text: /* ===========================================================
   * sw.js
   * ===========================================================
   * Copyright 2016 @huxpro
   * Licensed under Apache 2.0
   * service worker scripting
   * ========================================================== */
  
  // CACHE_NAMESPACE
  // CacheStorage is shared between all sites under same domain.
  // A namespace can prevent potential name conflicts and mis-deletion.
  const CACHE_NAMESPACE = 'main-'
  
  const CACHE = CACHE_NAMESPACE + 'precache-then-runtime';
  const PRECACHE_LIST = [
    "./",
    "./offline.html",
    "./js/jquery.min.js",
    "./js/bootstrap.min.js",
    "./js/hux-blog.min.js",
    "./js/snackbar.js",
    "./img/icon_wechat.png",
    "./img/avatar_desheng_sexy.jpg",
    "./img/ElCapitan.jpg",
    "./img/404-bg.jpg",
    "./css/hux-blog.min.css",
    "./css/syntax.css",
    "./css/bootstrap.min.css"
    // "//cdnjs.cloudflare.com/ajax/libs/font-awesome/4.6.3/css/font-awesome.min.css",
    // "//cdnjs.cloudflare.com/ajax/libs/font-awesome/4.6.3/fonts/fontawesome-webfont.woff2?v=4.6.3",
    // "//cdnjs.cloudflare.com/ajax/libs/fastclick/1.0.6/fastclick.min.js"
  ]
  const HOSTNAME_WHITELIST = [
    self.location.hostname,
    "huangxuan.me",
    "yanshuo.io",
    "cdnjs.cloudflare.com"
  ]
  const DEPRECATED_CACHES = ['precache-v1', 'runtime', 'main-precache-v1', 'main-runtime']
  
  
  // The Util Function to hack URLs of intercepted requests
  const getCacheBustingUrl = (req) => {
    var now = Date.now();
    url = new URL(req.url)
  
    // 1. fixed http URL
    // Just keep syncing with location.protocol
    // fetch(httpURL) belongs to active mixed content.
    // And fetch(httpRequest) is not supported yet.
    url.protocol = self.location.protocol
  
    // 2. add query for caching-busting.
    // Github Pages served with Cache-Control: max-age=600
    // max-age on mutable content is error-prone, with SW life of bugs can even extend.
    // Until cache mode of Fetch API landed, we have to workaround cache-busting with query string.
    // Cache-Control-Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=453190
    url.search += (url.search ? '&' : '?') + 'cache-bust=' + now;
    return url.href
  }
  
  // The Util Function to detect and polyfill req.mode="navigate"
  // request.mode of 'navigate' is unfortunately not supported in Chrome
  // versions older than 49, so we need to include a less precise fallback,
  // which checks for a GET request with an Accept: text/html header.
  const isNavigationReq = (req) => (req.mode === 'navigate' || (req.method === 'GET' && req.headers.get('accept').includes('text/html')))
  
  // The Util Function to detect if a req is end with extension
  // Accordin to Fetch API spec <https://fetch.spec.whatwg.org/#concept-request-destination>
  // Any HTML's navigation has consistently mode="navigate" type="" and destination="document"
  // including requesting an img (or any static resources) from URL Bar directly.
  // So It ends up with that regExp is still the king of URL routing ;)
  // P.S. An url.pathname has no '.' can not indicate it ends with extension (e.g. /api/version/1.2/)
  const endWithExtension = (req) => Boolean(new URL(req.url).pathname.match(/\.\w+$/))
  
  // Redirect in SW manually fixed github pages arbitray 404s on things?blah
  // what we want:
  //    repo?blah -> !(gh 404) -> sw 302 -> repo/?blah
  //    .ext?blah -> !(sw 302 -> .ext/?blah -> gh 404) -> .ext?blah
  // If It's a navigation req and it's url.pathname isn't end with '/' or '.ext'
  // it should be a dir/repo request and need to be fixed (a.k.a be redirected)
  // Tracking https://twitter.com/Huxpro/status/798816417097224193
  const shouldRedirect = (req) => (isNavigationReq(req) && new URL(req.url).pathname.substr(-1) !== "/" && !endWithExtension(req))
  
  // The Util Function to get redirect URL
  // `${url}/` would mis-add "/" in the end of query, so we use URL object.
  // P.P.S. Always trust url.pathname instead of the whole url string.
  const getRedirectUrl = (req) => {
    url = new URL(req.url)
    url.pathname += "/"
    return url.href
  }
  
  
  /**
   *  @Lifecycle Install
   *  Precache anything static to this version of your app.
   *  e.g. App Shell, 404, JS/CSS dependencies...
   *
   *  waitUntil() : installing ====> installed
   *  skipWaiting() : waiting(installed) ====> activating
   */
  self.addEventListener('install', e => {
    e.waitUntil(
      caches.open(CACHE).then(cache => {
        return cache.addAll(PRECACHE_LIST)
          .then(self.skipWaiting())
          .catch(err => console.log(err))
      })
    )
  });
  
