rx-postmessenger

Minimal RxJS adapter for the Window # postMessage API for request-response streams and notification streams across frame windows.

In short

An RxPostmessenger class instance establishes one end of a connection between 2 window objects, using the Window # postMessage API. Each instance provides methods to initiate outgoing messages and handle incoming messages, both categorized by channel name.

Each instance targets one single Window object. It propagates incoming MessageEvents only from that specific window object while it’s serving documents from one single origin.

RxJS Interoperability

	RxPostmessenger v1.x	RxPostmessenger v2.x	RxPostmessenger v3.x
RxJS v5.x	✅	❌	❌
RxJS v6.x	❌	✅	✅
RxJS v7.x	❌	❌	✅

View changelog.

Installation

$ npm install rx-postmessenger --save

Contents / API

Static methods

Method	Description
connect()	Connect Window objects by creating messenger instances.

Messenger Instance methods

Method	Description
notify()	Send notifications to the connected window.
notifications()	Listen for inbound notifications.
request()	Send requests to the connected window.
requests()	Listen for inbound requests.

Request Instance methods

Method	Description
respond()	Respond to the request with a certain payload.

---

Usage

import RxPostmessenger from 'rx-postmessenger';

Static methods

Connecting 2 Window objects

RxPostmessenger.connect(otherWindow: Window, origin: string): RxPostmessenger.Messenger

Both ends of the connection should implement this package. One in a parent project (that implements the iframe), and one in a child project (that’s being served by the iframe). Creating a new messenger is straightforward:

At parent window - https://parent-project.com

const childMessenger = RxPostmessenger.connect(
    someIFrame.contentWindow,
    'https://child-project.com'
);

At child window - https://child-project.com

const parentMessenger = RxPostmessenger.connect(
    window.parent,
    'https://parent-project.com'
);

Messenger Instance methods

Sending notifications

Messenger.notify<T>(channel: string, payload?: T): void

The messenger instances give you a way to send notifications to the other Window through the notify() method.
The notify method is void – notifications are fire-and-forget.
Use request() instead if you require data back.
Consider an example where we want to notify a child window of price changes:

childMessenger.notify('price-changed', {
    oldPrice: 12.50,
    newPrice: 14.50,
});

Listening for inbound notifications

Messenger.notifications<T = any>(channel: string): Observable<T>

The child project can request an Observable stream for a certain notification channel.
In this case we’re interested in 'price-changed' events, but only the ones where the price increased.
The ability to use RxJS operators can help us out:

parentMessenger.notifications('price-changed').pipe(
    filter(({ oldPrice, newPrice }) => newPrice > oldPrice),
    map(({ oldPrice, newPrice }) => newPrice - oldPrice),
).subscribe((increase) => console.log(`Price increased with €${increase}!`));

// > 'Price increased with €2!'

Sending requests

Messenger.request<T = any, U = any>(channel: string, payload?: T): Observable<U>

RxPostmessenger also supports request - response communication.
At the requester side a request is initiated by calling the request() method with 1 or 2 arguments.
The first is a request alias (actually just another channel) of our choice.

A notification-channel and a request-channel can both have the same channel name without any problem.

An observable is returned that emits the response when arrived, and then completes.
Let’s request a greeting from the child window, and tell it to localize the response to 'en':

const greetingResponse$ = childMessenger.request('greeting', {
    language: 'en',
});

We can then subscribe to the greeting response stream.
Provided that the greeting says something nice, we’ll log it for everyone to see:

greetingResponse$.pipe(
    filter((greeting) => isNiceGreeting(greeting)),
).subscribe(console.log);

// > 'Hi parent!'

Listening for inbound requests

Messenger.requests<T = any, U = any>(channel: string): Observable<RxPostmessenger.Request<T, U>>

No greeting would ever be received by parentMessenger when the child project does not listen
for requests to handle and respond to. Let’s not be rude and create a request stream for
'greeting' requests, and subscribe to it. We’ll pass the RxPostmessenger.Request objects
that the subscription receives into a function handleGreetingRequest():

parentMessenger
    .requests('greeting')
    .subscribe(handleGreetingRequest);

---

Request instance methods

Sending request responses

RxPostmessenger.Request<T, U> ~ respond(payload: U): void

The requests method returns an observable of RxPostmessenger.Request objects.
They provide a single method respond that accepts one argument: the response payload.
Let’s use the method on the requests we give to handleGreetingRequest:

const handleGreetingRequest = (request) => {

    // The data that was sent along with the request
    const requestPayload = request.payload;

    // A hypothetical greeting translator
    const localizedGreeting = translateGreeting(
        'Hi parent!',
        requestPayload.language
    );

    // Eventually respond to the request with some data (payload)
    request.respond(localizedGreeting);
};

License

The MIT License (MIT). See license file for more information.