Observe when any Ember.ObjectController in Ember.ArrayController changes

I'm trying to set up mass-action checkboxes (for something like Delete Selected Items) in an Ember App. The idea is to make a mass-action dropdown show or hide if any one of the checkboxes are selected. I haven't put in the dropdown yet since I can't figure out how to observe all elements in the array. How can I:

1. Set up the Array controller so it observes all client objects


2. Get the number of checked items when it changes


3. ALSO: Am I on track with the conventions of how I'm approaching the client itemController?

app/templates/clients.hbs

<section id="clients">
    <h4>my clients</h4>
    <ul>
    {{#each itemController="clients/client"}}
        <li>
            {{input type="checkbox" name="markedForDeletion" checked=markedForDeletion}}
            {{#link-to 'clients.show' this}}
                {{clientName}}
            {{/link-to}}
        </li>
    {{/each}}

    </ul>
</section>
{{#link-to 'clients.new'}}Add client{{/link-to}}
{{outlet}}

router.js

import Ember from 'ember';
import config from './config/environment';

var Router = Ember.Router.extend({
  location: config.locationType
});

export default Router.map(function() {
  this.resource('clients', function() {
    this.route('show', {path: '/:client_id'});
    this.route('new');
  });
});

app/routes/clients.js

import Ember from 'ember';
export default Ember.Route.extend({
    model: function() {
        return this.store.find('client');
    }
});

app/models/client.js

import DS from 'ember-data';
export default DS.Model.extend({
  clientName: DS.attr('string'),
  clientEmail: DS.attr('string')
});

app/controllers/clients.js

import Ember from 'ember';
export default Ember.ArrayController.extend({
    checkBoxesChanged: function() {
    // This fires only once, when the /clients/ route is activated
    console.log('markedForDeletion in array changed');
  }.observes('@each.clients')
});

app/controllers/clients/client.js

import Ember from 'ember';
export default Ember.ObjectController.extend({
    markedForDeletion: true,
    markedForDeletionChanged: function(){
        // This fires correctly
        console.log('markedForDeletion object changed');
    }.observes('markedForDeletion')
});