tellme.js

Person = Ember.Object.extend({
  fullName: function() {
    var gender = this.get('gender'), parts, married;

    if (gender === "M") {
      parts = ["Mr."];
    } else if (gender === "F") {
      married = this.get('married');
      parts = [married ? "Mrs." : "Ms."];
    } else {
      parts = []
    }

    parts.push(this.get('firstName'));
    parts.push(this.get('lastName'));
  }.property(),

  fullNameDidChange: function() {
    alert("my full name has changed yo!");
  }.observes('fullName')
});

// "prewarming" Person would mean executing the computed property against
// a new object, hoping it was valid (aside: it wouldn't be in many cases,
// but let's pretend). This would result in dependencies of gender, firstName
// and lastName.

// In practice, simply prewarming Person wouldn't work anyway, because:

// the fullName property here depends on gender, firstName and lastName
var yehuda = Person.create({ firstName: "Yehuda", lastName: "Katz" });

// the fullName property here depends on gender, firstName, lastName AND married
var leah = Person.create({ gender: "F", firstName: "Leah", lastName: "Silber", married: false });

// In both cases, note that we have not yet evaluated fullName

leah.set('married', true)

// expected alert, but how can you get it without prewarming when you created?

// Note that computed properties can be arbitrarily expensive, and triggering all
// computed properties when an object was created means triggering computed
// properties that may never be used.