export declare class CheckSessionIFrame { private _callback; private _client_id; private _interval; private _stopOnError; private _frame_origin; private _frame; private _boundMessageEvent; private _timer; private _session_state; constructor(callback: () => Promise | void, client_id: string, url: string, interval?: number, stopOnError?: boolean); load(): Promise; private _message; start(session_state: string): void; stop(): void; }