Flipper as PWA

Summary:
^

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

Reviewed By: passy

Differential Revision: D45693382

fbshipit-source-id: 5a2e6c213a7e7e2cf9cd5f3033cff3e5291a2a92

desktop/flipper-server-client/src/FlipperServerClient.tsx

@@ -30,10 +30,10 @@ export type {FlipperServer, FlipperServerCommands, FlipperServerExecOptions};
 export function createFlipperServer(
   host: string,
   port: number,
-  args: string,
+  args: URLSearchParams,
   onStateChange: (state: FlipperServerState) => void,
 ): Promise<FlipperServer> {
-  const socket = new ReconnectingWebSocket(`ws://${host}:${port}${args}`);
+  const socket = new ReconnectingWebSocket(`ws://${host}:${port}?${args}`);
   return createFlipperServerWithSocket(socket as WebSocket, onStateChange);
 }
 

desktop/flipper-server-core/src/utils/certificateUtils.tsx

@@ -16,7 +16,7 @@ import {
 } from './openssl-wrapper-with-promises';
 import path from 'path';
 import tmp, {FileOptions} from 'tmp';
-import {reportPlatformFailures} from 'flipper-common';
+import {FlipperServerConfig, reportPlatformFailures} from 'flipper-common';
 import {isTest} from 'flipper-common';
 import {flipperDataFolder} from './paths';
 import * as jwt from 'jsonwebtoken';
@@ -265,38 +265,68 @@ const writeToTempFile = async (content: string): Promise<string> => {
   return path;
 };
 
-const getStaticFilePath = (filename: string): string => {
-  return path.resolve(getFlipperServerConfig().paths.staticPath, filename);
-};
-
 const tokenFilename = 'auth.token';
-const getTokenPath = (): string => {
+const getTokenPath = (config: FlipperServerConfig): string => {
-  const config = getFlipperServerConfig();
   if (config.environmentInfo.isHeadlessBuild) {
-    return getStaticFilePath(tokenFilename);
+    return path.resolve(config.paths.staticPath, tokenFilename);
   }
 
   return getFilePath(tokenFilename);
 };
+const manifestFilename = 'manifest.json';
+const getManifestPath = (config: FlipperServerConfig): string => {
+  return path.resolve(config.paths.staticPath, manifestFilename);
+};
+
+const exportTokenToManifest = async (
+  config: FlipperServerConfig,
+  token: string,
+) => {
+  const manifestPath = getManifestPath(config);
+  try {
+    const manifestData = await fs.readFile(manifestPath, {
+      encoding: 'utf-8',
+    });
+    const manifest = JSON.parse(manifestData);
+    manifest.token = token;
+
+    const newManifestData = JSON.stringify(manifest, null, 4);
+
+    await fs.writeFile(manifestPath, newManifestData);
+  } catch (e) {
+    console.error(
+      'Unable to export authentication token to manifest, may be non existent.',
+    );
+  }
+};
 
 export const generateAuthToken = async () => {
+  const config = getFlipperServerConfig();
+
   const privateKey = await fs.readFile(serverKey);
   const token = jwt.sign({unixname: os.userInfo().username}, privateKey, {
     algorithm: 'RS256',
     expiresIn: '21 days',
   });
 
-  await fs.writeFile(getTokenPath(), token);
+  await fs.writeFile(getTokenPath(config), token);
+
+  if (config.environmentInfo.isHeadlessBuild) {
+    await exportTokenToManifest(config, token);
+  }
 
   return token;
 };
 
 export const getAuthToken = async () => {
-  if (!(await fs.pathExists(getTokenPath()))) {
+  const config = getFlipperServerConfig();
+  const tokenPath = getTokenPath(config);
+
+  if (!(await fs.pathExists(tokenPath))) {
     return generateAuthToken();
   }
 
