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+/*
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+Copyright 2015, 2019, 2020 Google LLC. All Rights Reserved.
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+ Licensed under the Apache License, Version 2.0 (the "License");
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+ you may not use this file except in compliance with the License.
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+ You may obtain a copy of the License at
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+ http://www.apache.org/licenses/LICENSE-2.0
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+ Unless required by applicable law or agreed to in writing, software
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+ distributed under the License is distributed on an "AS IS" BASIS,
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+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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+ See the License for the specific language governing permissions and
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+ limitations under the License.
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+*/
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+
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+const CACHE_NAME = 'offline';
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+
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+// Customize this with a different URL if needed.
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+const OFFLINE_URL = 'static/offline.html';
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+
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+self.addEventListener('install', event => {
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+ event.waitUntil(
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+ (async () => {
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+ const cache = await caches.open(CACHE_NAME);
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+
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+
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+ // Setting {cache: 'reload'} in the new request will ensure that the
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+ // response isn't fulfilled from the HTTP cache; i.e., it will be from
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+ // the network.
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+ await cache.add(new Request(OFFLINE_URL, { cache: 'reload' }));
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+ })()
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+ );
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+
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+ // Force the waiting service worker to become the active service worker.
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+ self.skipWaiting();
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+});
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+
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+self.addEventListener('activate', event => {
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+ event.waitUntil(
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+ (async () => {
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+ // Enable navigation preload if it's supported.
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+ // See https://developers.google.com/web/updates/2017/02/navigation-preload
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+ if ('navigationPreload' in self.registration) {
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+ await self.registration.navigationPreload.enable();
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+ }
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+ })()
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+ );
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+
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+ // Tell the active service worker to take control of the page immediately.
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+ self.clients.claim();
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+});
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+
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+self.addEventListener('fetch', event => {
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+ // We only want to call event.respondWith() if this is a navigation request
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+ // for an HTML page.
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+ if (event.request.mode === 'navigate') {
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+ event.respondWith((async () => {
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+ try {
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+ // First, try to use the navigation preload response if it's supported.
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+ const preloadResponse = await event.preloadResponse;
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+
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+ if (preloadResponse) {
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+ return preloadResponse;
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+ }
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+
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+ // Always try the network first.
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+ const networkResponse = await fetch(event.request);
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+
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+ return networkResponse;
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+ } catch (error) {
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+ // catch is only triggered if an exception is thrown, which is likely
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+ // due to a network error.
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+ // If fetch() returns a valid HTTP response with a response code in
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+ // the 4xx or 5xx range, the catch() will NOT be called.
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+ console.log('Fetch failed; returning offline page instead.', error);
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+
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+ const cache = await caches.open(CACHE_NAME);
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+ const cachedResponse = await cache.match(OFFLINE_URL);
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+
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+ return cachedResponse;
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+ }
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+ })());
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+ }
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+
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+ // If our if() condition is false, then this fetch handler won't intercept the
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+ // request. If there are any other fetch handlers registered, they will get a
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+ // chance to call event.respondWith(). If no fetch handlers call
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+ // event.respondWith(), the request will be handled by the browser as if there
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+ // were no service worker involvement.
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+});
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Tudor-Ovidiu Avram
4 years ago
