Person = Ember.Object.extend({

  // these will be supplied by `create`

  firstName: null,

  lastName: null,

​

  fullName: function() {

    var firstName = this.get('firstName');

    var lastName = this.get('lastName');

​

    return firstName + ' ' + lastName;

  }.property('firstName', 'lastName')

});

​

var person = Person.create({

  firstName: "Yehuda",

  lastName: "Katz"

});

​

person.addObserver('fullName', function() {

    alert('hello');

});

​

//person.get('fullName'); // if I add this line, observer will really fire

person.set('firstName', "Brohuda"); // observer will fire