概念
Service Worker 是HTML5 的一个新特性,主要用来做持久的离线缓存。
应用场景
在公司业务中,因为经常要处理性能优化方面的需求,使用传统的性能优化手段,满足了大多数业务场景。但是,如果目标用户手机性能以及网络普遍较差的情况下(例如东南亚、印度等海外市场),瓶颈就在于DNS查询,TCP的建立时间,采用常规的优化手段就显得捉襟见肘。此时,我们项目组有尝试采用离线缓存方案,即将静态资源缓存到本地,通过拦截代理请求,读取本地文件,加快访问速度。
特征
- Service Worker拥有自己独立的 worker 线程,独立于当前网页线程
- 离线缓存静态资源
- 可自定义响应内容
- 必须在HTTPS环境下工作或 localhost / 127.0.0.1 (自身安全机制)
- 无法直接操作DOM
- Service Worker安装(installing)完成后,就会一直存在,除非手动卸载(unregister)
- 通过Promise异步实现
生命周期
service worker的生命周期晚去独立于网页
- 检测浏览器是否支持:首先,检测当前环境是否支持 service worker,可以使用 'serviceWorker' in navigator 进行检测。
- 注册:如果支持,可以使用 navigator.serviceWorker.register('./sw.js'),在当前主线程中注册 service worker。如果注册成功,service worker 则在 ServiceWorkerGlobalScope环境中运行; 需要注意的是: 当前环境无法操作DOM,且和主线程之间相互独立(即线程之间不会相互阻塞)。
- 安装:然后,后台开始安装service worker,一般在此过程中,开始缓存一些静态资源文件。
- 激活:安装成功之后,准备进行激活 service worker,通常在激活状态下,主要进行缓存清理,更新service worker等操作。
- 使用:激活成功后,,service worker 就可以控制当前页面了。需要注意的是,只有在service worker成功激活后,才具有控制页面的能力,一般在第一次访问页面时,service worker第一次创建成功,并没有激活,只有当刷新页面,再次访问之后,才具有控制页面的能力。
- 卸载:
// This optional code is used to register a service worker.
// register() is not called by default.
// This lets the app load faster on subsequent visits in production, and gives
// it offline capabilities. However, it also means that developers (and users)
// will only see deployed updates on subsequent visits to a page, after all the
// existing tabs open on the page have been closed, since previously cached
// resources are updated in the background.
// To learn more about the benefits of this model and instructions on how to
// opt-in, read https://bit.ly/CRA-PWA
const isLocalhost = Boolean(
window.location.hostname === 'localhost' ||
// [::1] is the IPv6 localhost address.
window.location.hostname === '[::1]' ||
// 127.0.0.0/8 are considered localhost for IPv4.
window.location.hostname.match(/^127(?:\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}$/)
)
export function register(config) {
if (process.env.NODE_ENV === 'production' && 'serviceWorker' in navigator) {
// The URL constructor is available in all browsers that support SW.
const publicUrl = new URL(process.env.PUBLIC_URL, window.location.href)
if (publicUrl.origin !== window.location.origin) {
// Our service worker won't work if PUBLIC_URL is on a different origin
// from what our page is served on. This might happen if a CDN is used to
// serve assets; see https://github.com/facebook/create-react-app/issues/2374
return
}
window.addEventListener('load', () => {
const swUrl = `${process.env.PUBLIC_URL}/service-worker.js`
if (isLocalhost) {
// This is running on localhost. Let's check if a service worker still exists or not.
checkValidServiceWorker(swUrl, config)
// Add some additional logging to localhost, pointing developers to the
// service worker/PWA documentation.
navigator.serviceWorker.ready.then(() => {
// console.log('This web app is being served cache-first by a service ' + 'worker. To learn more, visit https://bit.ly/CRA-PWA')
})
} else {
// Is not localhost. Just register service worker
registerValidSW(swUrl, config)
}
})
}
}
function registerValidSW(swUrl, config) {
navigator.serviceWorker
.register(swUrl)
.then(registration => {
registration.onupdatefound = () => {
const installingWorker = registration.installing
if (installingWorker == null) {
return
}
installingWorker.onstatechange = () => {
if (installingWorker.state === 'installed') {
if (navigator.serviceWorker.controller) {
// At this point, the updated precached content has been fetched,
// but the previous service worker will still serve the older
// content until all client tabs are closed.
// console.log('New content is available and will be used when all ' + 'tabs for this page are closed. See https://bit.ly/CRA-PWA.')
// Execute callback
if (config && config.onUpdate) {
config.onUpdate(registration)
}
} else {
// At this point, everything has been precached.
// It's the perfect time to display a
// "Content is cached for offline use." message.
console.log('Content is cached for offline use.')
// Execute callback
if (config && config.onSuccess) {
config.onSuccess(registration)
}
}
}
}
}
})
.catch(error => {
console.error('Error during service worker registration:', error)
})
}
function checkValidServiceWorker(swUrl, config) {
// Check if the service worker can be found. If it can't reload the page.
fetch(swUrl, {
headers: { 'Service-Worker': 'script' }
})
.then(response => {
// Ensure service worker exists, and that we really are getting a JS file.
const contentType = response.headers.get('content-type')
if (response.status === 404 || (contentType != null && contentType.indexOf('javascript') === -1)) {
// No service worker found. Probably a different app. Reload the page.
navigator.serviceWorker.ready.then(registration => {
registration.unregister().then(() => {
window.location.reload()
})
})
} else {
// Service worker found. Proceed as normal.
registerValidSW(swUrl, config)
}
})
.catch(() => {
console.log('No internet connection found. App is running in offline mode.')
})
}
export function unregister() {
if ('serviceWorker' in navigator) {
navigator.serviceWorker.ready.then(registration => {
registration.unregister()
})
}
}