angular-docs-cn/aio/content/guide/service-worker-getting-started.md

# Getting started with service workers


This document explains how to enable Angular service worker support in your CLI projects. It then uses a simple example to show you a service worker in action, demonstrating loading and basic caching. 

#### Prerequisites

A basic understanding of the following:
* [Introduction to Angular service workers](guide/service-worker-intro).
* Angular v6, including Angular CLI v6.

<hr />


## Adding a service worker to your project

To set up the Angular service worker in your project, use the CLI command `ng add @angular/pwa`. It takes care of configuring your app to use service workers by adding the `service-worker` package along 
with setting up the necessary support files.

```sh
ng add  @angular/pwa --project *project-name* 
```

The above command completes the following actions:

1. Adds the `@angular/service-worker` package to your project. 
2. Enables service worker build support in the CLI.
3. Imports and registers the service worker in the app module.
4. Updates the `index.html` file:
    * Includes a link to add the `manifest.json` file.
    * Adds meta tags for `theme-color`.
5. Installs icon files to support the installed Progressive Web App (PWA).
6. Creates the service worker configuration file called [`ngsw-config.json`](/guide/service-worker-config), which specifies the caching behaviors and other settings. 


 Now, build the project: 

```sh
ng build --prod
```

The CLI project is now set up to use the Angular service worker.


## Service worker in action: a tour

This section demonstrates a service worker in action, 
using an example application. 

### Serving with `http-server`

Because `ng serve` does not work with service workers, you must use a separate HTTP server to test your project locally. You can use any HTTP server. The example below uses the [http-server](https://www.npmjs.com/package/http-server) package from npm. To reduce the possibility of conflicts, test on a dedicated port.

To serve with `http-server`, change to the directory containing your web files and start the web server: 

```sh
cd dist/*project-name*
http-server -p 8080
```

### Initial load

With the server running, you can point your browser at http://localhost:8080/. Your application should load normally.

**Tip:** When testing Angular service workers, it's a good idea to use an incognito or private window in your browser to ensure the service worker doesn't end up reading from a previous leftover state, which can cause unexpected behavior.

### Simulating a network issue

To simulate a network issue, disable network interaction for your application. In Chrome: 

1. Select **Tools** > **Developer Tools** (from the Chrome menu located at the top right corner).
2. Go to the **Network tab**.
3. Check the **Offline box**.

<figure>
  <img src="generated/images/guide/service-worker/offline-checkbox.png" alt="The offline checkbox in the Network tab is checked">
</figure>

Now the app has no access to network interaction.

For applications that do not use the Angular service worker, refreshing now would display Chrome's Internet disconnected page that says "There is no Internet connection". 

With the addition of an Angular service worker, the application behavior changes. On a refresh, the page loads normally. 

If you look at the Network tab, you can verify that the service worker is active.

<figure>
  <img src="generated/images/guide/service-worker/sw-active.png" alt="Requests are marked as from ServiceWorker">
</figure>

Notice that under the "Size" column, the requests state is `(from ServiceWorker)`. This means that the resources are not being loaded from the network. Instead, they are being loaded from the service worker's cache.


### What's being cached?

Notice that all of the files the browser needs to render this application are cached. The `ngsw-config.json` boilerplate configuration is set up to cache the specific resources used by the CLI:

* `index.html`.
* `favicon.ico`.
* Build artifacts (JS and CSS bundles).
* Anything under `assets`.

### Making changes to your application

Now that you've seen how service workers cache your application, the 
next step is understanding how updates work. 

1. If you're testing in an incognito window, open a second blank tab. This will keep the incognito and the cache state alive during your test.

2. Close the application tab, but not the window. This should also close the Developer Tools. 

3. Shut down `http-server`.

4. Next, make a change to the application, and watch the service worker install the update.

5. Open `src/app/app.component.html` for editing.

6. Change the text `Welcome to {{title}}!` to `Bienvenue à {{title}}!`.

7. Build and run the server again:

```sh
ng build --prod
cd dist
http-server -p 8080
```

### Updating your application in the browser

Now look at how the browser and service worker handle the updated application.

1. Open http://localhost:8080 again in the same window. What happens?

<figure>
  <img src="generated/images/guide/service-worker/welcome-msg-en.png" alt="It still says Welcome to Service Workers!">
</figure>

What went wrong? Nothing, actually. The Angular service worker is doing its job and serving the version of the application that it has **installed**, even though there is an update available. In the interest of speed, the service worker doesn't wait to check for updates before it serves the application that it has cached.

If you look at the `http-server` logs, you can see the service worker requesting `/ngsw.json`. This is how the service worker checks for updates.

2. Refresh the page.

<figure>
  <img src="generated/images/guide/service-worker/welcome-msg-fr.png" alt="The text has changed to say Bienvenue à app!">
</figure>

The service worker installed the updated version of your app *in the background*, and the next time the page is loaded or reloaded, the service worker switches to the latest version.

<hr />

## More on Angular service workers

You may also be interested in the following:
* [Communicating with service workers](guide/service-worker-communications).