fix(service-worker): check for updates on navigation

Currently the Service Worker checks for updates only on SW startup,
an event which happens frequently but also nondeterministically. This
makes it hard for developers to observe the update process or reason
about how updates will be delivered to users. This problem is
exacerbated by the DevTools behavior of keeping the SW alive
indefinitely while opened, effectively preventing the page from
updating at all.

This change causes the SW to additionally check for updates on
navigation requests (app page reloads). This creates deterministic
update behavior, and is much easier for developers to reason about.
It does leave the old update-on-SW-startup behavior in place, as
removing that would be a breaking change.

Fixes #20877

aio/content/guide/service-worker-devops.md

@@ -32,12 +32,10 @@ server can ensure that the Angular app always has a consistent set of files.
 
 #### Update checks
 
-Every time the Angular service worker starts, it checks for updates to the 
-app by looking for updates to the `ngsw.json` manifest. 
-
-Note that the service worker starts periodically throughout the usage of 
-the app because the web browser terminates the service worker if the page 
-is idle beyond a given timeout.
+Every time the user opens or refreshes the application, the Angular service worker
+checks for updates to the app by looking for updates to the `ngsw.json` manifest. If
+an update is found, it is downloaded and cached automatically, and will be served
+the next time the application is loaded.
 
 ### Resource integrity
 
@@ -276,8 +274,8 @@ with service workers. Such tools can be powerful when used properly,
 but there are a few things to keep in mind.
 
 * When using developer tools, the service worker is kept running 
-in the background and never restarts. For the Angular service 
-worker, this means that update checks to the app will generally not happen.
+in the background and never restarts. This can cause behavior with Dev
+Tools open to differ from behavior a user might experience.
 
 * If you look in the Cache Storage viewer, the cache is frequently 
 out of date. Right click the Cache Storage title and refresh the caches.

packages/service-worker/worker/src/driver.ts

@@ -88,6 +88,11 @@ export class Driver implements Debuggable, UpdateSource {
 
   private lastUpdateCheck: number|null = null;
 
+  /**
+   * Whether there is a check for updates currently scheduled due to navigation.
+   */
+  private scheduledNavUpdateCheck: boolean = false;
+
   /**
    * A scheduler which manages a queue of tasks that need to be executed when the SW is
    * not doing any other work (not processing any other requests).
@@ -327,6 +332,15 @@ export class Driver implements Debuggable, UpdateSource {
       return this.safeFetch(event.request);
     }
 
+    // On navigation requests, check for new updates.
+    if (event.request.mode === 'navigate' && !this.scheduledNavUpdateCheck) {
+      this.scheduledNavUpdateCheck = true;
+      this.idle.schedule('check-updates-on-navigation', async() => {
+        this.scheduledNavUpdateCheck = false;
+        await this.checkForUpdate();
+      });
+    }
+
     // Decide which version of the app to use to serve this request. This is asynchronous as in
     // some cases, a record will need to be written to disk about the assignment that is made.
     const appVersion = await this.assignVersion(event);

packages/service-worker/worker/test/happy_spec.ts

@@ -337,6 +337,41 @@ export function main() {
       serverUpdate.assertNoOtherRequests();
     });
 
+    async_it('checks for updates on navigation', async() => {
+      expect(await makeRequest(scope, '/foo.txt')).toEqual('this is foo');
+      await driver.initialized;
+      server.clearRequests();
+
+      expect(await makeRequest(scope, '/foo.txt', 'default', {
+        mode: 'navigate',
+      })).toEqual('this is foo');
+
+      scope.advance(12000);
+      await driver.idle.empty;
+
+      server.assertSawRequestFor('ngsw.json');
+    });
+
+    async_it('does not make concurrent checks for updates on navigation', async() => {
+      expect(await makeRequest(scope, '/foo.txt')).toEqual('this is foo');
+      await driver.initialized;
+      server.clearRequests();
+
+      expect(await makeRequest(scope, '/foo.txt', 'default', {
+        mode: 'navigate',
+      })).toEqual('this is foo');
+
+      expect(await makeRequest(scope, '/foo.txt', 'default', {
+        mode: 'navigate',
+      })).toEqual('this is foo');
+
+      scope.advance(12000);
+      await driver.idle.empty;
+
+      server.assertSawRequestFor('ngsw.json');
+      server.assertNoOtherRequests();
+    });
+
     async_it('preserves multiple client assignments across restarts', async() => {
       expect(await makeRequest(scope, '/foo.txt')).toEqual('this is foo');
       await driver.initialized;