Angular 的 Service Worker 简介
Angular service worker introduction
Service Worker 可以增强传统的 Web 发布模式，并使应用程序能够提供可与原生代码媲美的高可靠、高性能的用户体验。为 Angular 应用添加 Service Worker 是把应用转换成渐进式应用（PWA）的步骤之一。
Service workers augment the traditional web deployment model and empower applications to deliver a user experience with the reliability and performance on par with natively-installed code. Adding a service worker to an Angular application is one of the steps for turning an application into a Progressive Web App (also known as a PWA).
简单来说，Service Worker 就是一段运行在 Web 浏览器中，并为应用管理缓存的脚本。
At its simplest, a service worker is a script that runs in the web browser and manages caching for an application.
Service Worker 的功能就像一个网络代理。它们会拦截所有由应用发出的 HTTP 请求，并选择如何给出响应。 比如，它们可以查询局部缓存，如果有缓存的响应数据，就用它做出响应。 这种代理行为不会局限于通过程序调用 API（比如 fetch）发起的请求，还包括 HTML 中对资源的引用，甚至对 index.html 的首次请求。 基于 Service Worker 的缓存是完全可编程的，并且不依赖于服务端指定的那些控制缓存策略的头。
Service workers function as a network proxy. They intercept all outgoing HTTP requests made by the application and can choose how to respond to them. For example, they can query a local cache and deliver a cached response if one is available. Proxying isn't limited to requests made through programmatic APIs, such as fetch; it also includes resources referenced in HTML and even the initial request to index.html. Service worker-based caching is thus completely programmable and doesn't rely on server-specified caching headers.
不像应用中的其它脚本（如 Angular 的应用包），Service Worker 在用户关闭浏览器页标签时仍然会被保留。 下次浏览器加载本应用时，Service Worker 会首先加载，然后拦截加载本应用时的对每一项资源的请求。 如果这个 Service Worker 就是为此而设计的，它就能完全满足应用加载时的需求，而不需要依赖网络。
Unlike the other scripts that make up an application, such as the Angular application bundle, the service worker is preserved after the user closes the tab. The next time that browser loads the application, the service worker loads first, and can intercept every request for resources to load the application. If the service worker is designed to do so, it can completely satisfy the loading of the application, without the need for the network.
即使在快速可靠的网络中，往返延迟也可能在加载应用程序时产生显著的延迟。使用 Service Worker 来减少对网络的依赖可以显着改善用户体验。
Even across a fast reliable network, round-trip delays can introduce significant latency when loading the application. Using a service worker to reduce dependency on the network can significantly improve the user experience.
Angular 中的 Service Worker
Service workers in Angular
作为单页面应用，Angular 应用可以受益于 Service Worker 的优势。 从 Angular v5.0.0 开始，Angular 提供了一份 Service Worker 的实现。 Angular 开发人员可以利用 Service Worker，并受益于其增强的可靠性和性能，而无需再针对底层 API 写代码。
Angular applications, as single-page applications, are in a prime position to benefit from the advantages of service workers. Starting with version 5.0.0, Angular ships with a service worker implementation. Angular developers can take advantage of this service worker and benefit from the increased reliability and performance it provides, without needing to code against low-level APIs.
Angular 的 Service Worker 的设计目标是优化那些使用慢速、不可靠网络的最终用户的体验，同时还要尽可能减小提供过期内容的风险。
Angular's service worker is designed to optimize the end user experience of using an application over a slow or unreliable network connection, while also minimizing the risks of serving outdated content.
Angular 的 Service Worker 的行为遵循下列设计目标：
The Angular service worker's behavior follows that design goal:
像安装原生应用一样缓存应用。该应用作为整体被缓存，它的所有文件作为整体进行更新。
Caching an application is like installing a native application. The application is cached as one unit, and all files update together.
正在运行的应用使用所有文件的同一版本继续运行。不要突然开始接收来自新版本的、可能不兼容的缓存文件。
A running application continues to run with the same version of all files. It does not suddenly start receiving cached files from a newer version, which are likely incompatible.
当用户刷新本应用时，他们会看到最新的被完全缓存的版本。新的页标签中会加载最新的缓存代码。
When users refresh the application, they see the latest fully cached version. New tabs load the latest cached code.
在更改发布之后，相对较快的在后台进行更新。在一次完整的更新完成之前，仍然使用应用的上一个版本。
Updates happen in the background, relatively quickly after changes are published. The previous version of the application is served until an update is installed and ready.
只要有可能，Service Worker 就会尽量节省带宽。它只会下载那些发生了变化的资源。
The service worker conserves bandwidth when possible. Resources are only downloaded if they've changed.
要支持这些行为，Angular 的 Service Worker 会从服务器上下载一个 manifest 文件。 这个 manifest 文件描述要缓存的资源，并包含每个文件内容的哈希值。 当发布了应用的一个新版本时，manifest 的内容就会改变，通知 Service Worker 应该下载并缓存应用的一个新版本了。 这个 manifest 是从 CLI 生成的一个名叫 ngsw-config.json 的文件中生成的。
To support these behaviors, the Angular service worker loads a manifest file from the server. The manifest describes the resources to cache and includes hashes of every file's contents. When an update to the application is deployed, the contents of the manifest change, informing the service worker that a new version of the application should be downloaded and cached. This manifest is generated from a CLI-generated configuration file called ngsw-config.json.
