change value and recheck.js

SC.RunLoop.begin();
Arrayobservers.setPath('objects.1.foo', 5); // don't know what you are actually doing 
SC.RunLoop.end();


Arrayobservers.contentNotifyingObjects.get('computedProperty'); //should be 6

define.js

Arrayobservers = SC.Application.create({

  NAMESPACE: 'Arrayobservers',
  VERSION: '0.1.0',

  store: SC.Store.create().from(SC.Record.fixtures)
  
});

Arrayobservers.objects = [SC.Object.create({foo: 1}), SC.Object.create({foo: 2})];

Arrayobservers.objectsController = SC.ArrayController.create();

Arrayobservers.contentNotifyingObjects = SC.Controller.create({
  objectsBinding: SC.Binding.oneWay('Arrayobservers.objectsController*content'),

  // or perhaps this instead of the other one.
  //objectsBinding: SC.Binding.oneWay('Arrayobservers*objectsController),

  computedProperty: function() {
      console.log('computedProperty', this.get('objects').getEach('foo').get('@sum'));
  }.property('objects')
});

SC.RunLoop.begin();
Arrayobservers.setPath('objectsController.content', Arrayobservers.objects);
SC.RunLoop.end();

firstcheck.js

Arrayobservers.contentNotifyingObjects.get('computedProperty'); //should be 3