Introduce flush_output_buffer to append the buffer to the response body then start a new buffer. Useful for pushing custom parts to the response body without disrupting template rendering.

actionpack/lib/action_view/helpers/capture_helper.rb

@@ -131,6 +131,14 @@ module ActionView
       ensure
         self.output_buffer = old_buffer
       end
+
+      # Add the output buffer to the response body and start a new one.
+      def flush_output_buffer #:nodoc:
+        if output_buffer && output_buffer != ''
+          response.body_parts << output_buffer
+          self.output_buffer = ''
+        end
+      end
     end
   end
 end

actionpack/test/template/output_buffer_test.rb

@@ -0,0 +1,35 @@
+require 'abstract_unit'
+
+class OutputBufferTest < ActionController::TestCase
+  class TestController < ActionController::Base
+    def index
+      render :text => 'foo'
+    end
+  end
+
+  tests TestController
+
+  def test_flush_output_buffer
+    # Start with the default body parts
+    get :index
+    assert_equal ['foo'], @response.body_parts
+    assert_nil @response.template.output_buffer
+
+    # Nil output buffer is skipped
+    @response.template.flush_output_buffer
+    assert_nil @response.template.output_buffer
+    assert_equal ['foo'], @response.body_parts
+
+    # Empty output buffer is skipped
+    @response.template.output_buffer = ''
+    @response.template.flush_output_buffer
+    assert_equal '', @response.template.output_buffer
+    assert_equal ['foo'], @response.body_parts
+
+    # Flushing appends the output buffer to the body parts
+    @response.template.output_buffer = 'bar'
+    @response.template.flush_output_buffer
+    assert_equal '', @response.template.output_buffer
+    assert_equal ['foo', 'bar'], @response.body_parts
+  end
+end