Pete LePage
Thomas Steiner
Google 助理應用程式、Slack 應用程式、Zoom 應用程式，以及手機或電腦上的幾乎任何其他平台專屬應用程式有什麼共通點？沒錯，這些要求至少都會為你提供「東西」。即使沒有網路連線，您還是可以開啟 Google 助理應用程式、進入 Slack 或啟動 Zoom。您可能無法獲得特別有意義的任何體驗，甚至無法達成您想要的目標，但至少能取得一些東西，應用程式就能全權掌控。
相較之下，在網頁版上，離線時不會顯示任何內容。當然是 Chrome 提供的離線恐龍遊戲，
含有自訂 Service Worker 的離線備用頁面
但不一定要以這種方式提交。透過服務工作站和 Cache Storage API，您可以為使用者提供自訂的離線體驗。這可以是簡單的品牌頁面，其中顯示使用者目前離線的資訊，但也可以是更有創意的解決方案，例如知名的Trivago 離線迷宮遊戲 (設有手動「重新連線」按鈕和自動重新連線倒數計時)。
註冊 Service Worker
實現上述目標的方法是透過 Service Worker。您可以從主頁面註冊 Service Worker，如下列程式碼範例所示。通常在應用程式載入後執行此操作。
window.addEventListener("load", () => {
if ("serviceWorker" in navigator) {
navigator.serviceWorker.register("service-worker.js");
}
});
Service Worker 程式碼
實際 Service Worker 檔案的內容看似有點複雜，但在以下範例中的註解應該可以清除。核心概念是預先快取名為 offline.html 的檔案，這個檔案只會在「失敗」的導覽要求時提供，並讓瀏覽器處理所有其他情況：
/*
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.
});
離線備用頁面
offline.html 檔案可讓您盡情揮灑創意，配合您的需求調整，並新增您的品牌。以下範例說明可能的最小值。這示範了根據按下按鈕而手動重新載入，以及根據 online 事件和一般伺服器輪詢自動重新載入的情況。
<!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>
操作示範
您可以在下方內嵌的示範中查看離線備用頁面的實際運作情形。如果您有興趣，可以在 Glitch 中探索原始碼。
說明如何安裝應用程式的附註
現在您的網站有離線備用網頁，建議您瞭解後續步驟。如要讓您的應用程式可供安裝，您必須新增網頁應用程式資訊清單，並視需要提供安裝策略。
使用 Workbox.js 放送離線備用頁面的附註
您可能聽過 Workbox。 Workbox 是一組 JavaScript 程式庫，可在網頁應用程式中新增離線支援。如果您希望自行編寫少許 Service Worker 程式碼，可以將 Workbox 方案用於僅限離線頁面。
接下來，請參閱這篇文章，瞭解如何定義應用程式的安裝策略。