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Volodymyr Yepishev
Volodymyr Yepishev

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Iframe Microfrontends: Make React App Proper Iframe

The repo is here.
The commit for this part of the tutorial is here :)

It is time for our final part, in which we will add functionality to the React app we have, so it can determine if it is a standalone app and use its own means of fetching activities, or if it is a part of something else, and delegate the request to the parent window.

The key point here is our use-activity-provider module. The only thing this folder exports to the rest of the application is the useActivityProvider hook, which returns a function, which corresponds to GetActivity interface. The rest is concealed under the hood of the module. What that means is that we simply need to craft another function for communicating with the parent window, which would correspond to GetActivity interface and then return it from our useActivityProvider hook in cases when our React app detects it is inside an iframe.

Sounds simple, right?

In order to do that we will need two more hooks inside use-activity-provider module, which will be working under its hood. The first one will do nothing but receiving messages which come down from the parent window, and the other one will serve as an adapter to pipe these messages to the familiar GetActivity interface, which the rest of the application is expected.

Finally, useActivityProvider will be granted the logic to tell if the app is standalone or inside an inframe, and will get to pick which one of the two functions returning Promise to provide to the application.

These two new hooks deserve a module of their own, since they encapsulate a good chunk of logic, so we'll be placing them inside use-ask-for-activity folder in use-activity-provider.

We'll start with the simpler hook, the one which receives activities from messages:

npm run nx -- g @nrwl/react:hook use-activity-from-message --directory=app/use-activity-provider/use-ask-for-activity --project=react-app --skipTests=true --export=false --flat
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Now let's populate the file with logic. We will utilize useEffect, useCallback and useState hooks:

// ./apps/react-app/src/app/use-activity-provider/use-ask-for-activity/use-activity-from-message.ts
import { useState, useCallback, useEffect } from 'react';

import { ActivityItem } from '@demo--nx-iframe-microfrontends/models';

export function useActivityFromMessage(): ActivityItem | null {
  const [activity, setActivity] = useState<ActivityItem | null>(null);

  const logMessage = useCallback((event: { data: ActivityItem }) => {
  }, []);

  useEffect(() => {
    window.addEventListener('message', logMessage);
    return () => {
      window.removeEventListener('message', logMessage);
  }, [logMessage]);

  return activity;
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Looks fairly straightforward, doesn't it? We add a listener and every time activity comes down (for the sake of simplicity we are not performing any checks here, i.e. if it is really ActivityItem, etc.), we throw it into useState and send it further to whoever is using the hook. This hook has no idea how the acitvity is further delivered and that's the marvel of it.

Now we need our last hook, which will provide means for requesting activity from the parent window and return the result which it will obtain from our recently created useActivityFromMessage.

I suck at naming, so I will call it useAskForActivity :)

npm run nx -- g @nrwl/react:hook use-ask-for-activity --directory=app/use-activity-provider/use-ask-for-activity --project=react-app --skipTests=true --export=false --flat
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This one is going to be a bit more tricky: we will need it to return a promise, but we would have to manually resolve it with the result coming from useActivityFromMessage. Luckily we can easily obtain a reference to resolve of a Promise and keep it preserve using useRef hook :)

// ./apps/react-app/src/app/use-activity-provider/use-ask-for-activity/use-ask-for-activity.ts
import { useEffect, useRef } from 'react';

import { ActivityItem } from '@demo--nx-iframe-microfrontends/models';

import { GetActivity } from '../../models/get-activity.model';
import { useActivityFromMessage } from './use-activity-from-message';

export function useAskForActivity(): GetActivity {
  const activity = useActivityFromMessage();

  const megares = useRef<(activity: ActivityItem) => void>();

  useEffect(() => {
    if (activity) {
  }, [activity]);

  return (): Promise<ActivityItem> => {
        message: 'plz give some activity, bro?',
    return new Promise<ActivityItem>((res) => {
      activityResolver.current = res;
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So as you see when the returned function is invoked by a consumer, it will message parent window, create a new Promise, store its resolve to useRef resolver and trigger it once activity comes from useActivityFromMessage!

All what's left is to tweak useActivityProvider to determine whether our app is standalone or iframe, we could use window location for the check and then return the correct version of GetActivity implementation:

// ./apps/react-app/src/app/use-activity-provider/use-activity-provider.ts
import { GetActivity } from '../models/get-activity.model';
import { fetchActivity } from './fetch-activity.util';
import { useAskForActivity } from './use-ask-for-activity/use-ask-for-activity';

export function useActivityProvider(): GetActivity {
  const askForActivity = useAskForActivity();
  const isStandaloneApplication = window.location === window.parent.location;

  return isStandaloneApplication ? fetchActivity : askForActivity;
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So now you have it, http://localhost:4201/ run Angular application with React inside an iframe requesting Angular to do http requests, and at the same time there's a standalone React app http://localhost:4200/ which functions independently.

Cool, eh? :)

And that's how this series ends :>

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