Flipper as PWA

Summary:
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Reference: https://docs.google.com/document/d/1flQJUzTe4AuQz3QCpvbloQycenHsu7ZxbKScov7K7ao

Reviewed By: passy

Differential Revision: D45693382

fbshipit-source-id: 5a2e6c213a7e7e2cf9cd5f3033cff3e5291a2a92

desktop/static/service-worker.js

@@ -0,0 +1,70 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// OFFLINE_VERSION is used as an update marker so that on subsequent installations
+// the newer version of the file gets updated.
+// eslint-disable-next-line header/header, no-unused-vars
+const OFFLINE_VERSION = 1;
+const CACHE_NAME = 'offline';
+const OFFLINE_URL = 'offline.html';
+
+self.addEventListener('install', (event) => {
+    console.log('Flipper service worker installed');
+
+    event.waitUntil((async () => {
+      const cache = await caches.open(CACHE_NAME);
+      // Setting {cache: 'reload'} in the new request will ensure that the response
+      // isn't fulfilled from the HTTP cache; i.e., it will be from the network.
+      await cache.add(new Request(OFFLINE_URL, {cache: 'reload'}));
+    })());
+    // Force the waiting service worker to become the active service worker.
+    self.skipWaiting();
+});
+
+self.addEventListener('activate', (event) => {
+  console.log('Flipper service worker activate');
+  event.waitUntil((async () => {
+    // Enable navigation preload if it's supported.
+    // See https://developers.google.com/web/updates/2017/02/navigation-preload
+    if ('navigationPreload' in self.registration) {
+      await self.registration.navigationPreload.enable();
+    }
+  })());
+
+  // Tell the active service worker to take control of the page immediately.
+  self.clients.claim();
+});
+
+self.addEventListener('fetch', (event) => {
+  // We only want to call event.respondWith() if this is a navigation request
+  // for an HTML page.
+  if (event.request.mode === 'navigate') {
+    event.respondWith((async () => {
+      try {
+        // First, try to use the navigation preload response if it's supported.
+        const preloadResponse = await event.preloadResponse;
+        if (preloadResponse) {
+          return preloadResponse;
+        }
+
+        // Always try the network first (try flipper server)
+        const networkResponse = await fetch(event.request);
+        return networkResponse;
+      } catch (error) {
+        // Catch is only triggered if an exception is thrown, which is likely
+        // due to a network error.
+        // If fetch() returns a valid HTTP response with a response code in
+        // the 4xx or 5xx range, the catch() will NOT be called.
+        console.log('Fetch failed; returning offline page instead.', error);
+
+        const cache = await caches.open(CACHE_NAME);
+        const cachedResponse = await cache.match(OFFLINE_URL);
+        return cachedResponse;
+      }
+    })());
+  }
+});