-  const token = await fs.readFile(getTokenPath());
+  const token = await fs.readFile(tokenPath);
   return token.toString();
 };
 

desktop/flipper-server/src/findInstallation.tsx

@@ -0,0 +1,32 @@
+/**
+ * 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.
+ *
+ * @format
+ */
+
+import {FlipperServerImpl} from 'flipper-server-core';
+import path from 'path';
+import fs from 'fs-extra';
+import os from 'os';
+
+export async function findInstallation(
+  server: FlipperServerImpl,
+): Promise<string | undefined> {
+  if (server.config.environmentInfo.os.platform !== 'darwin') {
+    return;
+  }
+
+  const appPath = path.join(
+    os.homedir(),
+    'Applications',
+    'Chrome Apps.localized',
+    'Flipper.app',
+  );
+  const appPlistPath = path.join(appPath, 'Contents', 'Info.plist');
+  if (await fs.pathExists(appPlistPath)) {
+    return appPath;
+  }
+}

desktop/flipper-server/src/index.tsx

@@ -26,6 +26,7 @@ import {
 import {isTest} from 'flipper-common';
 import exitHook from 'exit-hook';
 import {getAuthToken} from 'flipper-server-core';
+import {findInstallation} from './findInstallation';
 
 const argv = yargs
   .usage('yarn flipper-server [args]')
@@ -158,6 +159,8 @@ async function start() {
     await attachDevServer(app, server, socket, rootPath);
   }
   await readyForIncomingConnections(flipperServer, companionEnv);
+
+  return flipperServer;
 }
 
 process.on('uncaughtException', (error) => {
@@ -178,19 +181,28 @@ process.on('unhandledRejection', (reason, promise) => {
 });
 
 start()
-  .then(async () => {
+  .then(async (flipperServer) => {
     if (!argv.tcp) {
-      console.log('Flipper server started and listening');
+      console.log('Flipper server started');
       return;
     }
     console.log(
-      'Flipper server started and listening at port ' + chalk.green(argv.port),
+      'Flipper server started and is listening at port ' +
+        chalk.green(argv.port),
     );
     const token = await getAuthToken();
-    const url = `http://localhost:${argv.port}?token=${token}`;
+    const searchParams = new URLSearchParams({token: token ?? ''});
+    const url = new URL(`http://localhost:${argv.port}?${searchParams}`);
     console.log('Go to: ' + chalk.green(chalk.bold(url)));
-    if (argv.open) {
+    if (!argv.open) {
-      open(url);
+      return;
+    }
+
+    if (argv.bundler) {
+      open(url.toString());
+    } else {
+      const path = await findInstallation(flipperServer);
+      open(path ?? url.toString());
     }
   })
   .catch((e) => {

desktop/flipper-ui-browser/src/index.tsx

@@ -11,7 +11,10 @@ import {getLogger, Logger, setLoggerInstance} from 'flipper-common';
 import {initializeRenderHost} from './initializeRenderHost';
 import {createFlipperServer, FlipperServerState} from 'flipper-server-client';
 
-document.getElementById('root')!.innerText = 'flipper-ui-browser started';
+const loadingContainer = document.getElementById('loading');
+if (loadingContainer) {
+  loadingContainer.innerText = 'Loading...';
+}
 
 async function start() {
   // @ts-ignore
@@ -27,20 +30,30 @@ async function start() {
   const logger = createDelegatedLogger();
   setLoggerInstance(logger);
 
