title: Symfony2: update a form from event and get an object
body: <p>I have a form and a listener within it. As I have "choices" fields whose available value may depend on other field's values, I listen to the PRE_SUBMIT event. There, I re-create the choice fields with the new value.</p>
  
  <p>In this listener, to analyse the user data, I do:</p>
  
  <pre><code>$event-&gt;getData()
  </code></pre>
  
  <p>But, from this event, it returns me an array. 
  The constraint is that I need an object, in order to compute the available values. 
  As far as I know, from the SUBMIT event I could get an object, but fields cannot be added any more to the form.</p>
  
  <p>Is there no way to get an object ?</p>