foo.js

// This is the presenter class that stores the state
App.FooPresenter = Em.ObjectProxy.create({
  textAreaContent: '',
  isExpanded: false
});


// This is your controller - be sure to set the content to a list of models in your route
App.FoosController = Em.ArrayController.extend({
  presenters: function() {
    return this.get('content').map(function(foo){
      return App.FooPresenter.create({content: foo});
    });
  }.property('content.@each')
});

// This is your template

{{#each presenters}}
  name: {{name}}
  {{#if isExpanded}}
    <textarea>
      {{textAreaContent}}
    </textarea>
  {{/if}}
{{/each}}