  
  /**
   *  @Lifecycle Activate
   *  New one activated when old isnt being used.
   *
   *  waitUntil(): activating ====> activated
   */
  self.addEventListener('activate', event => {
    // delete old deprecated caches.
    caches.keys().then(cacheNames => Promise.all(
      cacheNames
        .filter(cacheName => DEPRECATED_CACHES.includes(cacheName))
        .map(cacheName => caches.delete(cacheName))
    ))
    console.log('service worker activated.')
    event.waitUntil(self.clients.claim());
  });
  
  
  var fetchHelper = {
  
    fetchThenCache: function(request){
      // Requests with mode "no-cors" can result in Opaque Response,
      // Requests to Allow-Control-Cross-Origin: * can't include credentials.
      const init = { mode: "cors", credentials: "omit" } 
  
      const fetched = fetch(request, init)
      const fetchedCopy = fetched.then(resp => resp.clone());
  
      // NOTE: Opaque Responses have no hedaders so [[ok]] make no sense to them
      //       so Opaque Resp will not be cached in this case.
      Promise.all([fetchedCopy, caches.open(CACHE)])
        .then(([response, cache]) => response.ok && cache.put(request, response))
        .catch(_ => {/* eat any errors */})
      
      return fetched;
    },
  
    cacheFirst: function(url){
      return caches.match(url) 
        .then(resp => resp || this.fetchThenCache(url))
        .catch(_ => {/* eat any errors */})
    }
  }
  
  
  /**
   *  @Functional Fetch
   *  All network requests are being intercepted here.
   *
   *  void respondWith(Promise<Response> r);
   */
  self.addEventListener('fetch', event => {
    // logs for debugging
    //console.log(`fetch ${event.request.url}`)
    //console.log(` - type: ${event.request.type}; destination: ${event.request.destination}`)
    //console.log(` - mode: ${event.request.mode}, accept: ${event.request.headers.get('accept')}`)
  
    // Skip some of cross-origin requests, like those for Google Analytics.
    if (HOSTNAME_WHITELIST.indexOf(new URL(event.request.url).hostname) > -1) {
  
      // Redirect in SW manually fixed github pages 404s on repo?blah
      if (shouldRedirect(event.request)) {
        event.respondWith(Response.redirect(getRedirectUrl(event.request)))
        return;
      }
  
      // Cache-only Startgies for ys.static resources
      if (event.request.url.indexOf('ys.static') > -1){
        event.respondWith(fetchHelper.cacheFirst(event.request.url))
        return;
      }
  
      // Stale-while-revalidate for possiblily dynamic content
      // similar to HTTP's stale-while-revalidate: https://www.mnot.net/blog/2007/12/12/stale
      // Upgrade from Jake's to Surma's: https://gist.github.com/surma/eb441223daaedf880801ad80006389f1
      const cached = caches.match(event.request);
      const fetched = fetch(getCacheBustingUrl(event.request), { cache: "no-store" });
      const fetchedCopy = fetched.then(resp => resp.clone());
      
      // Call respondWith() with whatever we get first.
      // Promise.race() resolves with first one settled (even rejected)
      // If the fetch fails (e.g disconnected), wait for the cache.
      // If there’s nothing in cache, wait for the fetch.
      // If neither yields a response, return offline pages.
      event.respondWith(
        Promise.race([fetched.catch(_ => cached), cached])
          .then(resp => resp || fetched)
          .catch(_ => caches.match('offline.html'))
      );
  
      // Update the cache with the version we fetched (only for ok status)
      event.waitUntil(
        Promise.all([fetchedCopy, caches.open(CACHE)])
          .then(([response, cache]) => response.ok && cache.put(event.request, response))
          .catch(_ => {/* eat any errors */ })
      );
  
      // If one request is a HTML naviagtion, checking update!
      if (isNavigationReq(event.request)) {
        // you need "preserve logs" to see this log
        // cuz it happened before navigating
        console.log(`fetch ${event.request.url}`)
        event.waitUntil(revalidateContent(cached, fetchedCopy))
      }
    }
  });
  
  
  /**
   * Broadcasting all clients with MessageChannel API
   */
  function sendMessageToAllClients(msg) {
    self.clients.matchAll().then(clients => {
      clients.forEach(client => {
        console.log(client);
        client.postMessage(msg)
      })
    })
  }
  
  /**
   * Broadcasting all clients async
   */
  function sendMessageToClientsAsync(msg) {
    // waiting for new client alive with "async" setTimeout hacking
    // https://twitter.com/Huxpro/status/799265578443751424
    // https://jakearchibald.com/2016/service-worker-meeting-notes/#fetch-event-clients
    setTimeout(() => {
      sendMessageToAllClients(msg)
    }, 1000)
  }
  
  /**
   * if content modified, we can notify clients to refresh
   * TODO: Gh-pages rebuild everything in each release. should find a workaround (e.g. ETag with cloudflare)
   * 
   * @param  {Promise<response>} cachedResp  [description]
   * @param  {Promise<response>} fetchedResp [description]
   * @return {Promise}
   */
  function revalidateContent(cachedResp, fetchedResp) {
    // revalidate when both promise resolved
    return Promise.all([cachedResp, fetchedResp])
      .then(([cached, fetched]) => {
        const cachedVer = cached.headers.get('last-modified')
        const fetchedVer = fetched.headers.get('last-modified')
        console.log(`"${cachedVer}" vs. "${fetchedVer}"`);
        if (cachedVer !== fetchedVer) {
          sendMessageToClientsAsync({
            'command': 'UPDATE_FOUND',
            'url': fetched.url
          })
        }
      })
      .catch(err => console.log(err))
  }