Oren Farhi

Written by Oren Farhi, Senior Front End Engineer & Javascript Consultant, follow me on

Show A PWA Update with Redux, React Hooks & Service Workers

June 28, 2020

Show A PWA Update with Redux, React Hooks & Service Workers

In this article, i’m sharing the method to notify about an update for ReadM PWA. readm is using a service worker to add offline and cache support - so, when an update is available, it’s important to notify the user and have the most up to date code.

ReadM is a free friendly reading web app that motivates kids to practice, learn, read and speak English using real time feedback and providing positive experience.

Setting Up Offline in CRA

In the cra documentation, there’s a very good explanation to why you would consider adding offline support to your app - there are important considerations to think about and these docs are doing it well.

One of the requirements for creating a PWA and support offline is to have a service worker that supports that. CRA is already packed with a very good service worker abilities. In the main index.js there’s already a very clear disclaimer and an optional opt-in code to allow offline worker.

It clearly indicate the following:

// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: https://bit.ly/CRA-PWA
serviceWorker.unregister()

The actual code of the service and what it does is available in serviceWorker.js. To setup offline out of the box, the serviceWorker should simply call the register method:

serviceWorker.register();

Behind The Scenes - “Crash Course” on Service Worker in CRA

The Service Worker is a separate asynchronous layer that lives in parallel to the app’s DOM. In the code, it is declared and run by default after React initiates the app’s main render.

In ReadM, the main index.tsx includes a store provider that wraps the App in order to provide the redux store layer:

ReactDOM.render(
  <Provider store={store}>
    <Router>
      <App />
    </Router>
  </Provider>,
  document.querySelector("#root")
)

serviceWorker.register(config)

The config object that is passed to the register function, may include callbacks for interesting events that happen in the serviceWorker lifecycle. On of these events is the onUpdate(registration) event.

This function is called from within the service worker once the the service worker has already registered with the browser. Still, this is NOT an indication that something has changed.

the registration object is a service worker registration - an object that the service worker gets to activate and it allows to control several pages under the same domain and act as a sort of “event emitter” within the browser.

the code within serviceWorker.js listens to an onStateChange event and calls that onUpdate() callback.

Dispatch an update to Redux Store

lets get to business: we’re interested in the onUpdate() method. The code below is passed in the configuration to the Service Worker.

registration.waiting is a service worker that is returned from the registration object (a service worker on its own). Only if it exists, it will listen to any changes or updates in ReadM’s code and upon that will simply dispatch an updateRead() action to Redux store. the store object is available (as seen above) within the scope of index.ts.

serviceWorker.register({
  onUpdate: registration => {
    const waitingServiceWorker = registration.waiting

    if (waitingServiceWorker) {
      waitingServiceWorker.addEventListener("statechange", event => {
        if (event.target.state === "activated") {
          store.dispatch(updateReady())
        }
      })
    }
  },
})

In this case, I decided to take the simple route and signal an update with a boolean value.

Similarly, the onSuccess() callback is invoked once ReadM is registered for offline use. In this case, this information may be used to indicates the app is in offline mode.

// Pseudo code
serviceWorker.register({
  onSuccess: registration => {
    console.log("registered app for offline use. details:", registration)
  },
})

Separation of Concerns - Reacting with UI

Up until now we handled the technical layer for checking and verifying there’s an update. Now it’s time to UI into it.

In ReadM i took a simple approach - The App component is using Redux selector hook to get the value of update from the app’s main reducer (I simplified the code for this example - i’m actually using a store api hook):

const update = useSelector(selectUpdate)

Simply enough, the jsx includes a conditional rendering of a Toast component that notifies the user of an update and an action to take:

{
  update && (
    <Toast
      text="Update is available. Please Close all tabs and reload."
      header="Update"
      actionLabel="Update"
      onAction={() => window.location.reload()}
    />
  )
}

alt text

“Offline” Notes To Take

Before approaching this solution, it took some time to have a clear mind set of the problem - This is important for the App’s overall architecture and for me as the maintainer.

I wanted to have a clear separation of concerns - the logics layer and the UI layer. I was able to achieve that by having a layer in between - in this case - Redux (it can be any other layer).

ReadM is free - please try it out.


Oren Farhi

Written by Oren Farhi, Senior Front End Engineer & Javascript Consultant, follow me on

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