List with Add

components.accordion-item.js

// components/accordion-item.js

import Ember from 'ember';

export default Ember.Component.extend({
  tagName: 'div',
  item: null, // passed-in
  isExpanded: Ember.computed('activeItem', 'item', function() {
    return this.get('activeItem') === this.get('item');
  })
});

components.accordion-list.js

// components/accordion-list.js

import Ember from 'ember';

export default Ember.Component.extend({
  items: null,
  activeItem: null
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  activeItem: null,
  items: Ember.computed(function() {
    return [
      {title: "Item 1", details: "Details here"},
      {title: "Item 2", details: "Details here"},
      {title: "Item 3", details: "Details here"}
    ]
  }),
  actions: {
    addItem() {
      let newItem = {
        title: `Item ${this.get('items.length') + 1}`,
        details: 'Details here'
      }
      this.get('items').addObject(newItem);
      this.set('activeItem', newItem);
    },
    
    toggleActiveItem(item) {
      if (this.get('activeItem') !== item) {
        this.set('activeItem', item);
      } else {
        this.set('activeItem', null);
      } 
    }
  }
});

new-diagram1.txt

                             +---------------------------------+
                             |                                 |
                             |          Parent                 |
                             |                                 |
                             +---------------------------------+
                                                      ||
                                                      || items
                                                      ||
                                                      ||
                                                      ||
                                                     _++_
                                                     \  /
                                                      \/
+------------+    (sibling)  +--------------------------------+
| add|button +---------------+           accordion|list       |
+------------+               +--------------------------------+
                                    /\                 ||
                                   /__\                ||
                                    ||                 || item, activeItem
                                    ||                _++_
                   toggleActiveItem ||                \  /
                                    ||                 \/
                                    ||   +--------------------+
                                    |----+  accordion+item    |
                                    +----+                    |
                                         +--------------------+

new-diagram2.txt

                             +---------------------------------+
          addItem            |                                 |
      +--------------------->|          Parent                 |
      |                      |                                 |
      |                      +--------------+------------------+
      |                          ^          |
      |                  toggleActiveItem   | items, activeItem
      |                          |          v
+-----+------+               ----+-----------------------------+
| add-button |               |           accordion-list        |
+------------+               +---+------------------+----------+
                                 ^                  |
                                 |                  | item, activeItem
                            toggleActiveItem        |
                                 |                  v
                                 |        +--------------------+
                                 |        |  accordion-item    |
                                 +--------+                    |
                                          +--------------------+

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.header {
  background: #eee;
  padding: 10px;
}

.details {
  padding: 10px 10px 20px 10px;
}

templates.application-added.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{accordion-list
    items=items}}
<button {{action "addItem"}}>Add Item </button>
<br>
<br>

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{accordion-list
    activeItem=activeItem
    items=items
    toggleActiveItem=(action 'toggleActiveItem')}}
<button {{action "addItem"}}>Add Item </button>
<br>
<br>

templates.components.accordion-item.hbs

<!-- components/accordion-item.hbs -->

<div class='header' {{action headerClicked}}>{{item.title}} (click to {{if isExpanded 'close' 'open'}})</div>
{{#if isExpanded}}
  <div class='details'>{{item.details}}</div>
{{/if}}

templates.components.accordion-list.hbs

<!-- components/accordion-list.hbs -->
<div>
{{#each items as |item|}}
  {{accordion-item
      item=item
      activeItem=activeItem
      headerClicked=(action attrs.toggleActiveItem item)}}
{{/each}}
</div>

twiddle.json

{
  "version": "0.10.4",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.7.0",
    "ember-data": "2.7.0",
    "ember-template-compiler": "2.7.0"
  },
  "addons": {}
}