import { SnapshotOutOf } from './SnapshotOf';
/**
 * Listener function for onSnapshot.
 */
export type OnSnapshotListener<T> = (sn: SnapshotOutOf<T>, prevSn: SnapshotOutOf<T>) => void;
/**
 * Disposer function for onSnapshot.
 */
export type OnSnapshotDisposer = () => void;
/**
 * Adds a reaction that will trigger every time an snapshot changes.
 *
 * @template T Object type.
 * @param nodeOrFn Object to get the snapshot from or a function to get it.
 * @param listener Function that will be triggered when the snapshot changes.
 * @returns A disposer.
 */
export declare function onSnapshot<T extends object>(nodeOrFn: T | (() => T), listener: OnSnapshotListener<T>): OnSnapshotDisposer;