Service Worker Sample: Cache Video Sample

Available in Chrome 52+ | View on GitHub | Browse Samples

Background

This sample demonstrates basic service worker registration, in conjunction with pre-fetching a video and its poster image.

Visit chrome://inspect/#service-workers and click on the "inspect" link below the registered service worker to view logging statements for the various actions the service-worker.js script is performing. Since most of the action takes place during the service worker's install handler, and that is only executed the very first time you visit this page (unless the service worker script changes), it can be helpful to inspect the logging in a Chrome Incognito window.

To try out caching, disconnect from your network then reload this page.

index.js

// Helper function which returns a promise which resolves once the service worker registration
// is past the "installing" state.
function waitUntilInstalled(registration) {
  return new Promise(function(resolve, reject) {
    if (registration.installing) {
      // If the current registration represents the "installing" service worker, then wait
      // until the installation step (during which the resources are pre-fetched) completes
      // to display the file list.
      registration.installing.addEventListener('statechange', function(e) {
        if (e.target.state === 'installed') {
          resolve();
        } else if (e.target.state === 'redundant') {
          reject();
        }
      });
    } else {
      // Otherwise, if this isn't the "installing" service worker, then installation must have been
      // completed during a previous visit to this page, and the resources are already pre-fetched.
      // So we can show the list of files right away.
      resolve();
    }
  });
}

if ('serviceWorker' in navigator) {
  navigator.serviceWorker.register('./service-worker.js', {
    scope: './'
  })
    .then(waitUntilInstalled)
    // .then(showFilesList)
    .catch(function(error) {
      // Something went wrong during registration. The service-worker.js file
      // might be unavailable or contain a syntax error.
      document.querySelector('#status').textContent = error;
    });
} else {
  // The current browser doesn't support service workers.
  var aElement = document.createElement('a');
  aElement.href =
    'http://www.chromium.org/blink/serviceworker/service-worker-faq';
  aElement.textContent =
    'Service workers are not supported in the current browser.';
  document.querySelector('#status').appendChild(aElement);
}

document.querySelector('video').onloadedmetadata = function() {
  var fileName = this.currentSrc.replace(/^.*[\\\/]/, '');
  document.querySelector('#currentSrc').textContent = 'Video src: ' + fileName;
};

service-worker.js

/*
 Copyright 2014 Google Inc. All Rights Reserved.
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at
 http://www.apache.org/licenses/LICENSE-2.0
 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 */

// While overkill for this specific sample in which there is only one cache,
// this is one best practice that can be followed in general to keep track of
// multiple caches used by a given service worker, and keep them all versioned.
// It maps a shorthand identifier for a cache to a specific, versioned cache name.

// Note that since global state is discarded in between service worker restarts, these
// variables will be reinitialized each time the service worker handles an event, and you
// should not attempt to change their values inside an event handler. (Treat them as constants.)

// If at any point you want to force pages that use this service worker to start using a fresh
// cache, then increment the CACHE_VERSION value. It will kick off the service worker update
// flow and the old cache(s) will be purged as part of the activate event handler when the
// updated service worker is activated.
var CACHE_VERSION = 2;
var CURRENT_CACHES = {
  prefetch: 'prefetch-cache-v' + CACHE_VERSION
};

self.addEventListener('install', function(event) {
  var urlsToPrefetch = [
    './',
    'index.js',
    '../../styles/main.css',
    'static/poster.jpg',
    // The videos are stored remotely with CORS enabled.
    'https://prefetch-video-sample.storage.googleapis.com/gbike.webm',
    'https://prefetch-video-sample.storage.googleapis.com/gbike.mp4'
  ];

  // All of these logging statements should be visible via the "Inspect" interface
  // for the relevant SW accessed via chrome://serviceworker-internals
  console.log('Handling install event. Resources to prefetch:', urlsToPrefetch);

  self.skipWaiting();

  event.waitUntil(
    caches.open(CURRENT_CACHES.prefetch).then(function(cache) {
      return cache.addAll(urlsToPrefetch);
    })
  );
});

self.addEventListener('activate', function(event) {
  // Delete all caches that aren't named in CURRENT_CACHES.
  // While there is only one cache in this example, the same logic will handle the case where
  // there are multiple versioned caches.
  var expectedCacheNames = Object.keys(CURRENT_CACHES).map(function(key) {
    return CURRENT_CACHES[key];
  });

  event.waitUntil(
    caches.keys().then(function(cacheNames) {
      return Promise.all(
        cacheNames.map(function(cacheName) {
          if (expectedCacheNames.indexOf(cacheName) === -1) {
            // If this cache name isn't present in the array of "expected" cache names, then delete it.
            console.log('Deleting out of date cache:', cacheName);
            return caches.delete(cacheName);
          }
        })
        );
    })
    );
});

self.addEventListener('fetch', function(event) {
  console.log('Handling fetch event for', event.request.url);

  if (event.request.headers.get('range')) {
    var pos =
    Number(/^bytes\=(\d+)\-$/g.exec(event.request.headers.get('range'))[1]);
    console.log('Range request for', event.request.url,
      ', starting position:', pos);
    event.respondWith(
      caches.open(CURRENT_CACHES.prefetch)
      .then(function(cache) {
        return cache.match(event.request.url);
      }).then(function(res) {
        if (!res) {
          return fetch(event.request)
          .then(res => {
            return res.arrayBuffer();
          });
        }
        return res.arrayBuffer();
      }).then(function(ab) {
        return new Response(
          ab.slice(pos),
          {
            status: 206,
            statusText: 'Partial Content',
            headers: [
              // ['Content-Type', 'video/webm'],
              ['Content-Range', 'bytes ' + pos + '-' +
                (ab.byteLength - 1) + '/' + ab.byteLength]]
          });
      }));
  } else {
    console.log('Non-range request for', event.request.url);
    event.respondWith(
    // caches.match() will look for a cache entry in all of the caches available to the service worker.
    // It's an alternative to first opening a specific named cache and then matching on that.
    caches.match(event.request).then(function(response) {
      if (response) {
        console.log('Found response in cache:', response);
        return response;
      }
      console.log('No response found in cache. About to fetch from network...');
      // event.request will always have the proper mode set ('cors, 'no-cors', etc.) so we don't
      // have to hardcode 'no-cors' like we do when fetch()ing in the install handler.
      return fetch(event.request).then(function(response) {
        console.log('Response from network is:', response);

        return response;
      }).catch(function(error) {
        // This catch() will handle exceptions thrown from the fetch() operation.
        // Note that a HTTP error response (e.g. 404) will NOT trigger an exception.
        // It will return a normal response object that has the appropriate error code set.
        console.error('Fetching failed:', error);

        throw error;
      });
    })
    );
  }
});