[Sass Extensions] Allow the asset host and cache busting strategies to be defined as part of the compass configuration. Closes #20.

lib/compass.rb

@@ -1,7 +1,7 @@
 module Compass
 end
 
-['dependencies', 'sass_extensions', 'core_ext', 'version'].each do |file|
+['dependencies', 'sass_extensions', 'core_ext', 'version', 'errors'].each do |file|
   require File.join(File.dirname(__FILE__), 'compass', file)
 end
 

lib/compass/configuration.rb

@@ -117,7 +117,41 @@ module Compass
       end
     end
 
+    # When called with a block, defines the asset host url to be used.
+    # The block must return a string that starts with a protocol (E.g. http).
+    # The block will be passed the root-relative url of the asset.
+    # When called without a block, returns the block that was previously set.
+    def asset_host(&block)
+      if block_given?
+        @asset_host = block
+      else
+        @asset_host
+      end
+    end
+
+    # When called with a block, defines the cache buster strategy to be used.
+    # The block must return nil or a string that can be appended to a url as a query parameter.
+    # The returned string must not include the starting '?'.
+    # The block will be passed the root-relative url of the asset.
+    # If the block accepts two arguments, it will also be passed a File object
+    # that points to the asset on disk -- which may or may not exist.
+    # When called without a block, returns the block that was previously set.
+    def asset_cache_buster(&block)
+      if block_given?
+        @asset_cache_buster = block
+      else
+        @asset_cache_buster
+      end
+    end
+
+
     def serialize
+      if asset_cache_buster
+        raise Compass::Error, "Cannot serialize a configuration with asset_cache_buster set."
+      end
+      if asset_host
+        raise Compass::Error, "Cannot serialize a configuration with asset_host set."
+      end
       contents = ""
       required_libraries.each do |lib|
         contents << %Q{require '#{lib}'\n}
@@ -179,6 +213,8 @@ module Compass
       ATTRIBUTES.each do |attr|
         send("#{attr}=", nil)
       end
+      @asset_cache_buster = nil
+      @asset_host = nil
       self.required_libraries = []
     end
 

lib/compass/sass_extensions/functions/image_url.rb

@@ -1,35 +1,51 @@
 module Compass::SassExtensions::Functions::ImageUrl
   def image_url(path)
     path = path.value # get to the string value of the literal.
+    # Short curcuit if they have provided an absolute url.
     if absolute_path?(path)
+      puts "#{path} is absolute"
       return Sass::Script::String.new("url(#{path})")
     end
-    http_images_path = if Compass.configuration.http_images_path == :relative
+
+    # Compute the path to the image, either root relative or stylesheet relative
+    # or nil if the http_images_path is not set in the configuration.
+    http_images_path = if relative?
       compute_relative_path
     else
       Compass.configuration.http_images_path
     end
 
+    # Compute the real path to the image on the file stystem if the images_dir is set.
     real_path = if Compass.configuration.images_dir
       File.join(Compass.configuration.project_path, Compass.configuration.images_dir, path)
     end
 
+    # prepend the path to the image if there's one
     if http_images_path
       http_images_path = "#{http_images_path}/" unless http_images_path[-1..-1] == "/"
       path = "#{http_images_path}#{path}"
     end
 
-    if real_path && File.exists?(real_path)
-      path += "?#{File.mtime(real_path).strftime("%s")}"
-    elsif real_path
-      $stderr.puts "WARNING: '#{File.basename(path)}' was not found in #{File.dirname(real_path)}"
+    # Compute the asset host unless in relative mode.
+    asset_host = (compute_asset_host(path) unless relative?)
+
+    # Compute and append the cache buster if there is one.
+    if buster = compute_cache_buster(path, real_path)
+      path += "?#{buster}"
     end
 
+    # prepend the asset host if there is one.
+    path = "#{asset_host}#{'/' unless path[0..0] == "/"}#{path}" if asset_host
+
     Sass::Script::String.new("url(#{path})")
   end
 
   private
 
+  def relative?
+    Compass.configuration.http_images_path == :relative
+  end
+
   def absolute_path?(path)
     path[0..0] == "/" || path[0..3] == "http"
   end
@@ -40,4 +56,33 @@ module Compass::SassExtensions::Functions::ImageUrl
       Pathname.new(images_path).relative_path_from(Pathname.new(File.dirname(target_css_file))).to_s
     end
   end
+
+  def compute_cache_buster(path, real_path)
+    if Compass.configuration.asset_cache_buster
+      args = [path]
+      if Compass.configuration.asset_cache_buster.arity > 1
+        args << (File.new(real_path) if real_path)
+      end
+      Compass.configuration.asset_cache_buster.call(*args)
+    elsif real_path
+      default_cache_buster(path, real_path)
+    end
+  end
+
+  def default_cache_buster(path, real_path)
+    if File.readable?(real_path)
+      File.mtime(real_path).strftime("%s") 
+    else
+      $stderr.puts "WARNING: '#{File.basename(path)}' was not found (or cannot be read) in #{File.dirname(real_path)}"
+    end
+  end
+
+  def compute_asset_host(path)
+    if Compass.configuration.asset_host
+      Compass.configuration.asset_host.call(path)
+    else
+      nil
+    end
+  end
+
 end

test/compass_test.rb

@@ -9,7 +9,7 @@ class CompassTest < Test::Unit::TestCase
   end
 
   def teardown
-    teardown_fixtures :blueprint, :yui, :empty, :compass
+    teardown_fixtures :blueprint, :yui, :empty, :compass, :image_urls
   end
 
   def teardown_fixtures(*project_names)
@@ -55,6 +55,15 @@ class CompassTest < Test::Unit::TestCase
     end
   end
 
+  def test_image_urls
+    within_project('image_urls') do |proj|
+      each_css_file(proj.css_path) do |css_file|
+        assert_no_errors css_file, 'image_urls'
+      end
+      assert_renders_correctly :screen
+    end
+  end
+
 private
   def assert_no_errors(css_file, project_name)
     file = css_file[(tempfile_path(project_name).size+1)..-1]

test/fixtures/stylesheets/image_urls/config.rb

@@ -0,0 +1,17 @@
+# Require any additional compass plugins here.
+project_type = :stand_alone
+css_dir = "tmp"
+sass_dir = "sass"
+images_dir = "images"
+output_style = :compact
+# To enable relative image paths using the images_url() function:
+# http_images_path = :relative
+http_images_path = "/images"
+
+asset_cache_buster do |path, file|
+  "busted=true"
+end
+
+asset_host do |path|
+  "http://assets%d.example.com" % (path.hash % 4)
+end

test/fixtures/stylesheets/image_urls/css/screen.css

@@ -0,0 +1 @@
+.showgrid { background-image: url(http://assets2.example.com/images/grid.png?busted=true); }

test/fixtures/stylesheets/image_urls/sass/screen.sass

@@ -0,0 +1,2 @@
+.showgrid
+  background-image= image_url("grid.png")