+  const params = new URL(location.href).searchParams;
+  let token = params.get('token');
+  if (!token) {
+    const manifestResponse = await fetch('manifest.json');
+    const manifest = await manifestResponse.json();
+    token = manifest.token;
+  }
+
+  const searchParams = new URLSearchParams({token: token ?? ''});
+
   const flipperServer = await createFlipperServer(
     location.hostname,
     parseInt(location.port, 10),
-    location.search,
+    searchParams,
     (state: FlipperServerState) => {
       switch (state) {
         case FlipperServerState.CONNECTING:
-          window.flipperShowError?.('Connecting to flipper-server...');
+          window.flipperShowError?.('Connecting to server...');
           break;
         case FlipperServerState.CONNECTED:
           window?.flipperHideError?.();
           break;
         case FlipperServerState.DISCONNECTED:
-          window?.flipperShowError?.('Lost connection to flipper-server');
+          window?.flipperShowError?.('Lost connection to server');
           break;
       }
     },

desktop/flipper-ui-core/src/chrome/PWAppInstallationWizard.tsx

@@ -0,0 +1,122 @@
+/**
+ * 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.
+ *
+ * @format
+ */
+
+import React from 'react';
+import {Modal, Button} from 'antd';
+import {Layout, _NuxManagerContext} from 'flipper-plugin';
+type Props = {
+  onHide: () => void;
+};
+
+const lastShownTimestampKey = 'flipper-pwa-wizard-last-shown-timestamp';
+export function shouldShowPWAInstallationWizard(): boolean {
+  if (window.matchMedia('(display-mode: standalone)').matches) {
+    return false;
+  }
+
+  let lastShownTimestampFromStorage = undefined;
+  try {
+    lastShownTimestampFromStorage = window.localStorage.getItem(
+      lastShownTimestampKey,
+    );
+  } catch (e) {}
+
+  if (lastShownTimestampFromStorage) {
+    const withinOneDay = (timestamp: number) => {
+      const Day = 1 * 24 * 60 * 60 * 1000;
+      const DayAgo = Date.now() - Day;
+
+      return timestamp > DayAgo;
+    };
+    const lastShownTimestamp = Number(lastShownTimestampFromStorage);
+
+    return !withinOneDay(lastShownTimestamp);
+  }
+
+  const lastShownTimestamp = Date.now();
+  try {
+    window.localStorage.setItem(
+      lastShownTimestampKey,
+      String(lastShownTimestamp),
+    );
+  } catch (e) {}
+
+  return true;
+}
+
+async function install(event: any) {
+  event.prompt();
+
+  (event.userChoice as any)
+    .then((choiceResult: any) => {
+      if (choiceResult.outcome === 'accepted') {
+        console.log('PWA installation, user accepted the prompt.');
+      } else {
+        console.log('PWA installation, user dismissed the prompt.');
+      }
+      (globalThis as any).PWAppInstallationEvent = null;
+    })
+    .catch((e: Error) => {
+      console.error('PWA failed to install with error', e);
+    });
+}
+
+export default function PWAInstallationWizard(props: Props) {
+  const contents = (
+    <Layout.Container gap>
+      <Layout.Container style={{width: '100%', paddingBottom: 15}}>
+        <>
+          Please install Flipper as a PWA. Installed Progressive Web Apps run in
+          a standalone window instead of a browser tab. They're launchable from
+          your home screen, dock, taskbar, or shelf. It's possible to search for
+          and jump between them with the app switcher, making them feel like
+          part of the device they're installed on. New capabilities open up
+          after a web app is installed. Keyboard shortcuts, usually reserved
+          when running in the browser, become available too.
+        </>
+      </Layout.Container>
+    </Layout.Container>
+  );
+
+  const footer = (
+    <>
+      <Button
+        type="ghost"
+        onClick={async () => {
+          props.onHide();
+        }}>
+        Not now
+      </Button>
+      <Button
+        type="primary"
+        onClick={async () => {
+          const installEvent = (globalThis as any).PWAppInstallationEvent;
+          if (installEvent) {
+            await install(installEvent).then(props.onHide);
+          }
+        }}>
+        Install
+      </Button>
+    </>
+  );
+
+  return (
+    <Modal
+      visible
+      centered
+      onCancel={() => {
+        props.onHide();
+      }}
+      width={570}
+      title="Install Flipper to Desktop"
+      footer={footer}>
+      {contents}
+    </Modal>
+  );
+}

