CycleTracker: Service workers

The service worker is what makes the application work offline while making sure the application is always up to date. To do this well, the service worker should include the following:

• Version number (or other identifier).
• List of resources to cache.
• Cache version name.

The service worker is also responsible for:

• Installing the cache when the app is installed.
• Updating itself and the other application file as needed.
• Removing cached files that are no longer used.

We achieve these tasks by reacting to three service worker events, including the

• fetch,
• install, and
• activate events.

Once the PWA is installed on the user's machine, the only way to inform the browser that there are updated files to be retrieved is for there to be a change in the service worker. If a change is made to any other PWA resource — if the HTML is updated, a bug is fixed in the CSS, a function is added to app.js, an image is compressed to reduce the file save, etc. — the service worker of your installed PWA will not know it needs to download updated resources. Only when the service worker is altered in any way, will the PWA know it may be time to update the cache; which is the service worker's task to initiate.

While changing any character may technically suffice, a PWA best practice is to create a version number constant that gets updated sequentially to indicate an update to the file. Updating a version number (or date), provides an official edit to the service worker even if nothing else is changed in the service worker itself and provides developers with a way of identifying app versions.

Task

Start a JavaScript file by including a version number:

const VERSION = "v1";

Save the file as sw.js

For a good offline experience, the list of cached files should include all the resources used within the PWA's offline experience. While the manifest file may have a multitude of icons listed in various sizes, the application cache only needs to include the assets used by the app in offline mode.

const APP_STATIC_RESOURCES = [
  "/",
  "/index.html",
  "/style.css",
  "/app.js",
  "/icon-512x512.png",
];

You don't need to include the various icons that are used by all the different operating systems and devices in the list. But do include any images that are used within the app, including assets to used within any splash pages that may be visible if the app is slow as the app loads or used in any "you need to connect to the internet for the full experience" type pages.

Do not include the service worker file in the list of resources to be cached.

Task

Add the list of resources to be cached for the CycleTracker PWA to sw.js.

Example solution

We include the static resources created in other sections of this tutorial that CycleTracker needs to function when offline. Our current sw.js file is:

const VERSION = "v1";

const APP_STATIC_RESOURCES = [
  "/",
  "/index.html",
  "/style.css",
  "/app.js",
  "/cycletracker.json",
  "/icons/wheel.svg",
];

We included the wheel.svg icon, even though our current application doesn't use it, in case you are enhancing the PWA UI, such as displaying the logo when there is no period data.

We have a version number and we have the files that need to be cached. Before caching the files, we need to create a name of the cache that will be used to store the app's static resources. This cache name should be versioned to ensure that when the app is updated, a new cache will be created and the old one will be deleted.

Task

Use the VERSION number to create a versioned CACHE_NAME, adding it as a constant to sw.js.

Example solution

We name our cache period-tracker- with the current VERSION appended. As the constant declaration is on a single line, we put it before the array of resources constant for better legibility.

const VERSION = "v1";
const CACHE_NAME = `period-tracker-${VERSION}`;

const APP_STATIC_RESOURCES = [ ... ];

We have successfully declared our constants; a unique identifier, the list of offline resources as an array, and the application's cache name that changes every time the identifier is updated. Now let's focus on installing, updating, and deleting unused cached resources.

When a user installs or simply visits a website with a service worker, an install event is fired in the service worker scope. We want to listen for this event, filling the cache with the PWA's static resources upon installation. Every time the service worker version is updated, the browser installs the new service worker and the install event occurs.

The install event happens when the app is used for the first time, or when a new version of the service worker is detected by the browser. When an older service worker is being replaced by a new one, the old service worker is used as the PWA's service worker until the new service work is activated.

Only available in secure contexts, the WorkerGlobalScope.caches property returns a CacheStorage object associated with the current context. The CacheStorage.open() method returns a Promise that resolves to the Cache object matching name of the cache, passed as a parameter.

The Cache.addAll() method takes an array of URLs as a parameter, retrieves them, then adds the responses to the given cache. The ExtendableEvent.waitUntil() method tells the browser that work is ongoing until the promise settles, and it shouldn't terminate the service worker if it wants that work to complete. While browsers are responsible for executing and terminating service workers when necessary, the waitUntil method is a request to the browser to not terminate the service worker while a task is being executed.

self.addEventListener("install", (e) => {
  e.waitUntil((async () => {
      const cache = await caches.open("cacheName_identifier");
      cache.addAll([
        "/",
        "/index.html"
        "/style.css"
        "/app.js"
      ]);
    })()
  );
});

Task

Add an install event listener that retrieves and stores the files listed in APP_STATIC_RESOURCES into the cache named CACHE_NAME.

