[PWA] 19. Cache the avatars

Cache the avatars is little different from cache photos. We need to serve the page with our cache data and also go to the network for fetch avatars in case some user like change their avatars frequently.

self.addEventListener('fetch', function(event) {
  var requestUrl = new URL(event.request.url);

  if (requestUrl.origin === location.origin) {
    if (requestUrl.pathname === '/') {
      event.respondWith(caches.match('/skeleton'));
      return;
    }
    if (requestUrl.pathname.startsWith('/photos/')) {
      event.respondWith(servePhoto(event.request));
      return;
    }
    // TODO: respond to avatar urls by responding with
    // the return value of serveAvatar(event.request)
    if(requestUrl.pathname.startsWith('/avatars/')){
      event.respondWith(serveAvatar(event.request));
      return;
    }
  }

  event.respondWith(
    caches.match(event.request).then(function(response) {
      return response || fetch(event.request);
    })
  );
});

function serveAvatar(request) {
  // Avatar urls look like:
  // avatars/sam-2x.jpg
  // But storageUrl has the -2x.jpg bit missing.
  // Use this url to store & match the image in the cache.
  // This means you only store one copy of each avatar.
  var storageUrl = request.url.replace(/-dx.jpg$/, '');

  // TODO: return images from the "wittr-content-imgs" cache
  // if they're in there. But afterwards, go to the network
  // to update the entry in the cache.
  //
  // Note that this is slightly different to servePhoto!
  return caches.open(contentImgsCache)
      .then(function(cache){
         return cache.match(storageUrl).then(function(response){

             var netFetchResponse = fetch(request).then(function(networkResponse){
                 cache.put(storageUrl ,networkResponse.clone());
                 return networkResponse;
             });

             return response || netFetchResponse;
         })
      })
}