New Twiddle

controllers.application.js

import Ember from 'ember';
import SelectionHandler from '../utils/selection';

export default Ember.Controller.extend({
  content: [1, 2, 3, 4, 5],
  init() {
    this._super(...arguments);
    this.set('selectionHandler', SelectionHandler.create());
    Ember.bind(this, 'selectionHandler.content', 'content');
  },
  
  actions: {
    selectItem(item) {
      this.get('selectionHandler').select(item);
    },
    
    removeItem(item) {
      let content = this.get('content');
      this.set('content', content.removeObject(item));
    }
  }
});

templates.application.hbs

<h2>Click to (de)select</h2>
<br>
Content: {{content.length}}
{{#each content as |item|}}
	<div>
  	<span {{action "selectItem" item}}>Item {{item}}</span>
    <button {{action "removeItem" item}}>X</button>
  </div>
{{/each}}
<br>
Selection content: {{selectionHandler.content.length}}<br>
Selection selected: {{selectionHandler.selection.length}}

{{#each selectionHandler.selection as |item|}}
	<div>Item {{item}}</div>
{{/each}}

twiddle.json

{
  "version": "0.10.5",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.8.0",
    "ember-data": "2.8.0",
    "ember-template-compiler": "2.8.0",
    "ember-testing": "2.8.0"
  },
  "addons": {}
}

utils.selection.js

import Ember from 'ember';
import get from 'ember-metal/get';
import set from 'ember-metal/set';

import computed from 'ember-computed';

export default Ember.Object.extend({
  content: null,
  _selection: null,
  
  selection: computed('_selection.[]', 'content.[]', function() {
    let content = get(this, 'content');
    let selection = get(this, '_selection');
    
    if (!selection || !content) {
      return Ember.A();
    }
    
    return content.filter((item) => {
      return selection.indexOf(item) !== -1;
    });
  }),

  init() {
    this._super(...arguments);
    set(this, 'content', Ember.A());
    set(this, '_selection', Ember.A());
  },

  setContent(list) {
    if (list && get(list, 'length')) {
      set(this, 'content', list);
    }
  },

  select(item) {
    let selection = get(this, '_selection');

    if (selection.includes(item)) {
      selection.removeObject(item);
    } else {
      selection.pushObject(item);
    }
  },

  isSelected(item) {
    return get(this, 'selection').includes(item);
  },

  selectAll() {
    set(this, '_selection', get(this, 'content'));
  },

  clear() {
    set(this, '_selection', Ember.A());
  }
});