run helper on update

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle',
  stuff: ['grapes', 'animals', 'cheese'],
  selectedStuff: ['animals'],
  actions: {
    toggleThing(thing){
      if(this.get('selectedStuff').contains(thing)){
        console.log('remove ' + thing);
        this.get('selectedStuff').removeObject(thing);
      } else {
        console.log('add ' + thing);
        this.get('selectedStuff').pushObject(thing);
      }
    }
  }
});

helpers.is-in.js

import Ember from 'ember';

export function isIn(params) {
  return params[0].contains(params[1]);
}

export default Ember.Helper.extend({
  compute: function([array, item]) {
    this.set('_array', array);

    return array.contains(item);
  },

  recomputeOnArrayChange: Ember.observer('_array.[]', function() {
    this.recompute();
  })
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
<ul>
{{#each stuff as |thing|}}
  <li {{action 'toggleThing' thing preventDefault=false}}>
    <input type='checkbox' checked={{is-in selectedStuff thing}} />{{thing}}
  </li>
{{/each}}
</ul>
<br>
<br>
Selected Stuff:
<ul>
{{#each selectedStuff as |thing|}}
  <li>{{thing}}</li>
{{/each}}
</ul>

twiddle.json

{
  "version": "0.4.7",
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.6/ember.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/1.13.7/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.6/ember-template-compiler.js"
  }
}