Web In Play의 새로운 기능

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self.addEventListener('fetch', event => {
  event.respondWith((async () => {
    try {
      return await fetchAndHandleError(event.request);
    } catch {
      // Failed to load from the network. User is offline or the response
      // has a status code that triggers the Quality Criteria.
      // Try loading from cache.
      const cachedResponse = await caches.match(event.request);
      if (cachedResponse) {
        return cachedResponse;
      }
      // Response was not found on the cache. Send the error / offline
      // page. OFFLINE_PAGE should be pre-cached when the service worker
      // is activated.
      return await caches.match(OFFLINE_PAGE);
    }
  })());
});

async function fetchAndHandleError(request) {
  const cache = await caches.open(RUNTIME_CACHE);
  const response = await fetch(request);

  // Throw an error if the response returns one of the status
  // that trigger the Quality Criteria.
  if (response.status === 404 ||
      response.status >= 500 && response.status < 600) {
    throw new Error(`Server responded with status: ${response.status}`);
  }

  // Cache the response if the request is successful.
  cache.put(request, response.clone());
  return response;
}

export class FallbackOnErrorPlugin {
  constructor(offlineFallbackUrl, notFoundFallbackUrl, serverErrorFallbackUrl) {
    this.notFoundFallbackUrl = notFoundFallbackUrl;
    this.offlineFallbackUrl = offlineFallbackUrl;
    this.serverErrorFallbackUrl = serverErrorFallbackUrl;
  }

  checkTrustedWebActivityCrash(response) {
    if (response.status === 404 || response.status >= 500 && response.status <= 600) {
      const type = response.status === 404 ? 'E_NOT_FOUND' : 'E_SERVER_ERROR';
      const error = new Error(`Invalid response status (${response.status})`);
      error.type = type;
      throw error;
    }
  }

  // This is called whenever there's a network response,
  // but we want special behavior for 404 and 5**.
  fetchDidSucceed({response}) {
    // Cause a crash if this is a Trusted Web Activity crash.
    this.checkTrustedWebActivityCrash(response);

    // If it's a good response, it can be used as-is.
    return response;
  }

  // This callback is new in Workbox v6, and is triggered whenever
  // an error (including a NetworkError) is thrown when a handler runs.
  handlerDidError(details) {
    let fallbackURL;
    switch (details.error.details.error.type) {
      case 'E_NOT_FOUND': fallbackURL = this.notFoundFallbackUrl; break;
      case 'E_SERVER_ERROR': fallbackURL = this.serverErrorFallbackUrl; break;
      default: fallbackURL = this.offlineFallbackUrl;
    }

    return caches.match(fallbackURL, {
      // Use ignoreSearch as a shortcut to work with precached URLs
      // that have _WB_REVISION parameters.
      ignoreSearch: true,
    });
  }
}