title: Mutual Exclusion in an Ember ArrayController
body: <p>I have a list of items in an array controller. Clicking on an item makes it "active", but I’d like only one item to be active at any a time (akin to radio buttons).</p>
  
  <p>I have this working by storing the active item in a computed property, then toggling its active state in an action on the array controller, see: <a href="http://emberjs.jsbin.com/wavay/2/edit" rel="nofollow">http://emberjs.jsbin.com/wavay/2/edit</a></p>
  
  <p>However, this doesn’t handle the case where an item is made active by some other means i.e. <em>not</em> through the action.</p>
  
  <p>I have experimented with observing the <code>isActive</code> change (using <code>.observesBefore('@each.isActive')</code>), and flipping the state of the <code>activeItem</code>, but of course, this approach causes an infinite loop.</p>
  
  <p>Is there a better way?</p>