/**
 * @module Viewer
 * Provies a facility to trigger and handle cancellation requests.
 */
/**
 * Cancellation token.
 */
export declare class CancellationToken {
    private _isCancellationRequested;
    private _cancellationPromise;
    private _cancel;
    /** The parent token */
    readonly parentToken?: CancellationToken;
    private constructor();
    /**
     * Creates a cancellation token and trigger function.
     * @param parentToken The parent token.
     */
    static create(parentToken?: CancellationToken): {
        token: CancellationToken;
        cancel: (e: any) => void;
    };
    /**
     * Gets true if cancellation is being requested.
     */
    get isCancellationRequested(): boolean;
    /**
     * Gets a promise that is triggered when cancellation is requested.
     */
    get promise(): Promise<any>;
    /**
     * Registers a new cancellation handler. Handler is invoked when cancellation is requested.
     * @param callback
     */
    register(callback: (reason: any) => any): void;
}