RoR Redis subscription rspec stub

I'm working on finding a stub for:

 @redis.subscribe(key) do |on|   on.message do |channel, message|      handle_message(message)   end end

However I am unsure what type of object on is? How can we iterate on two fields channel, and message without a .each block? This seems to be a pretty common chunk of code if anyone had any examples for some unit tests.