Importing `ServiceWorkerModule` into your `AppModule` doesn't just register the service worker, it also provides a few services you can use to interact with the service worker and control the caching of your app.
The `SwUpdate` service gives you access to events that indicate when the service worker has discovered an available update for your app or when it has activated such an update—meaning it is now serving content from that update to your app.
The `SwUpdate` service supports four separate operations:
* Getting notified of *available* updates. These are new versions of the app to be loaded if the page is refreshed.
* Getting notified of update *activation*. This is when the service worker starts serving a new version of the app immediately.
* Asking the service worker to check the server for new updates.
* Asking the service worker to activate the latest version of the app for the current tab.
### Available and activated updates
The two update events, `available` and `activated`, are `Observable` properties of `SwUpdate`:
It's possible to ask the service worker to check if any updates have been deployed to the server.
The service worker checks for updates during initialization and on each navigation request—that is, when the user navigates from a different address to your app.
However, you might choose to manually check for updates if you have a site that changes frequently or want updates to happen on a schedule.
This method returns a `Promise` which indicates that the update check has completed successfully, though it does not indicate whether an update was discovered as a result of the check. Even if one is found, the service worker must still successfully download the changed files, which can fail. If successful, the `available` event will indicate availability of a new version of the app.
In order to avoid negatively affecting the initial rendering of the page, `ServiceWorkerModule` waits for up to 30 seconds by default for the app to stabilize, before registering the ServiceWorker script.
Constantly polling for updates, for example, with [setInterval()](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setInterval) or RxJS' [interval()](https://rxjs.dev/api/index/function/interval), will prevent the app from stabilizing and the ServiceWorker script will not be registered with the browser until the 30 seconds upper limit is reached.
Calling `activateUpdate()` without reloading the page could break lazy-loading in a currently running app, especially if the lazy-loaded chunks use filenames with hashes, which change every version.
Therefore, it is recommended to reload the page once the promise returned by `activateUpdate()` is resolved.
In some cases, the version of the app used by the service worker to serve a client might be in a broken state that cannot be recovered from without a full page reload.
For example, imagine the following scenario:
- A user opens the app for the first time and the service worker caches the latest version of the app.
Let's assume the app's cached assets include `index.html`, `main.<main-hash-1>.js` and `lazy-chunk.<lazy-hash-1>.js`.
- The user closes the app and does not open it for a while.
- After some time, a new version of the app is deployed to the server.
This newer version includes the files `index.html`, `main.<main-hash-2>.js` and `lazy-chunk.<lazy-hash-2>.js` (note that the hashes are different now, because the content of the files has changed).
The old version is no longer available on the server.
- In the meantime, the user's browser decides to evict `lazy-chunk.<lazy-hash-1>.js` from its cache.
Browsers may decide to evict specific (or all) resources from a cache in order to reclaim disk space.
- The user opens the app again.
The service worker serves the latest version known to it at this point, namely the old version (`index.html` and `main.<main-hash-1>.js`).
- At some later point, the app requests the lazy bundle, `lazy-chunk.<lazy-hash-1>.js`.
- The service worker is unable to find the asset in the cache (remember that the browser evicted it).
Nor is it able to retrieve it from the server (since the server now only has `lazy-chunk.<lazy-hash-2>.js` from the newer version).
In the above scenario, the service worker is not able to serve an asset that would normally be cached.
That particular app version is broken and there is no way to fix the state of the client without reloading the page.
In such cases, the service worker notifies the client by sending an `UnrecoverableStateEvent` event.
You can subscribe to `SwUpdate#unrecoverable` to be notified and handle these errors.
