Pete LePage
Thomas Steiner
Google 助理应用、Slack 应用、Zoom 应用以及您手机或计算机上的几乎所有其他平台专用应用有什么共同之处？没错，他们至少会给您一些东西。 即使没有网络连接，您仍然可以打开 Google 助理应用、进入 Slack 或启动 Zoom。您可能不会获得任何特别有意义的东西，甚至可能无法实现预期目标，但至少您会得到一些结果，并且应用会掌控一切。
相比之下，在传统的 Web 上，当您离线时，系统不会向您显示任何内容。Chrome 会提供离线小恐龙游戏，但仅此而已。
带有自定义 Service Worker 的离线后备页面
不过，情况并非一定如此。借助服务工作器和 Cache Storage API，您可以为用户提供量身定制的离线体验。这可以是一个简单的品牌页面，其中包含用户当前处于离线状态的信息，但也可以是更具创意的解决方案，例如，著名的 trivago 离线迷宫游戏，其中包含手动重新连接按钮和自动重新连接尝试倒计时。
注册 Service Worker
实现此目的的方法是通过 Service Worker。您可以从主页面注册服务工件，如下方代码示例所示。通常，您可以在应用加载完毕后执行此操作。
window.addEventListener("load", () => {
if ("serviceWorker" in navigator) {
navigator.serviceWorker.register("service-worker.js");
}
});
Service Worker 代码
乍一看，实际的 Service Worker 文件的内容可能有点复杂，但以下示例中的注释应该能解决问题。核心思想是预缓存一个名为 offline.html 的文件，该文件仅在失败的导航请求中提供，并让浏览器处理所有其他情况：
/*
Copyright 2015, 2019, 2020, 2021 Google LLC. All Rights Reserved.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/
// Incrementing OFFLINE_VERSION will kick off the install event and force
// previously cached resources to be updated from the network.
// This variable is intentionally declared and unused.
// Add a comment for your linter if you want:
// eslint-disable-next-line no-unused-vars
const OFFLINE_VERSION = 1;
const CACHE_NAME = "offline";
// Customize this with a different URL if needed.
const OFFLINE_URL = "offline.html";
self.addEventListener("install", (event) => {
event.waitUntil(
(async () => {
const cache = await caches.open(CACHE_NAME);
// Setting {cache: 'reload'} in the new request ensures 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) => {
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) => {
// Only 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.
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;
}
})()
);
}
// If our if() condition is false, then this fetch handler won't
// intercept the request. If there are any other fetch handlers
// registered, they will get a chance to call event.respondWith().
// If no fetch handlers call event.respondWith(), the request
// will be handled by the browser as if there were no service
// worker involvement.
});
离线后备网页
您可以在 offline.html 文件中发挥创意，根据自己的需求进行调整，并添加品牌信息。以下示例展示了最基本的可用功能。
该示例演示了基于按钮按下的手动重新加载，以及基于 online 事件和定期服务器轮询的自动重新加载。
<!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: helvetica, arial, sans-serif;
margin: 2em;
}
h1 {
font-style: italic;
color: #373fff;
}
p {
margin-block: 1rem;
}
button {
display: block;
}
</style>
</head>
<body>
<h1>You are offline</h1>
<p>Click the button below to try reloading.</p>
<button type="button">⤾ Reload</button>
<!-- Inline the page's JavaScript file. -->
<script>
// Manual reload feature.
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.
window.addEventListener('online', () => {
window.location.reload();
});
// Check if the server is responding and 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('.');
// Verify we get a valid response from the server
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>
演示
您可以在下方嵌入的演示版中查看线下后备页面的实际运作情况。如果您有兴趣，可以探索 Glitch 上的源代码。
关于使应用可安装的注意事项
现在，您的网站已设置离线后备页面，您可能想知道后续步骤。如需使您的应用可安装，您需要添加 Web 应用清单，并选择性地制定安装策略。
关于使用 Workbox.js 提供离线后备页面的附注
您可能听说过 Workbox。Workbox 是一组 JavaScript 库，用于为 Web 应用添加离线支持。如果您希望自己编写的服务工件代码更少，可以使用适用于仅限离线页面的 Workbox 方案。
接下来，了解如何为您的应用定义安装策略。