Make GOV.UK Frontend assets paths work with http prefix

[lfdebrux]

What’s changed

You no longer need to change the Sass variable govuk-assets-path when using a http prefix; the tech docs gem will add the prefix for you.

Identifying a user need

Users who want to deploy their tech docs to GitHub pages need to handle an additional path prefix to all URLs; they may want to use the http_prefix setting for this. PR #197 added this http prefix to all assets from the tech docs gem, but required the user to also override govuk-assets-path themselves so that assets from GOV.UK Frontend had the correct path. This PR should mean that this additional step is no longer required, making configuration easier for users.

[lfdebrux]

If I understand what this is doing correctly, then I think there's a neater way to do this that doesn't need us to monkey patch the asset_path function.

GOV.UK Frontend allows us to define which function to use when generating paths for images ($govuk-image-url-function) and the font ($govuk-font-url-function).

If no custom function is set, the default behaviour is just to prefix the $filename with $govuk-images-path / $govuk-fonts-path (which default to "#{$govuk-assets-path}images/" and "#{$govuk-assets-path}fonts/" respectively).

I think we can instead pass our own function instead which does the same thing but then calls asset-path with the resolved path including the filename. So something like this:

$govuk-assets-path: "/assets/govuk/assets/";

@function tech-docs-image-handler($filename) {
  @return asset-path($govuk-image-url-function + $filename);
}

$govuk-image-url-function: 'tech-docs-image-handler';

As ever, happy to talk through it if it's helpful!

Thanks for this @36degrees, this is much nicer!

I've pushed a different version of the code now based on your suggestion.

[lfdebrux]

I'm guessing there's no easy way to test this, given the functionality we're interested in is really part of Sprockets?

Otherwise I'm happy with this approach, bar a couple of very minor comments.

Ah tests is a good idea, I'll add some.