isabella232 / framebus

A message bus that operates across iframes

Framebus

Framebus allows you to easily send messages across frames (and iframes) with a simple bus.

In one frame:

var Framebus = require("framebus");
var bus = new Framebus();

bus.emit("message", {
  from: "Ron",
  contents: "they named it...San Diago",
});

In another frame:

var Framebus = require("framebus");
var bus = new Framebus();

bus.on("message", function (data) {
  console.log(data.from + " said: " + data.contents);
});

The Framebus class takes a configuration object, where all the params are optional.

type FramebusOptions = {
  origin?: string, // default: "*"
  channel?: string, // no default
  verifyDomain?: (url: string) => boolean, // no default
};

The origin sets the framebus instance to only operate on the chosen origin.

The channel namespaces the events called with on and emit so you can have multiple bus instances on the page and have them only communicate with busses with the same channel value.

If a verifyDomain is passed, then the on listener will only fire if the domain of the origin of the post message matches the location.href value of page or the function passed for verifyDomain returns true.

var bus = new Framebus({
  verifyDomain: function (url) {
    // only return true if the domain of the url matches exactly
    url.indexOf("https://my-domain") === 0;
  },
});

API

target(options: FramebusOptions): framebus

returns: a chainable instance of framebus that operates on the chosen origin.

This method is used in conjuction with emit, on, and off to restrict their results to the given origin. By default, an origin of '*' is used.

framebus
  .target({
    origin: "https://example.com",
  })
  .on("my cool event", function () {});
// will ignore all incoming 'my cool event' NOT from 'https://example.com'

Argument	Type	Description
options	FramebusOptions	See above section for more details

emit('event', data? , callback?): boolean

returns: true if the event was successfully published, false otherwise

Argument	Type	Description
event	String	The name of the event
data	Object	The data to give to subscribers
callback(data)	Function	Give subscribers a function for easy, direct replies

on('event', fn): boolean

returns: true if the subscriber was successfully added, false otherwise

Unless already bound to a scope, the listener will be executed with this set to the MessageEvent received over postMessage.

Argument	Type	Description
event	String	The name of the event
fn(data?, callback?)	Function	Event handler. Arguments are from the emit invocation
↳ this	scope	The MessageEvent object from the underlying postMessage

off('event', fn): boolean

returns: true if the subscriber was successfully removed, false otherwise

Argument	Type	Description
event	String	The name of the event
fn	Function	The function that was subscribed

include(popup): boolean

returns: true if the popup was successfully included, false otherwise

var popup = window.open("https://example.com");

framebus.include(popup);
framebus.emit("hello popup and friends!");

Argument	Type	Description
popup	Window	The popup refrence returned by window.open

teardown(): void

Calls off on all listeners used for this bus instance and makes subsequent calls to all methods noop.

bus.on("event-name", handler);

// event-name listener is torn down
bus.teardown();

// these now do nothing
bus.on("event-name", handler);
bus.emit("event-name", data);
bus.off("event-name", handler);

Pitfalls

These are some things to keep in mind while using framebus to handle your event delegation

Cross-site scripting (XSS)

framebus allows convenient event delegation across iframe borders. By default it will broadcast events to all iframes on the page, regardless of origin. Use the optional target() method when you know the exact domain of the iframes you are communicating with. This will protect your event data from malicious domains.

Data is serialized as JSON

framebus operates over postMessage using JSON.parse and JSON.stringify to facilitate message data passing. Keep in mind that not all JavaScript objects serialize cleanly into and out of JSON, such as undefined.

Asynchronicity

Even when the subscriber and publisher are within the same frame, events go through postMessage. Keep in mind that postMessage is an asynchronous protocol and that publication and subscription handling occur on separate iterations of the event loop (MDN).

Published callback functions are an abstraction

When you specify a callback while using emit, the function is not actually given to the subscriber. The subscriber receives a one-time-use function that is generated locally by the subscriber's framebus. This one-time-use callback function is pre-configured to publish an event back to the event origin's domain using a UUID as the event name. The events occur as follows:

1. http://emitter.example.com publishes an event with a function as the event data

   var callback = function (data) {
     console.log("Got back %s as a reply!", data);
   };
   
   framebus.emit("Marco!", callback, "http://listener.example.com");

2. The framebus on http://emitter.example.com generates a UUID as an event name and adds the callback as a subscriber to this event.

3. The framebus on http://listener.example.com sees that a special callback event is in the event payload. A one-time-use function is created locally and given to subscribers of 'Marco!' as the event data.

4. The subscriber on http://listener.example.com uses the local one-time-use callback function to send data back to the emitter's origin

   framebus
     .target("http://emitter.example.com")
     .on("Marco!", function (callback) {
       callback("Polo!");
     });

5. The one-time-use function on http://listener.example.com publishes an event as the UUID generated in step 2 to the origin that emitted the event.

6. Back on http://emitter.example.com, the callback is called and unsubscribed from the special UUID event afterward.

Development and contributing

See CONTRIBUTING.md