ServiceWorker

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

1. Service Worker拥有自己独立的 worker 线程，独立于当前网页线程
2. 离线缓存静态资源
3. 可自定义响应内容
4. 必须在HTTPS环境下工作或 localhost / 127.0.0.1 （自身安全机制）
5. 无法直接操作DOM
6. Service Worker安装(installing)完成后，就会一直存在，除非手动卸载(unregister)
7. 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()
    })
  }
}