view_cache_dependency API

A declarative API for specifying dependencies that affect template
cache digest computation. In your controller, specify any of said
dependencies:

  view_cache_dependency { "phone" if using_phone? }

When the block is evaluated, the resulting value is included in the
cache digest calculation, allowing you to generate different digests
for effectively the same template. (Mostly useful if you're mucking
with template load paths.)

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Added view_cache_dependency API for declaring dependencies that affect
+    cache digest computation.
+
+    *Jamis Buck*
+
 *   `image_submit_tag` will set `alt` attribute from image source if not
     specified.
 

actionpack/lib/action_controller/caching.rb

@@ -70,12 +70,30 @@ def self.page_cache_extension
 
       config_accessor :perform_caching
       self.perform_caching = true if perform_caching.nil?
+
+      class_attribute :_view_cache_dependencies
+      self._view_cache_dependencies = []
+      helper_method :view_cache_dependencies if respond_to?(:helper_method)
+    end
+
+    module ClassMethods
+      def view_cache_dependency(&dependency)
+        self._view_cache_dependencies += [dependency]
+      end
+
+      def view_cache_dependencies
+        _view_cache_dependencies.map { |dep| instance_exec &dep }.compact
+      end
     end
 
     def caching_allowed?
       request.get? && response.status == 200
     end
 
+    def view_cache_dependencies
+      self.class.view_cache_dependencies
+    end
+
     protected
       # Convenience accessor.
       def cache(key, options = {}, &block)

actionpack/lib/action_view/digestor.rb

@@ -24,16 +24,17 @@ class Digestor
     @@cache = ThreadSafe::Cache.new
 
     def self.digest(name, format, finder, options = {})
-      @@cache["#{name}.#{format}"] ||= begin
+      cache_key = [name, format] + Array.wrap(options[:dependencies])
+      @@cache[cache_key.join('.')] ||= begin
         klass = options[:partial] || name.include?("/_") ? PartialDigestor : Digestor
-        klass.new(name, format, finder).digest
+        klass.new(name, format, finder, options).digest
       end
     end
 
-    attr_reader :name, :format, :finder
+    attr_reader :name, :format, :finder, :options
 
-    def initialize(name, format, finder)
-      @name, @format, @finder = name, format, finder
+    def initialize(name, format, finder, options={})
+      @name, @format, @finder, @options = name, format, finder, options
     end
 
     def digest
@@ -81,9 +82,11 @@ def source
       end
 
       def dependency_digest
-        dependencies.collect do |template_name|
+        template_digests = dependencies.collect do |template_name|
           Digestor.digest(template_name, format, finder, partial: true)
-        end.join("-")
+        end
+
+        (template_digests + injected_dependencies).join("-")
       end
 
       def render_dependencies
@@ -105,6 +108,10 @@ def render_dependencies
       def explicit_dependencies
         source.scan(EXPLICIT_DEPENDENCY).flatten.uniq
       end
+
+      def injected_dependencies
+        Array.wrap(options[:dependencies])
+      end
   end
 
   class PartialDigestor < Digestor # :nodoc:

actionpack/lib/action_view/helpers/cache_helper.rb

@@ -167,7 +167,7 @@ def fragment_name_with_digest(name) #:nodoc:
         if @virtual_path
           [
             *Array(name.is_a?(Hash) ? controller.url_for(name).split("://").last : name),
-            Digestor.digest(@virtual_path, formats.last.to_sym, lookup_context)
+            Digestor.digest(@virtual_path, formats.last.to_sym, lookup_context, dependencies: view_cache_dependencies)
           ]
         else
           name

actionpack/test/controller/caching_test.rb

@@ -296,6 +296,24 @@ def test_safe_buffer
   end
 end
 
+class ViewCacheDependencyTest < ActionController::TestCase
+  class NoDependenciesController < ActionController::Base
+  end
+
+  class HasDependenciesController < ActionController::Base
+    view_cache_dependency { "trombone" }
+    view_cache_dependency { "flute" }
+  end
+
+  def test_view_cache_dependencies_are_empty_by_default
+    assert NoDependenciesController.view_cache_dependencies.empty?
+  end
+
+  def test_view_cache_dependencies_are_listed_in_declaration_order
+    assert_equal %w(trombone flute), HasDependenciesController.view_cache_dependencies
+  end
+end
+
 class DeprecatedPageCacheExtensionTest < ActiveSupport::TestCase
   def test_page_cache_extension_binds_default_static_extension
     deprecation_behavior = ActiveSupport::Deprecation.behavior

actionpack/test/template/digestor_test.rb

@@ -138,6 +138,20 @@ def test_old_style_hash_in_render_invocation
     end
   end
 
+  def test_dependencies_via_options_results_in_different_digest
+    digest_plain        = digest("comments/_comment")
+    digest_fridge       = digest("comments/_comment", dependencies: ["fridge"])
+    digest_phone        = digest("comments/_comment", dependencies: ["phone"])
+    digest_fridge_phone = digest("comments/_comment", dependencies: ["fridge", "phone"])
+
+    assert_not_equal digest_plain, digest_fridge
+    assert_not_equal digest_plain, digest_phone
+    assert_not_equal digest_plain, digest_fridge_phone
+    assert_not_equal digest_fridge, digest_phone
+    assert_not_equal digest_fridge, digest_fridge_phone
+    assert_not_equal digest_phone, digest_fridge_phone
+  end
+
   private
     def assert_logged(message)
       old_logger = ActionView::Base.logger
@@ -164,8 +178,8 @@ def assert_digest_difference(template_name)
       ActionView::Digestor.cache.clear
     end
 
-    def digest(template_name)
-      ActionView::Digestor.digest(template_name, :html, FixtureFinder.new)
+    def digest(template_name, options={})
+      ActionView::Digestor.digest(template_name, :html, FixtureFinder.new, options)
     end
 
     def change_template(template_name)