Make GOV.UK Frontend assets paths work with http prefix

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 ## Unreleased
 
+### Fixes
+
+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.
+
+- [#292: Make GOV.UK Frontend assets path work with http prefix](https://github.com/alphagov/tech-docs-gem/pull/292)
+
 ## 3.2.1
 
 ### Fixes

lib/assets/stylesheets/_govuk_tech_docs.scss

@@ -1,4 +1,20 @@
-$govuk-assets-path: "/assets/govuk/assets/" !default;
+// Set $govuk-assets-path so that $govuk-images-path and $govuk-fonts-path
+// (used below, but set in govuk/base) are correct.
+$govuk-assets-path: "/assets/govuk/assets/";
+
+// Use Sprockets' asset url helpers to add the http prefix if present.
+// GOV.UK Frontend provides hooks for us to define the url lookup functions,
+// see https://frontend.design-system.service.gov.uk/sass-api-reference/
+@function tech-docs-image-url-function($filename) {
+  @return image-url($govuk-images-path + $filename);
+}
+
+@function tech-docs-font-url-function($filename) {
+  @return font-url($govuk-fonts-path + $filename);
+}
+
+$govuk-font-url-function: "tech-docs-font-url-function";
+$govuk-image-url-function: "tech-docs-image-url-function";
 
 // Include only the bits of GOV.UK Frontend we need
 $govuk-new-link-styles: true;

lib/govuk_tech_docs.rb

@@ -24,12 +24,24 @@
 require "govuk_tech_docs/api_reference/api_reference_extension"
 
 module GovukTechDocs
+  class GovukTechDocsExtension < Middleman::Extension
+    def after_configuration
+      if app.config[:govuk_assets_path]
+        warn "Warning: setting govuk_assets_path in config.rb is no longer necessary, and will have no effect."
+      end
+    end
+  end
+
+  ::Middleman::Extensions.register(:govuk_tech_docs, GovukTechDocsExtension)
+
   # Configure the tech docs template
   #
   # @param options [Hash]
   # @option options [Hash] livereload Options to pass to the `livereload`
   #   extension. Hash with symbols as keys.
   def self.configure(context, options = {})
+    context.activate :govuk_tech_docs
+
     context.activate :sprockets
 
     context.sprockets.append_path File.join(__dir__, "../node_modules/govuk-frontend/")