accordion thing

components.accordion-section.js

import Ember from 'ember';

export default Ember.Component.extend({

});

components.parent-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  selectedIndex: 0,
  
  init() {
    this._super();
    
    this.selectIndex(this.get('selectedIndex'));
  },
  
  selectIndex(index) {
    let oldIndex = this.get('selectedIndex');
    let oldItem = this.get('items').objectAt(oldIndex);

    oldItem.set('isSelected', false);

    this.set('selectedIndex', index);

    let newItem = this.get('items').objectAt(index);

    newItem.set('isSelected', true);
  }

});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  items: Ember.A([
    Ember.Object.create({
      label: "Section 1",
      text: "Section 1 text!",
      isExpanded: false
    }),
    Ember.Object.create({
      label: "Section 2",
      text: "Section 2 text!",
      isExpanded: false
    }),
    Ember.Object.create({
      label: "Section 3",
      text: "Section 3 text!",
      isExpanded: false
    })  
  ])
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}

{{parent-component items=items}}
<br>
<br>

templates.components.accordion-section.hbs

{{#if isExpanded}}
  <strong>{{item.label}}</strong>
{{else}}
  {{item.label}}
{{/if}}

templates.components.parent-component.hbs

<p>Selected: {{selectedIndex}}</p>

<ul>
{{#each items as |item index|}}
  <li onclick={{action selectIndex index}}>{{item.label}}</li>
{{/each}}
</ul>

<ul>
{{#each items as |item index|}}
  <li>{{accordion-section item=item isExpanded=item.isSelected}}</li>
{{/each}}
</ul>

twiddle.json

{
  "version": "0.9.3",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.6.0",
    "ember-data": "2.6.1",
    "ember-template-compiler": "2.6.0"
  }
}