Service workers

Up till now, I have mostly ignored the activate event. This is a mistake. The activate event signals that the service worker code has been updated, and so I might want to invalidate the cache and repopulate it.

Other important notes

• The page that calls register doesn't get the service worker! The page has to load through the service worker before it will use the service worker.
• A new version of the service worker is installed if/when the fetched version is different
  • This implies that we should be careful loading the sw from sw cache
• The new version doesn't start taking events until all pages using the old worker have closed
• Refreshing the page doesn't count as closing! You must navigate away or close the tab. (Irritating from a debugging point of view)
• force-reload (shift-reload?) bypasses the service worker

It looks like we're expected to use a different cache name with each different version of the sw script, and delete old caches from the activate event.

Todo: Work out how to programmatically set the cache version, ideally using either the hash of the sw code, or the slug of the timestamp.