so, how to use yieldTo (look for "spy.yieldTo" in http://sinonjs.org/docs/#stubs) ? Simple:

```javascript
MessageManager.getMessages.yieldTo('each', {receiver: '123', threadId: '42'});
MessageManager.getMessages.yieldTo('done')
```

easy right? Just use it after the call to `mTrigger('activity')`.

And the nice thing is that it keeps the asynchronous-like nature of the API. Your current code is calling `each` and `done` in a synchronous way (they'll execute before the code after the "getMessages" call) whereas using `yieldTo` is "asynchronous-like" (they'll execute after the code that is after the getMessages call executes) while retaining the synchronous behavior we love (because this makes reliable tests).

Not sure I'm that clear, don't hesitate to ask if you need any more explanation :)