Body... wanna *stream* my body? Body... such a thrill my body!

Added stream as class level method to make it explicit when to stream.
Render also accepts :stream as option.

actionpack/lib/action_controller.rb

@@ -31,6 +31,7 @@ module ActionController
     autoload :Rescue
     autoload :Responder
     autoload :SessionManagement
+    autoload :Streaming
     autoload :Testing
     autoload :UrlFor
   end

actionpack/lib/action_controller/base.rb

@@ -199,6 +199,7 @@ module ActionController
       Flash,
       RequestForgeryProtection,
       ForceSSL,
+      Streaming,
       DataStreaming,
       RecordIdentifier,
       HttpAuthentication::Basic::ControllerMethods,

actionpack/lib/action_controller/metal/streaming.rb

@@ -0,0 +1,57 @@
+require 'active_support/core_ext/file/path'
+require 'rack/chunked'
+
+module ActionController #:nodoc:
+  # Methods for sending streaming templates back to the client.
+  module Streaming
+    extend ActiveSupport::Concern
+
+    include AbstractController::Rendering
+    attr_internal :stream
+
+    module ClassMethods
+      # Render streaming templates. It accepts :only, :except, :if and :unless as options
+      # to specify when to stream, as in ActionController filters.
+      def stream(options={})
+        if defined?(Fiber)
+          before_filter :_stream_filter, options
+        else
+          raise "You cannot use streaming if Fiber is not available."
+        end
+      end
+    end
+
+    protected
+
+    # Mark following render calls as streaming.
+    def _stream_filter #:nodoc:
+      self.stream = true
+    end
+
+    # Consider the stream option when normalazing options.
+    def _normalize_options(options) #:nodoc:
+      super
+      options[:stream] = self.stream unless options.key?(:stream)
+    end
+
+    # Set proper cache control and transfer encoding when streaming
+    def _process_options(options) #:nodoc:
+      super
+      if options[:stream]
+        headers["Cache-Control"] ||= "no-cache"
+        headers["Transfer-Encoding"] = "chunked"
+        headers.delete("Content-Length")
+      end
+    end
+
+    # Call render_to_body if we are streaming instead of usual +render+.
+    def _render_template(options) #:nodoc:
+      if options.delete(:stream)
+        Rack::Chunked::Body.new view_context.render_body(options)
+      else
+        super
+      end
+    end
+  end
+end
+      

actionpack/lib/action_view/context.rb

@@ -2,6 +2,7 @@ module ActionView
   module CompiledTemplates #:nodoc:
     # holds compiled template code
   end
+
   # = Action View Context
   #
   # Action View contexts are supplied to Action Controller to render template.

actionpack/test/controller/new_base/render_streaming_test.rb

@@ -0,0 +1,62 @@
+require 'abstract_unit'
+
+module RenderStreaming
+  class BasicController < ActionController::Base
+    self.view_paths = [ActionView::FixtureResolver.new(
+      "render_streaming/basic/hello_world.html.erb" => "Hello world",
+      "layouts/application.html.erb" => "<%= yield %>, I'm here!"
+    )]
+
+    layout "application"
+    stream :only => :hello_world
+
+    def hello_world
+    end
+
+    def explicit
+      render :action => "hello_world", :stream => true
+    end
+
+    def no_layout
+      render :action => "hello_world", :stream => true, :layout => false
+    end
+
+    def explicit_cache
+      headers["Cache-Control"] = "private"
+      render :action => "hello_world", :stream => true
+    end
+  end
+
+  class StreamingTest < Rack::TestCase
+    test "rendering with streaming enabled at the class level" do
+      get "/render_streaming/basic/hello_world"
+      assert_body "b\r\nHello world\r\nb\r\n, I'm here!\r\n0\r\n\r\n"
+      assert_streaming!
+    end
+
+    test "rendering with streaming given to render" do
+      get "/render_streaming/basic/explicit"
+      assert_body "b\r\nHello world\r\nb\r\n, I'm here!\r\n0\r\n\r\n"
+      assert_streaming!
+    end
+
+    test "rendering with streaming do not override explicit cache control given to render" do
+      get "/render_streaming/basic/explicit_cache"
+      assert_body "b\r\nHello world\r\nb\r\n, I'm here!\r\n0\r\n\r\n"
+      assert_streaming! "private"
+    end
+
+    test "rendering with streaming no layout" do
+      get "/render_streaming/basic/no_layout"
+      assert_body "b\r\nHello world\r\n0\r\n\r\n"
+      assert_streaming!
+    end
+
+    def assert_streaming!(cache="no-cache")
+      assert_status 200
+      assert_equal nil, headers["Content-Length"]
+      assert_equal "chunked", headers["Transfer-Encoding"]
+      assert_equal cache, headers["Cache-Control"]
+    end
+  end if defined?(Fiber)
+end