Ember Computed Property on Array

I'm an Ember newbie, so forgive me if I've missed something obvious (I've spent time Googling this issue and still can't find a solution) but it seems to me that Ember computed properties aren't working as documented/intended on array properties like length.

I'm trying to build my own queue:

// app/custom-objects/processing-queue-item.js
import Ember from 'ember';

export default Ember.Object.extend({
  payload: null,
  extraContext: null,
  processingState: 'pending', // pending, succeeded, failed
  processingOutcome: null,    // null for pending, result for      succeeded, error for failed

  toString() {
    return `{ProcessingQueueItem: processingState=${this.get('processingState')}, processingOutcome=${this.get('processingOutcome')}, extraContext=${this.get('extraContext')}, payload=${this.get('payload')}}`;
  }
});

// app/custom-objects/processing-queue.js
import Ember from 'ember';
import ProcessingQueueItem from './processing-queue-item';

export default Ember.Object.extend(Ember.Enumerable, {
  queueName: null,

init() {
  this.set('items', []);
  this.get('items');
  this.get('items.length');
  this.get('length'); // Force observation
},

/*
 * Public API
 */

enqueue(payload, extraContext = null) {
 let itemToEnqueue = ProcessingQueueItem.create({ payload: payload, extraContext: extraContext });

 this.get('items').pushObject(itemToEnqueue);
 this.scheduleProcessing();

 return itemToEnqueue;
},

toString() {
  return `{ProcessingQueue: queueName=${this.get('queueName')}, length=${this.get('length')}}`;
},

 /*
  * Internal API
  */

scheduleProcessing() {
  Ember.run(() => {
    this.maybeProcessAnItem();
  });
},

maybeProcessAnItem() {
  console.log(`maybe process an item ${this}`);
},

/*
 * Ember.Enumerable mixin
 */

length: Ember.computed('items.length', function() {
  return this.get('items.length');
}),

nextObject(index, previousObject, context) {
  return this.get('items').nextObject(index, previousObject, context);
}
});

This class is incomplete, but I want to start displaying queue contents in a template to help with debugging but I can't get that to work. Here are my controller and template:

// app/controllers/dashboard.js
import Ember from 'ember';
import ProcessingQueue from '../custom-objects/processing-queue';

export default Ember.Controller.extend({
  init() {
  this._super(...arguments);
  this.set('processingQueue', ProcessingQueue.create({ queueName: 'DashboardQueue' }));
  this.get('processingQueue');
  this.get('processingQueue.length');
  this.get('queueLength');
 },

 queueLength: Ember.computed('processingQueue.length', function() {
   return this.get('processingQueue.length');
 }),
});

// app/templates/dashboard.hbs
<h1>Dashboard</h1>

<h2>Queue Length: ''</h2>

<p></p>

The problem is, in the <h2>Queue Length: ''</h2>, the queue length is always undefined until I add items to the queue. But this is not true, the queue has an empty array and a length of 0. Using $E from the dashboard controller from EmberInspector I can see that $E.get('processingQueue.length') and $E.get('queueLength') are both undefined.

What's strange is that as soon as I add items to the queue, the queue length becomes defined, 1, 2, 3, ... and keeps up and syncs the template as I add queue items. So the first $E.get('processingQueue').enqueue('foo') automagically updates the template to show a queue length of '0', then '1' and so on.

Why is it undefined though before I've enqueued any items? I tried adding gets all over the place according to Unconsumed Computed Properties Do No Trigger Observers but that doesn't seem to help.

Any ideas? It's entirely possible that I misunderstand something about computed properties here, but I don't understand what and why ... I've tried volatile(), [], @each and all that and I can't get that to make a difference either. Something is not right ...

Any help would be hugely appreciated and I'd be willing to add to the Wiki, write a blog post and maybe release my queue as open source as a thank you. :-)

Thanks! And thanks again for making Ember so awesome!