The core of implementing offline web applications is to cache resources through Service Worker and take over network requests. The specific steps are as follows: 1. Register and activate Service Worker, check supportability and register sw.js file on the page, and preload key resources during the installation stage; 2. Intercept requests and return cached content, prioritize data from the cache, and obtain it online when there is no cache, and can also be processed separately according to resource type; 3. Update cache policy and version management, ensure content updates by changing the cache name and cleaning the old cache during the activation stage; 4. Test whether the offline function is normal, use Chrome DevTools to simulate the offline environment and verify the cache loading effect, and use Lighthouse to check PWA compliance.
Using HTML5 Service Workers to implement offline web applications, the core of this is just one sentence: let the website continue to run even if it is disconnected from the Internet . Service Worker is like a background agent that can cache resources, take over network requests, and even push notifications.
Let’s talk about how to use it to implement offline functions from several practical perspectives.
1. Register and activate Service Worker
To use Service Worker, the first step is to register it in the main page. This step is very simple, just add a piece of code to JavaScript:
if ('serviceWorker' in navigator) {
navigator.serviceWorker.register('/sw.js').then(function(registration) {
console.log('Service Worker registered successfully');
}).catch(function(error) {
console.log('Register failed:', error);
});
} This code will check whether the browser supports Service Worker, and then try to register a file called sw.js This file is your Service Worker script.
After registration, Service Worker will enter the "installation" phase, where you can preload key resources such as HTML, CSS, JS, and images.
self.addEventListener('install', event => {
event.waitUntil(
caches.open('my-cache-v1').then(cache => {
return cache.addAll([
'/',
'/index.html',
'/style.css',
'/app.js'
]);
})
);
});The key to this step is: cache resources that need offline access in advance .
2. Intercept the request and return the cached content
After the installation is complete, Service Worker will be triggered every time the page initiates a request. You can control request behavior here, such as prioritizing data fetching from cache, or getting from server when the network is available.
A common strategy is to "first see if there is a cache, then no networking". The code is roughly like this:
self.addEventListener('fetch', event => {
event.respondWith(
caches.match(event.request).then(response => {
return response || fetch(event.request);
})
);
});You can also control different types of requests more carefully, such as using only cache for static resources, and API requests go to the network:
if (event.request.url.startsWith('https://api.example.com')) {
// Special handling API request} else {
// Ordinary resources are cached network}3. Update cache policy and version management
The cache is not static. When you update the website resources, the old cache must be updated as well. Otherwise, users may keep seeing the old version of content.
The usual practice is to give the cache a new name (such as my-cache-v2 ) and then clean the old cache during the installation phase:
const CACHE_NAME = 'my-cache-v2';
self.addEventListener('activate', event => {
event.waitUntil(
caches.keys().then(keys => {
return Promise.all(
keys.filter(key => key !== CACHE_NAME)
.map(key => caches.delete(key))
);
})
);
});This step is to avoid cache accumulation while ensuring that users can get the latest content.
4. Test whether the offline function is normal
After development, don't forget to test it. Chrome DevTools provides great tools to simulate offline environments:
- Open the Network panel
- Check Offline
- Refresh the page to see if it still loads normally
If the page can still be displayed and the resource is loaded from (ServiceWorker) , it means that the cache is in effect.
In addition, you can also use Lighthouse to check PWA compliance, including offline support.
In general, using Service Worker to implement offline web applications is not complicated, but attention is required to be paid to the design of the cache policy, version update mechanism and actual testing. As long as these details are handled, users can use your website smoothly even if the Internet is disconnected.
Basically that's it.
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