RemoteDocumentMessaging

Overview

This document describes a proposal for discovering and communicating with "remote" documents. In particular, this proposal lets a web page communicate with another document even if the web page does not have a direct script connection to that document.

Use Cases

The user opens the same word processing document hosted by a document editing web site in two different browser windows. The document editing web site wants to ensure that edits in one window a reflected in the other window without needing to round-trip through the server (e.g., because the user is offline). Unfortunately, the documents are in different units of related browsing contexts, which means the two documents cannot form a direct script connection. Using remote document messaging, the two documents can discover each other and update each other about edits performed by the user.

Specification

partial interface Window {
    MessagePort connectTo(DOMString target);
};

The connectTo method opens a message port to a browsing context with name target within the caller's origin (although not necessarily within the caller's unit of related browsing contexts).

1. Create a new MessagePort object owned by the global object of the incumbent script. Let this be the initiator's port.
2. Return the initiator's port and perform the remaining steps asynchronously.
3. Spin the event loop until a browsing context with name target has the same effective script origin as the incumbent script has an active document whose ready state has reached "interactive". Let this document be the responder's document.
4. Create a new MessagePort object owned by the global object of the responder's document. Let this be the responder's port.
5. Entangle the initiator's port and the responder's port.
6. Create a trusted event that uses the MessageEvent interface, with the name connect, which does not bubble, is not cancelable, has no default action, has a data attribute whose value is initialized to the empty string, has a ports attribute whose value is initialized to a read only array containing only the responder's port, and has a source attribute whose value is initialized to the responder's port, and queue a task to dispatch the event at global object of the responder's document.

Examples

var channel = new BroadcastChannel("channel-name");

channel.addEventListener("message", function(message) {
  // Do something interesting with |message|, which is a MessageEvent.
}, false);

// Send a message to each other BroadcastChannel instance with the same channel
// name in the same origin.
channel.postMessage("I think, therefore I am.");