desktop/flipper-ui-core/src/sandy-chrome/SandyApp.tsx

@@ -33,6 +33,9 @@ import {showChangelog} from '../chrome/ChangelogSheet';
 import PlatformSelectWizard, {
   hasPlatformWizardBeenDone,
 } from '../chrome/PlatformSelectWizard';
+import PWAInstallationWizard, {
+  shouldShowPWAInstallationWizard,
+} from '../chrome/PWAppInstallationWizard';
 import {getVersionString} from '../utils/versionString';
 import config from '../fb-stubs/config';
 import {WelcomeScreenStaticView} from './WelcomeScreen';
@@ -110,6 +113,11 @@ export function SandyApp() {
       ));
     }
 
+    if (shouldShowPWAInstallationWizard()) {
+      console.info('Attempt to install PWA, launch installation wizard.');
+      Dialog.showModal((onHide) => <PWAInstallationWizard onHide={onHide} />);
+    }
+
     showChangelog(true);
 
     // don't warn about logger, even with a new logger we don't want to re-register

desktop/scripts/build-flipper-server-release.tsx

@@ -224,6 +224,9 @@ async function copyStaticResources(outDir: string, versionNumber: string) {
     'icons.json',
     'index.web.dev.html',
     'index.web.html',
+    'manifest.json',
+    'offline.html',
+    'service-worker.js',
     'style.css',
   ];
   if (isFB) {

desktop/static/index.web.dev.html

@@ -4,9 +4,14 @@
 <head>
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
-  <link rel="stylesheet" href="style.css">
+
   <link rel="icon" href="icon.png">
+  <link rel="apple-touch-icon" href="/icon.png">
+  <link rel="stylesheet" href="style.css">
+  <link rel="manifest" href="manifest.json">
+
   <link id="flipper-theme-import" rel="stylesheet">
+
   <title>Flipper</title>
   <script>
     window.flipperConfig = {
@@ -34,38 +39,32 @@
       text-align: center;
     }
 
-    .__infinity-dev-box-error {
-      background-color: red;
-      font-family: monospace;
-      font-size: 12px;
-      z-index: 10;
-      bottom: 0;
-      left: 0;
-      position: absolute;
-    }
-
   </style>
 </head>
 
 <body>
   <div id="root">
     <div id="loading">
-      Loading...
+      Connecting...
     </div>
   </div>
 
-
-  <div class="__infinity-dev-box __infinity-dev-box-error" hidden>
-
-  </div>
-
   <script>
-      (function () {
+      (async function () {
-        // FIXME: needed to make Metro work
+        // Line below needed to make Metro work. Alternatives could be considered.
         window.global = window;
         let suppressErrors = false;
+        let connected = false;
 
-        const socket = new WebSocket(`ws://${location.host}${location.search}`);
+        const params = new URL(location.href).searchParams;
+        let token = params.get('token');
+        if (!token) {
+          const manifestResponse = await fetch('manifest.json');
+          const manifest = await manifestResponse.json();
+          token = manifest.token;
+        }
+
+        const socket = new WebSocket(`ws://${location.host}?token=${token}`);
         window.devSocket = socket;
 
         socket.addEventListener('message', ({ data: dataRaw }) => {
@@ -89,31 +88,30 @@
           }
         })
 
-        socket.addEventListener('error', () => {
+        socket.addEventListener('error', (e) => {
-          openError('WebSocket -> error');
+          if (!connected) {
-          suppressErrors = true;
+            openError('Socket failed to connect. Is the server running? Have you provided a valid authentication token?');
-        })
+          }
+          else {
+            openError('Socket failed with error.');
+          }
 
+          suppressErrors = true;
+        });
+
+        socket.addEventListener('open', () => {
+          connected = true;
+        })
 
         function openError(text) {
           if (suppressErrors) {
             return;
           }
 
