Offline Mode

Progressive Web Apps (PWA) are experiences that combine the best of the web with the best of apps. We can enhance our website with service workers to work offline or on low-quality networks.

It is also very easy to use it.

Create serviceWorker

Create a sw.js file in your documents root directory and copy the following code:

sw.js

/* ===========================================================
 * docsify sw.js
 * ===========================================================
 * Copyright 2016 @huxpro
 * Licensed under Apache 2.0
 * Register service worker.
 * ========================================================== */

const RUNTIME = 'docsify'
const HOSTNAME_WHITELIST = [
  self.location.hostname,
  'fonts.gstatic.com',
  'fonts.googleapis.com',
  'cdn.jsdelivr.net'
]

// The Util Function to hack URLs of intercepted requests
const getFixedUrl = (req) => {
  var now = Date.now()
  var 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
  if (url.hostname === self.location.hostname) {
    url.search += (url.search ? '&' : '?') + 'cache-bust=' + now
  }
  return url.href
}

/**
 *  @Lifecycle Activate
 *  New one activated when old isnt being used.
 *
 *  waitUntil(): activating ====> activated
 */
self.addEventListener('activate', event => {
  event.waitUntil(self.clients.claim())
})

/**
 *  @Functional Fetch
 *  All network requests are being intercepted here.
 *
 *  void respondWith(Promise<Response> r)
 */
self.addEventListener('fetch', event => {
  // Skip some of cross-origin requests, like those for Google Analytics.
  if (HOSTNAME_WHITELIST.indexOf(new URL(event.request.url).hostname) > -1) {
    // Stale-while-revalidate
    // 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 fixedUrl = getFixedUrl(event.request)
    const fetched = fetch(fixedUrl, { cache: 'no-store' })
    const fetchedCopy = fetched.then(resp => resp.clone())

    // Call respondWith() with whatever we get first.
    // 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(_ => { /* eat any errors */ })
    )

    // Update the cache with the version we fetched (only for ok status)
    event.waitUntil(
      Promise.all([fetchedCopy, caches.open(RUNTIME)])
        .then(([response, cache]) => response.ok && cache.put(event.request, response))
        .catch(_ => { /* eat any errors */ })
    )
  }
})

Register

Now, register it in your index.html. It only works on some modern browsers, so we need to judge:

index.html

<script>
  if (typeof navigator.serviceWorker !== 'undefined') {
    navigator.serviceWorker.register('sw.js')
  }
</script>

Enjoy it

Release your website and start experiencing magical offline feature. ghost You can turn off Wi-Fi and refresh the current site to experience it.