title: Find out which individual item changed in an Ember ArrayController
body: <p>In an <code>Ember.ArrayController</code>, I have a function that <code>.observes()</code>
  a property  on the entire array of the model for a property change.</p>
  
  <pre><code>var FoosController = Ember.ArrayController.extend(Ember.Evented, {
  
      observesEachFooBar: function() {
          var foos = this.get('model');
          foos.forEach(function(foo) {
              //test if this foo has changed, then do something
          });
      }.observes('model.@each.bar'),
  
  });
  </code></pre>
  
  <p>Here I am manually testing every single  <code>Foo</code> in its model.
  How can I avoid doing this, and just be given the individual one (or few)
  that have changed?</p>