Trigger asset caching from boot

[bgrgicak]

Motivation for the change, related issues

We would like for Playground boot to control all aspects of the boot process including offline asset caching.

To do this we need a way to communicate between bootPlaygroundRemote and the service worker.

Implementation details

worker-thread.ts can now trigger cacheOfflineModeAssets which allows bootPlaygroundRemote to control offline asset caching.

cacheOfflineModeAssets now uses fetch instead of calling cache.addAll directly.
Because we call cacheOfflineModeAssets from the end of the boot process, caching is already working and all requests are cached.

If any of the files were already cached, they would be returned by cache instead of making a network request.

Testing Instructions (or ideally a Blueprint)

• Follow testing instructions from Cache Playground assets to enable offline support #1535

[bgrgicak]

@adamziel @brandonpayton I'm exploring how to trigger cache.cacheOfflineModeAssets which is available in service-worker.ts from bootPlaygroundRemote.

What do you think about this approach? Do you have any other suggestions on how to do it?

Context:

• bootPlaygroundRemote is in the main thread
• worker-thread is a WebWorker
• service-worker is a ServiceWorker

WebWorkers and ServiceWorkers can't communicate directly, so messages need to be relayed through the main thread.

Alternatively we could skip worker-thread and send a message directly from bootPlaygroundRemote to service-worker

[bgrgicak]

This doesn't work for me. The Web Worker isn't loaded on the first load so it doesn't relay the message to the service worker.

[bgrgicak]

I will focus on wrapping up offline support without triggering it from boot and we can explore this next week.

[bgrgicak]

Let's trigger fetch from boot to cache assets.

#1535 (comment)

[bgrgicak]

The current implementation in #1535 seems good as is. Let's close this PR without merging.