Add stub directive

Fixes #202

lib/sprockets/context.rb

@@ -22,7 +22,8 @@ module Sprockets
   # assets. See `DirectiveProcessor` for an example of this.
   class Context
     attr_reader :environment, :pathname
-    attr_reader :_required_paths, :_dependency_paths, :_dependency_assets
+    attr_reader :_required_paths, :_stubbed_assets
+    attr_reader :_dependency_paths, :_dependency_assets
     attr_writer :__LINE__
 
     def initialize(environment, logical_path, pathname)
@@ -32,6 +33,7 @@ def initialize(environment, logical_path, pathname)
       @__LINE__     = nil
 
       @_required_paths    = []
+      @_stubbed_assets    = Set.new
       @_dependency_paths  = Set.new
       @_dependency_assets = Set.new([pathname.to_s])
     end
@@ -143,6 +145,14 @@ def require_asset(path)
       nil
     end
 
+    # `stub_asset` blacklists `path` from being included in the bundle.
+    # `path` must be an asset which may or may not already be included
+    # in the bundle.
+    def stub_asset(path)
+      @_stubbed_assets << resolve(path, :content_type => :self).to_s
+      nil
+    end
+
     # Tests if target path is able to be safely required into the
     # current concatenation.
     def asset_requirable?(path)

lib/sprockets/directive_processor.rb

@@ -340,6 +340,18 @@ def process_depend_on_asset_directive(path)
         context.depend_on_asset(path)
       end
 
+      # Allows dependency to be excluded from the asset bundle.
+      #
+      # The `path` must be a valid asset and may or may not already
+      # be part of the bundle. Once stubbed, it is blacklisted and
+      # can't be brought back by any other `require`.
+      #
+      #     //= stub "jquery"
+      #
+      def process_stub_directive(path)
+        context.stub_asset(path)
+      end
+
       # Enable Sprockets 1.x compat mode.
       #
       # Makes it possible to use the same JavaScript source

lib/sprockets/processed_asset.rb

@@ -94,27 +94,31 @@ def hash
 
     private
       def build_required_assets(environment, context)
-        @required_assets = []
-        required_assets_cache = {}
+        @required_assets = resolve_dependencies(environment, context._required_paths + [pathname.to_s]) -
+          resolve_dependencies(environment, context._stubbed_assets.to_a)
+      end
+
+      def resolve_dependencies(environment, paths)
+        assets = []
+        cache  = {}
 
-        (context._required_paths + [pathname.to_s]).each do |path|
+        paths.each do |path|
           if path == self.pathname.to_s
-            unless required_assets_cache[self]
-              required_assets_cache[self] = true
-              @required_assets << self
+            unless cache[self]
+              cache[self] = true
+              assets << self
             end
           elsif asset = environment.find_asset(path, :bundle => false)
             asset.required_assets.each do |asset_dependency|
-              unless required_assets_cache[asset_dependency]
-                required_assets_cache[asset_dependency] = true
-                @required_assets << asset_dependency
+              unless cache[asset_dependency]
+                cache[asset_dependency] = true
+                assets << asset_dependency
               end
             end
           end
         end
 
-        required_assets_cache.clear
-        required_assets_cache = nil
+        assets
       end
 
       def build_dependency_paths(environment, context)

test/fixtures/asset/stub/application.js

@@ -0,0 +1,3 @@
+//= require ./jquery
+//= require ./foo
+//= stub ./frameworks

test/fixtures/asset/stub/foo.js

@@ -0,0 +1 @@
+var Foo = {};

test/fixtures/asset/stub/frameworks.js

@@ -0,0 +1,2 @@
+//= require ./jquery
+//= require ./jquery-ui

test/fixtures/asset/stub/jquery-ui.js

@@ -0,0 +1,2 @@
+//= require ./jquery
+var jQuery.UI = {};

test/fixtures/asset/stub/jquery.js

@@ -0,0 +1 @@
+var jQuery = {};

test/fixtures/asset/stub/skip_jquery.js

@@ -0,0 +1,2 @@
+//= require ./jquery-ui
+//= stub ./jquery

test/test_asset.rb

@@ -732,6 +732,14 @@ def setup
     end
   end
 
+  test "stub single dependency" do
+    assert_equal "var jQuery.UI = {};\n\n\n", asset("stub/skip_jquery").to_s
+  end
+
+  test "stub dependency tree" do
+    assert_equal "var Foo = {};\n\n\n\n", asset("stub/application").to_s
+  end
+
   test "circular require raises an error" do
     assert_raise(Sprockets::CircularDependencyError) do
       asset("circle/a.js")