import { type Ref } from 'vue';
/**
 * Two-way state sync across browser tabs.
 * - Without callback: reactive ref synced across tabs.
 * - With callback: called when any tab updates this key — side effects.
 *
 * @example
 * // Reactive two-way sync
 * const cart = useSocketSync<Cart>('cart', { items: [] });
 * cart.value = { items: [1, 2, 3] }; // syncs to all tabs
 *
 * @example
 * // With side effect callback
 * const cart = useSocketSync<Cart>('cart', { items: [] }, (cart) => {
 *   document.title = `Cart (${cart.items.length})`;
 *   analytics.track('cart_updated');
 * });
 */
export declare function useSocketSync<T>(key: string, initialValue: T, callback?: (value: T) => void): Ref<T>;