Extend the filter for responsive images (srcset attribute)

srcset can include multiple URIs.
See https://html.spec.whatwg.org/multipage/images.html#srcset-attribute

We should probably do as is done in Html::transformRootRelativeUrlsToAbsolute(): explode the comma-separated string that's within srcset="" and operate on each URI within.

Right now this module doesn't track where the attribute value ends, though. It should probably also mimic transformRootRelativeUrlsToAbsolute() and use XPath. But maybe you can keep this change simpler for the moment and look for the end quote of the attribute string?

Note that if we do copy core for this, there's a bit of a mistake with where Html::transformRootRelativeUrlsToAbsolute() puts its srcset logic – it shouldn't be within the loop over attributes.

I opened an issue for that: 🐛 Logic error in Html::transformRootRelativeUrlsToAbsolute() Fixed .

I'm uploading for now a newer version of the patch to apply on 2.2.5 and I can check your replies about a different process.

@jeffschuler Maybe I’m missing something, but why isn’t Html::transformRootRelativeUrlsToAbsolute() used in the process() function instead of the current regex?

@Daniel Korte : that would definitely be more DRY - and ultimately might also solve a few other issues like 🐛 Do not convert Relative URLs beginning with // (v2.2.5) Needs review and 🐛 Part of the path is lost after image path conversion Needs review ...

But one reason is that transformRootRelativeUrlsToAbsolute() uses a fixed set of $urlAttributes, and I think it would be useful if this module were to ✨ Allow choice of attributes to process Needs review .

Ideally transformRootRelativeUrlsToAbsolute() would allow choosing params to include/exclude, but I'm not sure it's worth trying to change core for this use case.

@jeffschuler Got it. Sounds good. I think I can knock that out in the next day or so.

Ready to review!

Fix tests.

I had to update the tests so the Filter is actually loaded with the settings. The code is mostly mirrored from core/modules/filter/tests/src/Kernel/FilterKernelTest.php

@Daniel Korte. Cool. I thought it would be easier for @podarok to review and commit ✨ Allow choice of attributes to process Needs review separately since these issues accomplish different things, but let's see what he thinks.

@jeffschuler Yeah, sorry, I wasn’t sure if I should have created a new issue or posted on that issue since the process function is completely different now. Also, though I finally figured out the tests. The test string wasn’t correct and the configuration wasn’t set.

Applying the patch from 🐛 Logic error in Html::transformRootRelativeUrlsToAbsolute() Fixed to our custom transformRootRelativeUrlsToAbsolute() function...