Hi!

Working hard to see where AssetMapper adjustments need to be made. 3 changes proposed here:

1. AssetMapper's "public output path" awareness was moved to a new PublicAssetsPathResolver service
2. The asset compilers were updated to use relative paths when adjusting paths in files.
3. Drop the "mime type" stuff entirely - except for the "dev server" so it can set the content-type. Asset compiler's can use $mappedAsset->getPublicExtension() instead.

Why these changes?

1. Allow public/assets/ structure to flatten/change

The new PublicAssetsPathResolver + the "adjustment" of import/url paths in the compilers allows the "output" directory structure to be decoupled from the source structure. Currently the output structure matches your "source" structure, so you get things like public/assets/styles/app.css or public/assets/bundles/easyadmin/foo.css. We could add an option in the future (or the user could decorate PublicAssetsPathResolver immediately) to "flatten" everything into just public/assets/.

2. Allow for building of certain files, like .scss or .jsx

These changes allow users to create... more controversial functionality that could build certain files automatically. For example, you could have:

<link rel="stylesheet" href="{{ asset('styles/app.scss') }}">

Locally, I have a custom compiler that executes the dart-sass binary to convert this to CSS. Then, by decorating PublicAssetsPathResolver, I'm changing the public path for styles/app.scss so that it ends with .css. This is about 50 lines of code that results in the sass-compiled CSS file being properly rendered onto the page.

I have a similar compiler + path decorator locally that compiles .jsx => .js using the babel binary. Usage:

// assets/app.js
import HelloWorldComponent from './HelloReact.jsx';

// ... render like normal

I realize usage like this is more controversial... however it's also super pragmatic if you DO use Sass or have some React components. The alternative would be to run the sass binary manually, import the built .css file and probably ignore that file from git.

Thanks for your consideration :).

Cheers!