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import {
createContext,
useCallback,
useContext,
useEffect,
useMemo,
useSyncExternalStore,
type ReactNode,
} from "react";
export type ThemeMode = "dark" | "light";
const storageKey = "headless-commerce-theme";
const listeners = new Set<() => void>();
let fallbackTheme: ThemeMode | null = null;
/**
* Public theme context used by interactive controls.
*/
export interface ThemeContextValue {
readonly isDark: boolean;
readonly setTheme: (theme: ThemeMode) => void;
readonly theme: ThemeMode;
readonly toggleTheme: () => void;
}
/**
* Theme provider options for the production shell.
*/
export interface ThemeProviderProps {
readonly children: ReactNode;
}
const ThemeContext = createContext<ThemeContextValue | null>(null);
function canUseDom(): boolean {
return typeof window !== "undefined" && typeof document !== "undefined";
}
function getStoredTheme(): ThemeMode | null {
try {
const value = window.localStorage.getItem(storageKey);
return value === "dark" || value === "light" ? value : null;
} catch {
return null;
}
}
function getPreferredTheme(): ThemeMode {
const storedTheme = getStoredTheme();
Iif (storedTheme) {
return storedTheme;
}
return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
}
function applyTheme(theme: ThemeMode): void {
document.documentElement.classList.toggle("dark", theme === "dark");
document.documentElement.style.colorScheme = theme;
}
function emitThemeChange(): void {
listeners.forEach((listener) => listener());
}
function getClientThemeSnapshot(): ThemeMode {
Iif (!canUseDom()) {
return "light";
}
return getStoredTheme() ?? fallbackTheme ?? getPreferredTheme();
}
function getServerThemeSnapshot(): ThemeMode {
return "light";
}
function subscribeToTheme(listener: () => void): () => void {
listeners.add(listener);
const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
const handleSystemThemeChange = (event: MediaQueryListEvent): void => {
if (getStoredTheme()) {
return;
}
const nextTheme: ThemeMode = event.matches ? "dark" : "light";
applyTheme(nextTheme);
emitThemeChange();
};
const handleStorageChange = (event: StorageEvent): void => {
if (event.key !== storageKey) {
return;
}
fallbackTheme = null;
applyTheme(getPreferredTheme());
emitThemeChange();
};
mediaQuery.addEventListener("change", handleSystemThemeChange);
window.addEventListener("storage", handleStorageChange);
return () => {
listeners.delete(listener);
mediaQuery.removeEventListener("change", handleSystemThemeChange);
window.removeEventListener("storage", handleStorageChange);
};
}
/**
* Client-only theme provider with localStorage persistence and system preference fallback.
*/
export function ThemeProvider({ children }: ThemeProviderProps): ReactNode {
const theme = useSyncExternalStore(
subscribeToTheme,
getClientThemeSnapshot,
getServerThemeSnapshot,
);
useEffect(() => {
applyTheme(theme);
}, [theme]);
const setTheme = useCallback((nextTheme: ThemeMode) => {
fallbackTheme = nextTheme;
applyTheme(nextTheme);
try {
window.localStorage.setItem(storageKey, nextTheme);
fallbackTheme = null;
} catch {
// Ignore storage failures; the visual theme still updates for the current session.
}
emitThemeChange();
}, []);
const toggleTheme = useCallback(() => {
setTheme(theme === "dark" ? "light" : "dark");
}, [setTheme, theme]);
const value = useMemo<ThemeContextValue>(
() => ({
isDark: theme === "dark",
setTheme,
theme,
toggleTheme,
}),
[setTheme, theme, toggleTheme],
);
return <ThemeContext.Provider value={value}>{children}</ThemeContext.Provider>;
}
export function useTheme(): ThemeContextValue {
const context = useContext(ThemeContext);
Iif (!context) {
throw new Error("useTheme must be used inside ThemeProvider.");
}
return context;
}
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