与 Service Worker 通讯link

Service worker communicationlink

把 ServiceWorkerModule 导入到你的 AppModule 中不仅会注册 Service Worker，还会提供一些服务，让你能和 Service Worker 通讯，并控制你的应用缓存。

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 application.

前提条件link

Prerequisiteslink

对下列知识有基本的了解：

A basic understanding of the following:

• Service Worker 快速上手。

  Getting Started with Service Workers.

SwUpdate 服务link

SwUpdate servicelink

SwUpdate 服务让你能访问一些事件，这些事件会指出 Service Worker 何时发现了可用的更新或者一个更新何时可以被激活 —— 这意味着它现在可以通过更新后的版本提供服务了。

The SwUpdate service gives you access to events that indicate when the service worker has discovered an available update for your application or when it has activated such an update—meaning it is now serving content from that update to your application.

SwUpdate 服务支持四个独立的操作：

The SwUpdate service supports four separate operations:

• 获取出现可用更新的通知。如果要刷新页面，这些就是可加载的新版本。

  Getting notified of available updates. These are new versions of the application to be loaded if the page is refreshed.

• 获取更新被激活的通知。这时候 Service Worker 就可以立即使用这些新版本提供服务了。

  Getting notified of update activation. This is when the service worker starts serving a new version of the application immediately.

• 要求 Service Worker 向服务器查询是否有新版本。

  Asking the service worker to check the server for new updates.

• 要求 Service Worker 为当前标签页激活该应用的最新版本。

  Asking the service worker to activate the latest version of the application for the current tab.

有可用更新及已激活更新link

Available and activated updateslink

这两个更新事件 available 和 activated，都是 SwUpdate 的 Observable 属性：

The two update events, available and activated, are Observable properties of SwUpdate:

      
        content_copy
      
      @Injectable()
export class LogUpdateService {

  constructor(updates: SwUpdate) {
    updates.available.subscribe(event => {
      console.log('current version is', event.current);
      console.log('available version is', event.available);
    });
    updates.activated.subscribe(event => {
      console.log('old version was', event.previous);
      console.log('new version is', event.current);
    });
  }
}
    

你可以使用这些事件来通知用户有一个待做更新或当它们运行的代码已经过期时刷新页面。

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use these events to notify the user of a pending update or to refresh their pages when the code they are running is out of date.

检查更新link

Checking for updateslink

可以要求 Service Worker 检查是否有任何更新已经发布到了服务器上。 Service Worker 会在初始化和每次导航请求（也就是用户导航到应用中的另一个地址）时检查更新。 不过，如果你的站点更新非常频繁，或者需要按计划进行更新，你可能会选择手动检查更新。

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 application. 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.

通过调用 checkForUpdate() 方法来实现：

Do this with the checkForUpdate() method:

      
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      import { ApplicationRef, Injectable } from '@angular/core';
import { SwUpdate } from '@angular/service-worker';
import { concat, interval } from 'rxjs';
import { first } from 'rxjs/operators';

@Injectable()
export class CheckForUpdateService {

  constructor(appRef: ApplicationRef, updates: SwUpdate) {
    // Allow the app to stabilize first, before starting polling for updates with `interval()`.
    const appIsStable$ = appRef.isStable.pipe(first(isStable => isStable === true));
    const everySixHours$ = interval(6 * 60 * 60 * 1000);
    const everySixHoursOnceAppIsStable$ = concat(appIsStable$, everySixHours$);

    everySixHoursOnceAppIsStable$.subscribe(() => updates.checkForUpdate());
  }
}
    

该方法返回一个用来表示检查更新已经成功完成的 Promise，不过它不会指出是否确实发现了一个更新。 即使找到了一个，Service Worker 还必须成功下载更新过的文件，而这可能会失败。如果成功了，就会通过一个 available 事件来表明当前应用有一个可用的新版本。

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 application.

强制激活更新link

Forcing update activationlink

如果当前标签页需要立即更新到最新的应用版本，可以通过 activateUpdate() 方法来要求立即这么做：

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

      
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      @Injectable()
export class PromptUpdateService {

  constructor(updates: SwUpdate) {
    updates.available.subscribe(event => {
      if (promptUser(event)) {
        updates.activateUpdate().then(() => document.location.reload());
      }
    });
  }
}
    

处理不可恢复的状态link

Handling an unrecoverable statelink

在某些情况下，Service Worker 用来为客户端提供服务的应用版本可能处于损坏状态，如果不重新加载整个页面，则无法恢复该状态。

In some cases, the version of the application 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.

例如，设想以下情形：

For example, imagine the following scenario:

• 用户首次打开该应用，Service Worker 会缓存该应用的最新版本。假设应用要缓存的资源包括 index.html、main.<main-hash-1>.js 和 lazy-chunk.<lazy-hash-1>.js 。

  A user opens the application for the first time and the service worker caches the latest version of the application. Let's assume the application's cached assets include index.html, main.<main-hash-1>.js and lazy-chunk.<lazy-hash-1>.js.

• 用户关闭该应用程序，并且有一段时间没有打开它。

  The user closes the application and does not open it for a while.

• 一段时间后，会将新版本的应用程序部署到服务器。新版本中包含文件 index.html、main.<main-hash-2>.js 和 lazy-chunk.<lazy-hash-2>.js （请注意，哈希值现在已经不同了，因为文件的内容已经改变）。服务器上不再提供旧版本。

  After some time, a new version of the application 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.

• 同时，用户的浏览器决定从其缓存中清退 lazy-chunk.<lazy-hash-1>.js 浏览器可能决定从缓存中清退特定（或所有）资源，以便回收磁盘空间。

  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.

• 用户再次打开本应用。此时，Service Worker 将提供它所知的最新版本，当然，实际上对我们是旧版本（ index.html 和 main.<main-hash-1>.js ）。

  The user opens the application 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).

• 在稍后的某个时刻，该应用程序请求惰性捆绑包 lazy-chunk.<lazy-hash-1>.js 。

  At some later point, the application requests the lazy bundle, lazy-chunk.<lazy-hash-1>.js.

• Service Worker 无法在缓存中找到该资产（请记住浏览器已经将其清退了）。它也无法从服务器上获取它（因为服务器现在只有较新版本的 lazy-chunk.<lazy-hash-2>.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).

在上述情况下，Service Worker 将无法提供通常会被缓存的资产。该特定的应用程序版本已损坏，并且无法在不重新加载页面的情况下修复客户端的状态。在这种情况下，Service Worker 会通过发送 UnrecoverableStateEvent 事件来通知客户端。你可以订阅 SwUpdate#unrecoverable 以得到通知并处理这些错误。

In the above scenario, the service worker is not able to serve an asset that would normally be cached. That particular application 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.

      
        content_copy
      
      @Injectable()
export class HandleUnrecoverableStateService {
  constructor(updates: SwUpdate) {
    updates.unrecoverable.subscribe(event => {
      notifyUser(
        `An error occurred that we cannot recover from:\n${event.reason}\n\n` +
        'Please reload the page.');
    });
  }
}
    

关于 Angular Service Worker 的更多信息link

More on Angular service workerslink

你可能还对下列内容感兴趣：

You may also be interested in the following:

• 生产环境下的 Service Worker。

  Service Worker in Production.