Example solution

self.addEventListener("install", (event) => {
  event.waitUntil(
    (async () => {
      const cache = await caches.open(CACHE_NAME);
      cache.addAll(APP_STATIC_RESOURCES);
    })(),
  );
});

As mentioned, when an existing service worker is being replaced by a new one, the existing service worker is used as the PWA's service worker until the new service worker is activated. We use the activate event to delete old caches to avoid running out of space. We iterate over named Cache objects, deleting all but the current one, and then set the service worker as the controller for the PWA.

We listen for the current service worker's global scope activate event.

We get the names of the existing named caches. We use the CacheStorage.keys() method (again accessing CacheStorage through the WorkerGlobalScope.caches property) which returns a Promise that resolves with an array containing strings corresponding to all of the named Cache objects in the order they were created.

We use the Promise.all() method to iterate through that list of name cache promises. The all() method takes as input a list of iterable promises and returns a single Promise. For each name in the list of named caches, check if the cache is the currently active cache. If not, delete it with the Cache delete() method.

The last line, the await clients.claim() uses the claim() method of the Clients interface to enable our service worker to set itself as the controller for our client; the "client" referring to a running instance of the PWA. The claim() method enables the service worker to "claim control" of all clients within its scope. This way, clients loaded in the same scope don't need to be reloaded.

self.addEventListener("activate", (event) => {
  event.waitUntil(
    (async () => {
      const names = await caches.keys();
      await Promise.all(
        names.map((name) => {
          if (name !== CACHE_NAME) {
            return caches.delete(name);
          }
        }),
      );
      await clients.claim();
    })(),
  );
});

Task

Add the above activate eventListener to your sw.js file.

We can take advantage of the fetch event, to prevent an installed PWA from making requests if the user is online. Listening to the fetch event makes it possible to intercept all requests and respond with cached responses instead of going to the network. Most applications don't require this behavior. In fact, many business models want users to regularly make server requests for tracking and marketing purposes. So, while intercepting requests may be an anti-pattern for some, to improve the privacy of our CycleTracker app, we don't want the app to make unnecessary server requests.

As our PWA consists of a single page, for page navigation requests, we go back to the index.html home page. There are no other pages and we don't ever want to go to the server. If the Fetch API's Request readonly mode property is navigate, meaning it's looking for a web page, we use the FetchEvent's respondWith() method to prevent the browser's default fetch handling, providing our own response promise employing the caches.match() method.

For all other request modes, we open the caches as done in the install event response, instead passing the event request to the same match() method. It checks if the request is a key for a stored Response. If yes, it returns the cached response. If not, we return a 404 status as a response.

Using the Response() constructor to pass a null body and a status: 404 as options, doesn't mean there is an error in our PWA. Rather, everything we need should already be in the cache, and if it isn't, we're not going to the server to resolve this non-issue.

self.addEventListener("fetch", (event) => {
  // when seeking an HTML page
  if (event.request.mode === "navigate") {
    // Return to the index.html page
    event.respondWith(caches.match("/"));
    return;
  }

  // For every other request type
  event.respondWith(
    (async () => {
      const cache = await caches.open(CACHE_NAME);
      const cachedResponse = await cache.match(event.request.url);
      if (cachedResponse) {
        // Return the cached response if it's available.
        return cachedResponse;
      }
      // Respond with a HTTP 404 response status.
      return new Response(null, { status: 404 });
    })(),
  );
});

Your sw.js file should look similar to the following JavaScript. Note that when updating any of the resources listed in the APP_STATIC_RESOURCES array, the only constant or function that must be updated within this service worker is the value of VERSION.

// The version of the cache.
const VERSION = "v1";

// The name of the cache
const CACHE_NAME = `period-tracker-${VERSION}`;

// The static resources that the app needs to function.
const APP_STATIC_RESOURCES = [
  "/",
  "/index.html",
  "/app.js",
  "/style.css",
  "/icons/wheel.svg",
];

// On install, cache the static resources
self.addEventListener("install", (event) => {
  event.waitUntil(
    (async () => {
      const cache = await caches.open(CACHE_NAME);
      cache.addAll(APP_STATIC_RESOURCES);
    })(),
  );
});

// delete old caches on activate
self.addEventListener("activate", (event) => {
  event.waitUntil(
    (async () => {
      const names = await caches.keys();
      await Promise.all(
        names.map((name) => {
          if (name !== CACHE_NAME) {
            return caches.delete(name);
          }
        }),
      );
      await clients.claim();
    })(),
  );
});

