How can I determine whether a controller action has rendered (rather than redirected?)

The performed? method returns true in a controller if the application has redirected or rendered.

How can I determine whether the application rendered (rather than redirected)?

Judging from the performed? docs, I can look at @performed_render, but this seems hacky.

Edit: Why do I want to do this?

I'm trying to cache the output of a controller action only in the case that it results in a render (and not a redirect). Here's the code I'm using now: (based on render-caching)

  def render_with_cache(options = {})
    key = ([request.request_uri, request.format] | (options.delete(:key_additions) || [])).join("::")
    body = Rails.cache.read(key)

    if body
      render :text => body
    else
      yield if block_given?
      render unless performed?
      if @performed_render
        Rails.cache.write(key, response.body, options) # :expires_in
      end
    end
  end