import type { None, SubscribeToInAppMessageCallbackType } from "../types";
/**
 * Subscribe to receive in-app messages. The subscriber callback will be called whenever a new in-app message is
 * triggered. This method should be called before calling [[`openSession`]].
 *
 * @param callback - The callback function to handle the in-app message. This function will be
 *   called with an [[`InAppMessage`]] or a [[`ControlMessage`]] object.
 *
 * @returns The identifier of the subscription created. Returns undefined if the SDK has not been initialized.
 */
export declare function subscribeToInAppMessage(callback: SubscribeToInAppMessageCallbackType): string | None;