type OnRefreshFunction<KeyType, ReturnType> = (key?: KeyType, value?: ReturnType) => void | Promise<void>;
interface CacheOptions<KeyType, ReturnType, StorageEngineType extends (StorageEngine<ReturnType> | SyncStorageEngine<ReturnType>)> {
    freshseconds?: number;
    staleseconds?: number;
    storageClass?: StorageEngineType;
    onRefresh?: OnRefreshFunction<KeyType, ReturnType>;
}
export interface StorageEngine<StorageType> {
    get: (keystr: string) => Promise<StorageType | undefined>;
    set: (keystr: string, data: StorageType) => Promise<void>;
    del: (keystr: string) => Promise<void>;
    clear: () => Promise<void>;
}
export interface SyncStorageEngine<StorageType> {
    get: (keystr: string) => StorageType | undefined;
    set: (keystr: string, data: StorageType) => void;
    del: (keystr: string) => void;
    clear: () => void;
}
type OptionalArgPlus<T, V> = T extends undefined ? undefined extends T ? [V] : [T, V] : [T, V];
type OptionalArgBoth<T, V> = T extends undefined ? undefined extends T ? V extends undefined ? undefined extends V ? [] : [T, V] | [] : [T, V] : V extends undefined ? undefined extends V ? [T] | [] : [T, V] | [T] | [] : [T, V] : V extends undefined ? undefined extends V ? [T] : [T, V] | [T] : [T, V];
type FetcherFunction<KeyType, ReturnType, HelperType> = ((key: KeyType) => Promise<ReturnType>) | ((key: KeyType, helper: HelperType) => Promise<ReturnType>) | (() => Promise<ReturnType>);
export declare class Cache<KeyType = undefined, ReturnType = any, HelperType = undefined> {
    private fetcher;
    private options;
    private storage;
    private activeWork;
    private activeGets;
    private onRefresh?;
    constructor(fetcher: FetcherFunction<KeyType, ReturnType, HelperType>, options?: CacheOptions<KeyType, ReturnType, any>);
    get(...params: OptionalArgBoth<KeyType, HelperType>): Promise<ReturnType>;
    set(...params: OptionalArgPlus<KeyType, ReturnType>): Promise<void>;
    invalidate(key: KeyType | string): Promise<void>;
    clear(): Promise<void>;
    refresh(...params: OptionalArgBoth<KeyType, HelperType>): Promise<ReturnType>;
    private fresh;
    private valid;
}
export {};