolean): "dark" | "light" {
  if (mode === "system") {
    return systemIsDark ? "dark" : "light";
  }
  return mode;
}

/**
 * Apply theme attributes to document.documentElement
 * Call this immediately to prevent flash of wrong theme
 */
function applyThemeAttributes(themeMode: ThemeMode, colorTheme: ColorTheme, systemIsDark: boolean): void {
  if (!isBrowser) return;

  const effectiveMode = getEffectiveThemeMode(themeMode, systemIsDark);
  document.documentElement.setAttribute("data-theme", effectiveMode);
  document.documentElement.setAttribute("data-color-theme", colorTheme);
}

/**
 * Custom hook for theme management.
 *
 * Source of truth: backend global settings (`~/.pi/fusion/settings.json`).
 *
 * Behavior:
 * - Initializes from localStorage cache to avoid pre-hydration theme flash
 * - Hydrates from backend global settings on mount and reconciles cache
 * - Writes through on updates (state + localStorage cache + async backend update)
 */
export function useTheme(): UseThemeReturn {
  // Initialize from localStorage cache or defaults to avoid flash before hydration.
  const [themeMode, setThemeModeState] = useState<ThemeMode>(() => readCachedThemeMode());
  const [colorTheme, setColorThemeState] = useState<ColorTheme>(() => readCachedColorTheme());
  const [isHydrating, setIsHydrating] = useState(true);

  // Track system color scheme preference
  const [isSystemDark, setIsSystemDark] = useState<boolean>(() => {
    if (!isBrowser) return true;
    return window.matchMedia("(prefers-color-scheme: dark)").matches;
  });

  const themeModeRef = useRef(themeMode);
  const colorThemeRef = useRef(colorTheme);

  useEffect(() => {
    themeModeRef.current = themeMode;
  }, [themeMode]);

  useEffect(() => {
    colorThemeRef.current = colorTheme;
  }, [colorTheme]);

  // Hydrate canonical theme values from backend global settings.
  useEffect(() => {
    if (!isBrowser || !isHydrating) return;

    let cancelled = false;

    void fetchGlobalSettings()
      .then((globalSettings) => {
        if (cancelled) return;

        if (isValidThemeMode(globalSettings.themeMode)) {
          if (themeModeRef.current !== globalSettings.themeMode) {
            setThemeModeState(globalSettings.themeMode);
          }
          if (readCachedThemeMode() !== globalSettings.themeMode) {
            writeCachedThemeMode(globalSettings.themeMode);
          }
        }

        if (globalSettings.colorTheme && VALID_COLOR_THEMES.includes(globalSettings.colorTheme)) {
          if (colorThemeRef.current !== globalSettings.colorTheme) {
            setColorThemeState(globalSettings.colorTheme);
          }
          if (readCachedColorTheme() !== globalSettings.colorTheme) {
            writeCachedColorTheme(globalSettings.colorTheme);
          }
        }
      })
      .catch((error) => {
        console.warn("[useTheme] Failed to hydrate theme from global settings", error);
      })
      .finally(() => {
        if (!cancelled) {
          setIsHydrating(false);
        }
      });

    return () => {
      cancelled = true;
    };
  }, [isHydrating]);

  // Listen to system color scheme changes
  useEffect(() => {
    if (!isBrowser) return;

    const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
    const handleChange = (e: MediaQueryListEvent) => {
      setIsSystemDark(e.matches);
    };

    mediaQuery.addEventListener("change", handleChange);
    return () => mediaQuery.removeEventListener("change", handleChange);
  }, []);

  // Apply theme immediately on mount and when theme changes
  useIsomorphicLayoutEffect(() => {
    applyThemeAttributes(themeMode, colorTheme, isSystemDark);
  }, [themeMode, colorTheme, isSystemDark]);

  // Wrapper setters with write-through persistence.
  const setThemeMode = useCallback((mode: ThemeMode) => {
    setThemeModeState(mode);
    writeCachedThemeMode(mode);

    void updateGlobalSettings({ themeMode: mode }).catch((error) => {
      console.warn("[useTheme] Failed to persist themeMode to global settings", error);
    });
  }, []);

  const setColorTheme = useCallback((theme: ColorTheme) => {
    setColorThemeState(theme);
    writeCachedColorTheme(theme);

    void updateGlobalSettings({ colorTheme: theme }).catch((error) => {
      console.warn("[useTheme] Failed to persist colorTheme to global settings", error);
    });
  }, []);

  return {
    themeMode,
    colorTheme,
    setThemeMode,
    setColorTheme,
    isSystemDark,
  };
}

/**
 * Utility to apply theme before React hydration.
 *
 * This script intentionally reads from localStorage because it runs synchronously
 * before React boots; localStorage is treated as a backend-synced cache.
 */
export function getThemeInitScript(): string {
  return `
    (function() {
      try {
        var mode = localStorage.getItem('${THEME_MODE_STORAGE_KEY}') || 'dark';
        var colorTheme = localStorage.getItem('${COLOR_THEME_STORAGE_KEY}') || 'default';
        var validThemes = ${JSON.stringify(VALID_COLOR_THEMES)};
        if (!validThemes.includes(colorTheme)) {
          colorTheme = 'default';
        }
        var systemDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
        var effectiveMode = mode === 'system' ? (systemDark ? 'dark' : 'light') : mode;
        document.documentElement.setAttribute('data-theme', effectiveMode);
        document.documentElement.setAttribute('data-color-theme', colorTheme);
      } catch (e) {
        document.documentElement.setAttribute('data-theme', 'dark');
        document.documentElement.setAttribute('data-color-theme', 'default');
      }
    })();
  `;
}