import { KeyValueStorageEvent, KeyValueStorageInterface } from '../types';
/**
 * @internal
 */
export declare class KeyValueStorage implements KeyValueStorageInterface {
    storage?: Storage;
    listeners?: Set<(e: KeyValueStorageEvent) => Promise<void>>;
    /**
     * The bound `window` 'storage' event handler. It is created lazily on the
     * first successful browser subscription and cleared once the last listener
     * unsubscribes. Instances that never subscribe therefore carry no
     * function-valued own property, which keeps two structurally-identical
     * storage instances deep-equal for consumers that compare by value (e.g.
     * Jest's `toHaveBeenCalledWith`).
     */
    private storageListener?;
    constructor(storage?: Storage);
    /**
     * This is used to set a specific item in storage
     * @param {string} key - the key for the item
     * @param {object} value - the value
     * @returns {string} value that was set
     */
    setItem(key: string, value: string): Promise<void>;
    /**
     * This is used to get a specific key from storage
     * @param {string} key - the key for the item
     * This is used to clear the storage
     * @returns {string} the data item
     */
    getItem(key: string): Promise<string | null>;
    /**
     * This is used to remove an item from storage
     * @param {string} key - the key being set
     * @returns {string} value - value that was deleted
     */
    removeItem(key: string): Promise<void>;
    /**
     * This is used to clear the storage
     * @returns {string} nothing
     */
    clear(): Promise<void>;
    /**
     * This is used to allow listening for changes
     * @param {function} listener - the function called on storage change
     * @returns {function} an unsubscribe function that removes the listener. The
     * underlying `window` 'storage' event listener is attached lazily on the
     * first subscription and detached once the last listener unsubscribes. In
     * non-browser (SSR/native) environments nothing is attached and the returned
     * unsubscribe function is a safe no-op.
     */
    addListener(listener: (ev: KeyValueStorageEvent) => Promise<void>): () => void;
}