// On fetch, intercept server requests
// and respond with cached responses instead of going to network
self.addEventListener("fetch", (event) => {
  // As a single page app, direct app to always go to cached home page.
  if (event.request.mode === "navigate") {
    event.respondWith(caches.match("/"));
    return;
  }

  // For all other requests, go to the cache first, and then the network.
  event.respondWith(
    (async () => {
      const cache = await caches.open(CACHE_NAME);
      const cachedResponse = await cache.match(event.request.url);
      if (cachedResponse) {
        // Return the cached response if it's available.
        return cachedResponse;
      }
      // If resource isn't in the cache, return a 404.
      return new Response(null, { status: 404 });
    })(),
  );
});

When updating a service worker, the VERSION constant doesn't need to be updated, as any change in the content of the service worker script itself will trigger the browser to install the new service worker. However, it is a good practice to update the version number as it makes it easier for devs, including yourself, to see which version of the service worker is currently running in the browser, by checking the name of the Cache in the Application tool (or Sources tool).

Now that our service worker script is complete, we need to register the service worker.

We start by checking that the browser supports the Service Worker API by using feature detection for the presence of the serviceWorker property on the global navigator object:

<script>
  // Does "serviceWorker" exist
  if ("serviceWorker" in navigator) {
    // If yes, we register the service worker
  }
</script>

If the property is supported, we can then use the register() method of the service worker API's ServiceWorkerContainer interface.

<script>
  if ("serviceWorker" in navigator) {
    // Register the app's service worker
    // Passing the filename where that worker is defined.
    navigator.serviceWorker.register("sw.js");
  }
</script>

While the above suffices for the CycleTracker app needs, the register() method does return a Promise that resolves with a ServiceWorkerRegistration object. For a more robust application, error check the registration:

if ("serviceWorker" in navigator) {
  navigator.serviceWorker.register("sw.js").then(
    (registration) => {
      console.log("Service worker registration successful:", registration);
    },
    (error) => {
      console.error(`Service worker registration failed: ${error}`);
    },
  );
} else {
  console.error("Service workers are not supported.");
}

Open index.html and add the following <script> after the script to include app.js and before the closing </body> tag.

<!-- Register the app's service worker. -->
<script>
  if ("serviceWorker" in navigator) {
    navigator.serviceWorker.register("sw.js");
  }
</script>

You can try the fully functioning CycleTracker period tracking web app and view the web app source code on GitHub. Yes, it works, and it is now, officially, a PWA!

Because of the way we have set up the service worker, once it is registered, every request will pull from the cache instead of loading new content. When developing, you will be editing your code frequently. You likely want to test your edits in the browser regularly; likely with every save.

To get a new cache, you can change the version number and then do a hard browser refresh. The way you do a hard refresh depends on the browser and operating system:

• On Windows: Ctrl+F5, Shift+F5, or Ctrl+Shift+R.
• On macOS: Shift+Command+R.
• Safari on macOS: Option+Command+E to empty the cache, then Option+Command+R.
• On Mobile: Go to the browser (Android) or operating system (Samsung, iOS) settings, under advanced setting find the browser (iOS) or website data (Android, Samsung) site settings, and delete the data for CycleTracker, before reloading the page.

You likely don't want to update the version number with every save. Until you are ready to launch a new version of your PWA to production and give everyone a new version of your PWA, instead of changing the version number on save, you can unregister the service worker.

You can unregister a service worker by clicking on the unregister button in the browser developer tools. Hard refreshing the page will re-register the service worker and create a new cache.

In some developer tools, you can manually unregister a service worker, or you can select the service workers "update on reload" option which sets the developer tools to reset and re-activate the service worker on every reload as long as the developer tools are open. There is also an option to bypass the service worker and load resources from the network. This panel includes features we are not covering in this tutorial, but will be helpful as you create more advanced PWAs that include syncing and push, which are both covered in the offline and background operation guide.

The service worker window within the DevTools' application panel, provides a link to access to pop up window containing a list of all the registered service workers for the browser; not just the service worker for the application opened in the current tab. Each service worker list of workers has buttons to stop, start, or unregister that individual service worker.

In other words, as you are working on your PWA, you don't have to update the version number for every app view. But remember, when you are done with all your changes, update the service worker VERSION value before distributing the updated version of your PWA. If you forget, no one who has already installed your app or even visited your online PWA without installing it will ever get to see your changes!

At its core, a PWA is a web application that can be installed and that is progressively enhanced to work offline. We created a fully functional web application. We then added the two features - a manifest file and a service worker - required to convert it to a PWA. If you want to share your app with others, make it available via a secure connection. Alternatively, if you just want to use the cycle tracker yourself, create a local development environment, install the PWA, and enjoy! Once installed, you no longer need to run localhost.

Congratulations!

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