# Service worker communication

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.

#### Prerequisites

#### 前提条件

A basic understanding of the following:

对下列知识有基本的了解：

* [Getting Started with Service Workers](guide/service-worker-getting-started).

<hr />

## `SwUpdate` service

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&mdash;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`:

<code-example path="service-worker-getting-started/src/app/log-update.service.ts" linenums="false" title="log-update.service.ts" region="sw-update"> </code-example>

You can use these events to notify the user of a pending update or to refresh their pages when the code they are running is out of date.

### Checking for updates

It's possible to ask the service worker to check if any updates have been deployed to the server. You might choose to do this if you have a site that changes frequently or want updates to happen on a schedule.

Do this with the `checkForUpdate()` method:

<code-example path="service-worker-getting-started/src/app/check-for-update.service.ts" linenums="false" title="check-for-update.service.ts" region="sw-check-update"> </code-example>

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.

### Forcing update activation

If the current tab needs to be updated to the latest app version immediately, it can ask to do so with the `activateUpdate()` method:

<code-example path="service-worker-getting-started/src/app/prompt-update.service.ts" linenums="false" title="prompt-update.service.ts" region="sw-activate"> </code-example>

Doing this could break lazy-loading into currently running apps, especially if the lazy-loaded chunks use filenames with hashes, which change every version.

## More on Angular service workers

You may also be interested in the following:

* [Service Worker in Production](guide/service-worker-devops).