class ActionView::StreamingFlow

def get(key)

then it will begin waiting for the given key and yield.
is not available and we're inside the layout fiber,
Try to get stored content. If the content 
def get(key)
  return super if @content.key?(key)
  if inside_fiber?
    view = @view
    begin
      @waiting_for = key
      view.output_buffer, @parent = @child, view.output_buffer
      Fiber.yield
    ensure
      @waiting_for = nil
      view.output_buffer, @child = @parent, view.output_buffer
    end
  end
  super
end