Proposal: more advanced Service Worker support

Created by: MOZGIII

Is your proposal related to a problem?

Currently, the Service Worker support is very limited because the service-worker.js is generated by CRA and users can't customize it.

It's bad cause Service Workers can do much more than we currently use today, and by their inherent design, should allow app-specific logic.

Describe the solution you'd like

We're using workbox-webpack-plugin go generate service worker via it's GenerateSW.

Instead, we could switch to InjectManifest, and add a sample service worker file src/service-worker.js to all the app templates. Therefore users will be able to customize the behavior of the Service Worker by editing the src/service-worker.js file, and we can preserve the existing runtime defaults by putting the same logic at the src/service-worker.js template that GenerateSW generates by default.

Regarding TypeScript support, we should be able to transpile that service worker file too!

Additionally, we can benefit from this approach by disabling service worker generation logic if the src/service-worker.js is removed. This way we can solve long outstanding request to disable service worker generation.

Describe alternatives you've considered

I've looked into issues #5369 and #5359. They both essentially propose providing more flexibility by allowing webpack config customization. I think that would be a more generic solution, but it has some significant downsides:

We'd have to support unbound set of build configurations. My proposal allows flexibility while only having a single configuration. This can be arguably a strong point, and I've seen it a lot in the decision making of this project before. Yet, personally, I don't think it's that problematic.
We can provide better support using this proposal than by allowing webpack build configuration. By this, I mean that we can integrate the InjectManifest better if we only have that, in contrary to supporting both InjectManifest and GenerateSW, or even allowing users to provide an arbitrary plugin there. We can add the logic to disable service worker generation naturally, TypeScript support and possibly other built-time service worker injectors if we ever need to.
This proposal will also simplify bug fixing cause there will be less possible configuration states that the user can put the build system into. This kind of repeats the first point (1).

The #7866 might provide flexible webpack build process configuration, and relieve the pain points with the current service workers support. However, this proposal is focusing on more advanced support for Service Worker - not just solving the current pain points - and, in my opinion, has value independently of whether #7866 (or similar) is accepted or not.

Additional context

I'll be able to submit a PR if this proposal is accepted.