Service Workers - more support

[s-cork]

close #96, close #97, close #98

Managed to find some time last night to look at this

Have a fun working prototype here: https://anvil.works/build#clone:6KH7IIUS22JMNW54=664RQKGADXDEV6KC4XPXNMPR

Publish an enviorenment You'll need to change the env.ENDPOINT value You can install the app as PWA then upload some files, add them to IDB, sync them then repeat but go off-line close the app come on line wait a bit open the app - either the sync event will fire when you open the app, or it will have already fired while the app was off!

API Changes:

• add_listener -> subscribe, remove_listener -> unsubscribe
• @self.sync_event_handler - can be used as a decorator for a sync/periodicsync handler It's similar, but not quite the same as:

  
  def sync_event_handler(fn):
  def wrapper(e):
      # not the real implementation
      e.waitUntil(fn(e))
  return wrapper

self.sync_event_handler = sync_event_handler

@self.sync_event_handler def onsync(e):

do a bunch of things when a sync event fires

...

self.onsync = onsync

This Browser API means that if the sync event handler fails the syncManager will re-register this tag 👍 

[s-cork]

@rhurlbatt - you might want to check out the clone in this PR which uses the features added here Some instructions above You might also want to try opening the app in multiple tabs