/**
 * This module establishes a simple protocol for creating a MessageChannel between two
 * realms, as follows:
 * 1. The sending realm asserts that the receiving realm is ready to receive messages,
 * either by creating the realm itself (for example, by appending an iframe to the DOM),
 * or via some other means.
 * 2. The receiving realm calls `receiveMessagePort()` on startup in one of its scripts.
 * The script element in question should not have the `async` attribute.
 * 3. The sending realm calls `initializeMessageChannel()` which sends a message port to
 * the receiving realm. When the returned promise resolves, the sending realm and the
 * receiving realm have established a message channel.
 *
 * @module MessageChannel utilities
 */
export declare const MessageType: {
    readonly Initialize: "INIT_MESSAGE_CHANNEL";
    readonly Acknowledge: "ACK_MESSAGE_CHANNEL";
};
/**
 * Creates a message channel and sends one of the ports to the receiving realm. The
 * realm must be loaded, and it must have called {@link receiveMessagePort} to
 * receive the remote message port. Rejects if the first message received over the
 * channel is not an {@link AcknowledgeMessage}.
 *
 * A `portHandler` function can be specified to synchronously perform any work with
 * the local message port before the promise resolves.
 *
 * @param postMessage - A bound method for posting a message to the receiving realm.
 * Must be able to transfer a message port.
 * @param portHandler - A function that receives the local message port and returns a
 * value. Returns the local message port by default.
 * @returns A promise that resolves with the value returned by `portHandler`.
 */
export declare function initializeMessageChannel<Result = MessagePort>(postMessage: (message: unknown, transfer: Transferable[]) => void, portHandler?: (port: MessagePort) => Result): Promise<Result>;
type Listener = (message: MessageEvent) => void;
/**
 * Receives a message port from the sending realm, and sends an {@link AcknowledgeMessage}
 * over the port. Should be called in a script _without_ the `async` attribute on startup.
 * The sending realm must call {@link initializeMessageChannel} to send the message port
 * after this realm has loaded. Ignores any message events dispatched on the local
 * realm that are not an {@link InitializeMessage}.
 *
 * A `portHandler` function can be specified to synchronously perform any work with the
 * received port before the promise resolves.
 *
 * @param addListener - A bound method to add a message event listener to the sending
 * realm.
 * @param removeListener - A bound method to remove a message event listener from the
 * sending realm.
 * @param portHandler - A function that receives the message port and returns a value.
 * Returns the message port by default.
 * @returns A promise that resolves with the value returned by `portHandler`.
 */
export declare function receiveMessagePort<Result = MessagePort>(addListener: (listener: Listener) => void, removeListener: (listener: Listener) => void, portHandler?: (port: MessagePort) => Result): Promise<Result>;
export {};
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