-          const box = document.querySelector('.__infinity-dev-box-error');
+          const box = document.getElementById('loading');
-          box.removeAttribute('hidden');
           box.textContent = text;
-          box.appendChild(closeButton);
         }
         window.flipperShowError = openError;
-        window.flipperHideError = () => {
-          const box = document.querySelector('.__infinity-dev-box-error');
-          box.setAttribute('hidden', true);
-        }
-
-        const closeButton = document.createElement('button');
-        closeButton.addEventListener('click', window.flipperHideError);
-        closeButton.textContent = 'X';
 
         // load correct theme (n.b. this doesn't handle system value specifically, will assume light in such cases)
         try {
@@ -132,11 +130,31 @@
           script.src = window.flipperConfig.entryPoint;
 
           script.onerror = (e) => {
-            openError('Script failure. Check Chrome console for more info.');
+            openError('Failed to load entry point. Check Chrome console for more info.');
           };
 
           document.body.appendChild(script);
         }
+
+        if ('serviceWorker' in navigator) {
+          navigator.serviceWorker
+            .register('/service-worker.js')
+            .then(() => {
+              console.log('Flipper Service Worker has been registered');
+            })
+            .catch((e) => {
+              console.error('Flipper failed to register Service Worker', e);
+            });
+        }
+
+        window.addEventListener('beforeinstallprompt', (e) => {
+          console.log('Flipper PWA before install prompt with event', e);
+          // Prevent Chrome 67 and earlier from automatically showing the prompt.
+          e.preventDefault();
+          // Stash the event so it can be triggered later.
+          global.PWAppInstallationEvent = e;
+        });
+
         init();
       })();
   </script>

desktop/static/index.web.html

@@ -4,9 +4,14 @@
 <head>
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
-  <link rel="stylesheet" href="style.css">
+
   <link rel="icon" href="icon.png">
+  <link rel="apple-touch-icon" href="/icon.png">
+  <link rel="stylesheet" href="style.css">
+  <link rel="manifest" href="manifest.json">
+
   <link id="flipper-theme-import" rel="stylesheet">
+
   <title>Flipper</title>
   <script>
     window.flipperConfig = {
@@ -43,6 +48,7 @@
       left: 0;
       position: absolute;
     }
+
   </style>
 </head>
 
@@ -53,11 +59,7 @@
     </div>
   </div>
 
-
+  <div class="__infinity-dev-box __infinity-dev-box-error" hidden />
-  <div class="__infinity-dev-box __infinity-dev-box-error" hidden>
-
-  </div>
-
   <script>
       (function () {
         // FIXME: needed to make Metro work
@@ -69,11 +71,17 @@
             return;
           }
 
-          const box = document.querySelector('.__infinity-dev-box-error');
+          let box = document.getElementById('loading');
+          if (box) {
+            box.innerText = text;
+          }
+          else {
+            box = document.querySelector('.__infinity-dev-box-error');
             box.removeAttribute('hidden');
-          box.textContent = text;
+            box.innerText = text;
             box.appendChild(closeButton);
           }
+        }
         window.flipperShowError = openError;
         window.flipperHideError = () => {
           const box = document.querySelector('.__infinity-dev-box-error');
@@ -106,6 +114,25 @@
 
           document.body.appendChild(script);
         }
+
+        if ('serviceWorker' in navigator) {
+          navigator.serviceWorker
+            .register('/service-worker.js')
+            .then(() => {
+              console.log('Flipper Service Worker has been registered');
+            })
+            .catch((e) => {
+              console.error('Flipper failed to register Service Worker', e);
+            });
+        }
+
+        window.addEventListener('beforeinstallprompt', (e) => {
+          // Prevent Chrome 67 and earlier from automatically showing the prompt.
+          e.preventDefault();
+          // Stash the event so it can be triggered later.
+          global.PWAppInstallationEvent = e;
+        });
+
         init();
       })();
   </script>

