How to get objectAt to update like a computed property

I have a simple array of objects in my ember app

var foo = Ember.A([]);

When I push into this array it's auto updated when I bind to the array (foo above). But I have the need to pull out a single item and it looks like using foo.objectAt(0) won't "auto update" like just binding to the array itself.

What is a trick I can use to return the first item that pops into this array (like a traditional computed property would) ?