安装 Angular 的 Service Worker 就像引入一个 NgModule 一样简单。 除了使用浏览器注册 Angular 的 Service Worker 之外，还要制作一些可供注入的服务，它们可以与 Service Worker 交互，并控制它。 比如，应用可以要求当新的更新已经就绪时通知自己，或要求 Service Worker 检查服务器，看是否有可用的更新。
Installing the Angular service worker is as easy as including an NgModule. In addition to registering the Angular service worker with the browser, this also makes a few services available for injection which interact with the service worker and can be used to control it. For example, an application can ask to be notified when a new update becomes available, or an application can ask the service worker to check the server for available updates.
前提条件
Prerequisites
要使用 Angular Service Worker 的所有功能，请使用最新版本的 Angular 和 Angular CLI。
To make use of all the features of Angular service worker, use the latest versions of Angular and the Angular CLI.
为了注册 Service Worker，必须通过 HTTPS 而非 HTTP 访问该应用程序。浏览器会忽略通过不安全连接访问的页面上的 Service Worker。原因是 Service Worker 非常强大，因此需要格外小心，以确保 Service Worker 的脚本未被篡改。
In order for service workers to be registered, the application must be accessed over HTTPS, not HTTP. Browsers ignore service workers on pages that are served over an insecure connection. The reason is that service workers are quite powerful, so extra care needs to be taken to ensure the service worker script has not been tampered with.
这条规则有一个例外：为了方便本地开发，当访问 localhost 上的应用时，浏览器不要求安全连接。
There is one exception to this rule: to make local development easier, browsers do not require a secure connection when accessing an application on localhost.
浏览器支持
Browser support
要从 Angular Service Worker 中受益，你的应用必须在支持 Service Worker 的 Web 浏览器中运行。 当前，最新版本的 Chrome，Firefox，Edge，Safari，Opera，UC 浏览器（Android 版）和 Samsung Internet 支持 Service Worker。而 IE 和 Opera Mini 等浏览器不支持 Service Worker。
To benefit from the Angular service worker, your application must run in a web browser that supports service workers in general. Currently, service workers are supported in the latest versions of Chrome, Firefox, Edge, Safari, Opera, UC Browser (Android version) and Samsung Internet. Browsers like IE and Opera Mini do not support service workers.
如果用户通过不支持 Service Worker 的浏览器访问你的应用程序，则该 Service Worker 不会注册，并且不会发生相关行为，例如脱机缓存管理和推送通知。进一步来说：
If the user is accessing your application with a browser that does not support service workers, the service worker is not registered and related behavior such as offline cache management and push notifications does not happen. More specifically:
浏览器不会下载 Service Worker 脚本和
ngsw.json清单文件。The browser does not download the service worker script and
ngsw.jsonmanifest file.主动尝试与 Service Worker 进行交互，例如调用
SwUpdate.checkForUpdate()，会返回被拒绝的承诺（Promise）。Active attempts to interact with the service worker, such as calling
SwUpdate.checkForUpdate(), return rejected promises.相关服务（例如
SwUpdate.available）的可观察事件不会触发。The observable events of related services, such as
SwUpdate.available, are not triggered.
强烈建议你确保即使在浏览器中没有 Service Worker 支持的情况下，你的应用也可以正常运行。尽管不受支持的浏览器会忽略 Service Worker 缓存，但如果应用程序尝试与 Service Worker 进行交互，它将仍然会报告错误。例如，调用 SwUpdate.checkForUpdate() 将返回被拒绝的承诺。为避免此类错误，你可以使用 SwUpdate.isEnabled() 检查是否启用了 Angular Service Worker。
It is highly recommended that you ensure that your application works even without service worker support in the browser. Although an unsupported browser ignores service worker caching, it will still report errors if the application attempts to interact with the service worker. For example, calling SwUpdate.checkForUpdate() will return rejected promises. To avoid such an error, you can check whether the Angular service worker is enabled using SwUpdate.isEnabled().
要了解关于其它支持 Service Worker 的浏览器的更多信息，请参阅 Can I Use 页面和 MDN 文档。
To learn more about other browsers that are service worker ready, see the Can I Use page and MDN docs.
相关资源
Related resources
本节中的其余文章专门针对 Service Worker 的 Angular 实现。
The rest of the articles in this section specifically address the Angular implementation of service workers.
要了解更多关于 Service Worker 的普遍性信息，参阅 Service Worker 简介。
For more information about service workers in general, see Service Workers: an Introduction.
要了解关于浏览器支持度的更多信息，参阅 Service Worker 简介 中的浏览器支持部分、Jake Archibald 写的Serviceworker 好了吗？和 Can I Use。
For more information about browser support, see the browser support section of Service Workers: an Introduction, Jake Archibald's Is Serviceworker ready?, and Can I Use.
关于其它建议和范例，请参阅：
For additional recommendations and examples, see:
下一步
Next steps
要开始使用 Angular Serivce Worker，参阅 Service Worker 快速上手。
To begin using Angular service workers, see Getting Started with service workers.
