看的不是很懂

# The same as +content_for+ but when used with streaming flushes
      # straight back to the layout. In other words, if you want to
      # concatenate several times to the same buffer when rendering a given
      # template, you should use +content_for+, if not, use +provide+ to tell
      # the layout to stop looking for more contents.
      def provide(name, content = nil, &block)
        content = capture(&block) if block_given?
        result = @view_flow.append!(name, content) if content
        result unless content
      end