Method handleFetch

packages/service-worker/worker/src/driver.ts:579–643

(event: FetchEvent)

  }

  private async handleFetch(event: FetchEvent): Promise<Response> {
    try {
      // Ensure the SW instance has been initialized.
      await this.ensureInitialized(event);
    } catch {
      // Since the SW is already committed to responding to the currently active request,
      // respond with a network fetch.
      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);
    // If there's a configured max age, check whether this version is within that age.
    const isVersionWithinMaxAge =
      appVersion?.manifest.applicationMaxAge === undefined ||
      this.adapter.time - appVersion.manifest.timestamp < appVersion.manifest.applicationMaxAge;
    let res: Response | null = null;

    try {
      if (appVersion !== null && isVersionWithinMaxAge) {
        try {
          // Handle the request. First try the AppVersion. If that doesn't work, fall back on the
          // network.
          res = await appVersion.handleFetch(event.request, event);
        } catch (err: any) {
          if (err.isUnrecoverableState) {
            await this.notifyClientsAboutUnrecoverableState(appVersion, err.message);
          }
          if (err.isCritical) {
            // Something went wrong with handling the request from this version.
            this.debugger.log(err, `Driver.handleFetch(version: ${appVersion.manifestHash})`);
            await this.versionFailed(appVersion, err);
            return this.safeFetch(event.request);
          }
          throw err;
        }
      }

      // The response will be `null` only if no `AppVersion` can be assigned to the request or if
      // the assigned `AppVersion`'s manifest doesn't specify what to do about the request.
      // In that case, just fall back on the network.
      if (res === null) {
        return this.safeFetch(event.request);
      }

      // The `AppVersion` returned a usable response, so return it.
      return res;
    } finally {