import { SnapshotSubscriber } from "./Snapshots";
export declare type SnapshotUnsubscriber = () => void;
export declare type OnUpdatedCallback = <D>(value: D | undefined) => void;
export interface DataListener<D> {
    get(): D | undefined;
    unsubscribe(): void;
}
export declare class DefaultDataListener<D> implements DataListener<D> {
    private readonly subscriber;
    private readonly onUpdated?;
    private value;
    private unsubscriber;
    constructor(subscriber: SnapshotSubscriber<D>, onUpdated?: OnUpdatedCallback | undefined);
    get(): D | undefined;
    unsubscribe(): void;
}
export declare class NullDataListener<D> implements DataListener<D> {
    get(): D | undefined;
    unsubscribe(): void;
}
export declare class StaticDefaultDataListener<D> implements DataListener<D> {
    private readonly value;
    constructor(value: D | undefined);
    get(): D | undefined;
    unsubscribe(): void;
}