Use Ckeditor::Rails::AssetUrlProcessor

lib/ckeditor-rails/asset_url_processor.rb

@@ -0,0 +1,33 @@
+module Ckeditor
+  module Rails
+    # Rewrites urls in CSS files with the digested paths
+    class AssetUrlProcessor
+      REGEX = /url\(\s*["']?(?!(?:\#|data|http))([^"'\s)]+)\s*["']?\)/
+
+      def self.stylesheet_files
+        @stylesheet_files ||= ::Ckeditor::Rails::Asset.new.stylesheet_files
+      end
+
+      def self.call(input)
+        return { data: input[:data] } unless stylesheet_files.include?(input[:filename])
+
+        context = input[:environment].context_class.new(input)
+        path_prefix = "#{context.assets_prefix}/ckeditor"
+        matched_folders = input[:filename].match(/\/ckeditor\/(plugins|skins)\/([\w-]+)\//)
+
+        data = input[:data].gsub(REGEX) { |_match|
+          raw_asset_path = context.asset_path($1)
+          if raw_asset_path.starts_with?(path_prefix)
+            "url(#{raw_asset_path})"
+          elsif matched_folders
+            "url(#{path_prefix}/#{matched_folders[1]}/#{matched_folders[2]}#{raw_asset_path.gsub('/..', '')})"
+          else
+            "url(#{path_prefix}#{raw_asset_path})"
+          end
+        }
+
+        { data: data }
+      end
+    end
+  end
+end

lib/ckeditor-rails/engine.rb

@@ -1,10 +1,20 @@
+require 'ckeditor-rails/asset_url_processor'
+
 module Ckeditor
   module Rails
     class Engine < ::Rails::Engine
       initializer 'ckeditor.assets.precompile', group: :all do |app|
         app.config.assets.precompile += Ckeditor::Rails::Asset.new.files
       end
 
+      # Follow sprockets-rails 3.3.0+ to use postprocessor of Sprockets
+      # https://github.com/rails/sprockets-rails/blob/v3.3.0/lib/sprockets/railtie.rb#L121
+      initializer 'ckeditor.asset_url_processor' do |app|
+        if ::Sprockets.respond_to? :register_postprocessor
+          ::Sprockets.register_postprocessor 'text/css', ::Ckeditor::Rails::AssetUrlProcessor
+        end
+      end
+
       rake_tasks do
         load "ckeditor-rails/tasks.rake"
       end