class ActionView::StreamingTemplateRenderer

def delayed_render(buffer, template, layout, view, locals)
  # Wrap the given buffer in the StreamingBuffer and pass it to the
  # underlying template handler. Now, every time something is concatenated
  # to the buffer, it is not appended to an array, but streamed straight
  # to the client.
  output  = ActionView::StreamingBuffer.new(buffer)
  yielder = lambda { |*name| view._layout_for(*name) }
  ActiveSupport::Notifications.instrument(
    "render_template.action_view",
    identifier: template.identifier,
    layout: layout && layout.virtual_path,
    locals: locals
  ) do
    outer_config = I18n.config
    fiber = Fiber.new do
      I18n.config = outer_config
      if layout
        layout.render(view, locals, output, &yielder)
      else
        # If you don't have a layout, just render the thing
        # and concatenate the final result. This is the same
        # as a layout with just <%= yield %>
        output.safe_concat view._layout_for
      end
    end
    # Set the view flow to support streaming. It will be aware
    # when to stop rendering the layout because it needs to search
    # something in the template and vice-versa.
    view.view_flow = StreamingFlow.new(view, fiber)
    # Yo! Start the fiber!
    fiber.resume
    # If the fiber is still alive, it means we need something
    # from the template, so start rendering it. If not, it means
    # the layout exited without requiring anything from the template.
    if fiber.alive?
      content = template.render(view, locals, &yielder)
      # Once rendering the template is done, sets its content in the :layout key.
      view.view_flow.set(:layout, content)
      # In case the layout continues yielding, we need to resume
      # the fiber until all yields are handled.
      fiber.resume while fiber.alive?
    end
  end
end

def render_template(view, template, layout_name = nil, locals = {}) # :nodoc:
  return [super.body] unless layout_name && template.supports_streaming?
  locals ||= {}
  layout   = find_layout(layout_name, locals.keys, [formats.first])
  Body.new do |buffer|
    delayed_render(buffer, template, layout, view, locals)
  end
end