Add Engine#helpers method which loads all the engine's helpers

railties/lib/rails/engine.rb

@@ -286,6 +286,27 @@ module Rails
   #
   # This code will use <tt>my_engine.user_path(@user)</tt> to generate the proper route.
   #
+  # == Isolated engine's helpers
+  #
+  # Sometimes you may want to isolate engine, but use helpers that are defined for it.
+  # If you want to share just a few specific helpers you can add them to application's
+  # helpers in ApplicationController:
+  #
+  # class ApplicationController < ActionController::Base
+  #   helper MyEngine::SharedEngineHelper
+  # end
+  #
+  # If you want to include all of the engine's helpers, you can use #helpers method on egine's
+  # instance:
+  #
+  # class ApplicationController < ActionController::Base
+  #   helper MyEngine::Engine.helpers
+  # end
+  #
+  # It will include all of the helpers from engine's directory. Take into account that this does
+  # not include helpers defined in controllers with helper_method or other similar solutions,
+  # only helpers defined in helpers directory will be included.
+  #
   # == Migrations & seed data
   #
   # Engines can have their own migrations. The default path for migrations is exactly the same
@@ -384,6 +405,24 @@ module Rails
       @railties ||= self.class::Railties.new(config)
     end
 
+    def helpers
+      @helpers ||= begin
+        helpers = Module.new
+
+        helpers_paths = if config.respond_to?(:helpers_paths)
+          config.helpers_paths
+        else
+          paths["app/helpers"].existent
+        end
+
+        all = ActionController::Base.send(:all_helpers_from_path, helpers_paths)
+        ActionController::Base.send(:modules_for_helpers, all).each do |mod|
+          helpers.send(:include, mod)
+        end
+        helpers
+      end
+    end
+
     def app
       @app ||= begin
         config.middleware = config.middleware.merge_into(default_middleware_stack)

railties/test/railties/engine_test.rb

@@ -584,6 +584,51 @@ module RailtiesTest
       assert_equal Bukkits::Engine.instance, Rails::Engine.find(engine_path)
     end
 
+    test "gather isolated engine's helpers in Engine#helpers" do
+      @plugin.write "lib/bukkits.rb", <<-RUBY
+        module Bukkits
+          class Engine < ::Rails::Engine
+            isolate_namespace Bukkits
+          end
+        end
+      RUBY
+
+      app_file "app/helpers/some_helper.rb", <<-RUBY
+        module SomeHelper
+          def foo
+            'foo'
+          end
+        end
+      RUBY
+
+      @plugin.write "app/helpers/bukkits/engine_helper.rb", <<-RUBY
+        module Bukkits
+          module EngineHelper
+            def bar
+              'bar'
+            end
+          end
+        end
+      RUBY
+
+      @plugin.write "app/helpers/engine_helper.rb", <<-RUBY
+        module EngineHelper
+          def baz
+            'baz'
+          end
+        end
+      RUBY
+
+      add_to_config("config.action_dispatch.show_exceptions = false")
+
+      boot_rails
+      require "#{rails_root}/config/environment"
+
+      methods = Bukkits::Engine.helpers.public_instance_methods.sort
+      expected = ["bar", "baz"]
+      assert_equal expected, methods
+    end
+
   private
     def app
       Rails.application