gistfile1.js

var obj = SC.Object.extend({foo: null})
var array = [obj.create({foo: 'bar'})]
function arrayObserver(array) { console.log('called') }
var observer = SC.RangeObserver.create(array, SC.IndexSet.create(0, array.get('length')), undefined, arrayObserver, YES)
observer.beginObserving()
array.replace(0, obj.create({foo: 'baz'}))
array.objectAt(0).set('foo', 'fnord')