export declare type ValueCallback<T> = (value: T) => void;
export declare type ErrorCallback = (err: Error) => void;
export declare type ValueSink<T> = (err: Error | undefined, value?: T) => boolean;
export declare type UpdateFunction<T> = (sink: ValueSink<T>) => void;
export declare class Observer<T> {
    static create<T>(value: T): Observer<T>;
    private _isActive;
    private _value;
    private _previous;
    private _listeners;
    private _errorListeners;
    constructor(updater: UpdateFunction<T>, value?: T);
    destroy(): void;
    previous(): T | null;
    current(): T | null;
    onValue(cb: ValueCallback<T>): this;
    onError(cb: ErrorCallback): this;
    private update;
    private updateErrors;
}