title: Is there any way to wait to invoke callback until multiple backbone collection have synced?
body: <p>I'd like to run view.render() if both collectionA and collectionB have fired 'sync' event.
  Something like this.</p>
  
  <pre><code>collectionA = new CollectionA();
  collectionB = new CollectionB();
  
  view.listenTo([collectionA, collectionB], 'sync', view.render);
  
  collectionA.fetch(); // do not call view.render yet.
  collectionB.fetch(): // call view.render after fetch.
  </code></pre>
  
  <p>I know there is a way to use $.when() or _.after() like this answers.
  <a href="https://stackoverflow.com/questions/12168587/how-to-synchronize-multiple-backbone-js-fetches">How to synchronize multiple Backbone.js fetches?</a></p>
  
  <p>But I think these are not event-driven way.
  Is there any event-driven way on Backbone.js?</p>