import { ElementRef } from '@angular/core';
import { Observable } from 'rxjs';
import * as i0 from "@angular/core";
export declare class VisibilityService {
    private readonly observer;
    private readonly visibilityChangeSubject;
    observe(element: ElementRef): Observable<boolean>;
    unobserve(element: ElementRef): void;
    static ɵfac: i0.ɵɵFactoryDeclaration<VisibilityService, never>;
    static ɵprov: i0.ɵɵInjectableDeclaration<VisibilityService>;
}