example.js

var myContact = SC.Object.create({

  firstName: 'John',
  lastName:  'Doe',

  fullName: function() {
    return this.getEach('firstName', 'lastName').join(' ');
  }.property('firstName', 'lastName').cacheable(),

  fullNameDidChange: function() {
    console.log('FULL NAME CHANGE!');
  }.observes('fullName');

});

myContact.get('fullName');  // 'John Doe';

myContact.set('firstName', 'Frank');
>> FULL NAME CHANGE!! <-- because of dependent keys