desktop/static/manifest.json

@@ -0,0 +1,18 @@
+{
+    "$schema": "https://json.schemastore.org/web-manifest-combined.json",
+    "name": "Flipper",
+    "description": "Flipper is a platform for debugging iOS, Android and React Native apps",
+    "short_name": "Flipper",
+    "start_url": "index.web.html",
+    "display": "standalone",
+    "background_color": "#fff",
+    "theme_color": "#4267B2",
+    "orientation": "portrait-primary",
+    "icons": [
+      {
+        "src": "/icon.png",
+        "type": "image/png",
+        "sizes": "256x256"
+      }
+    ]
+  }

desktop/static/offline.html

@@ -0,0 +1,133 @@
+<!DOCTYPE html>
+<html lang="en">
+
+<head>
+  <meta charset="utf-8" />
+  <meta http-equiv="X-UA-Compatible" content="IE=edge" />
+  <meta name="viewport" content="width=device-width, initial-scale=1" />
+
+  <title>You are offline</title>
+
+  <!-- Inline the page's stylesheet. -->
+  <style>
+    body {
+      font-family: system-ui;
+      font-size: 13px;
+      cursor: default;
+      overflow: hidden;
+      line-height: 1;
+    }
+
+    #container {
+      -webkit-app-region: drag;
+      z-index: 999999;
+      position: absolute;
+      top: 0;
+      left: 0;
+      bottom: 0;
+      right: 0;
+      padding: 50px;
+      overflow: auto;
+      display: flex;
+      align-items: center;
+      justify-content: center;
+      font-size: 20px;
+      color: #525252;
+      text-align: center;
+    }
+
+    .console {
+      font-family: 'Fira Mono';
+      width: 600px;
+      height: 250px;
+      box-sizing: border-box;
+      margin: auto;
+
+    }
+
+    .console header {
+      border-top-left-radius: 15px;
+      border-top-right-radius: 15px;
+      background-color: #4267B2;
+      height: 45px;
+      line-height: 45px;
+      text-align: center;
+      color: #DDD;
+    }
+
+    .console .consolebody {
+      border-bottom-left-radius: 15px;
+      border-bottom-right-radius: 15px;
+      box-sizing: border-box;
+      padding: 0px 10px;
+      height: calc(100% - 40px);
+      overflow: scroll;
+      background-color: #000;
+      color: white;
+      text-align: left;
+    }
+
+  </style>
+</head>
+
+<body>
+  <div id="container">
+    <div>
+      <b>Oops! It seems Flipper is not running in the background</b>
+
+      <p>
+        Flipper will automatically reload once the connection is re-established.
+        Click the button below to try reloading manually.
+      </p>
+
+      <button type="button">⤾ Reload</button>
+
+      <p>Also, you can manually start Flipper.
+        From the terminal, please run:</p>
+
+      <div class="console">
+        <header>
+          <p>shell</p>
+        </header>
+        <div class="consolebody">
+          <p>> open -a 'Flipper' --args '--server'</p>
+        </div>
+      </div>
+    </div>
+
+  </div>
+
+  <script>
+    document.querySelector('button').addEventListener('click', () => {
+      window.location.reload();
+    });
+
+    // Listen to changes in the network state, reload when online.
+    // This handles the case when the device is completely offline
+    // i.e. no network connection.
+    window.addEventListener('online', () => {
+      window.location.reload();
+    });
+
+    // Check if the server is responding & reload the page if it is.
+    // This handles the case when the device is online, but the server
+    // is offline or misbehaving.
+    async function checkNetworkAndReload() {
+      try {
+        const response = await fetch('.');
+        if (response.status >= 200 && response.status < 500) {
+          window.location.reload();
+          return;
+        }
+      } catch {
+        // Unable to connect to the server, ignore.
+      }
+      window.setTimeout(checkNetworkAndReload, 2500);
+    }
+
+    checkNetworkAndReload();
+
+  </script>
+</body>
+
+</html>

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;
+      }
+    })());
+  }
+});