ComputedProperty

A computed property transforms an object literal with object's accessor function(s) into a property.

By default the function backing the computed property will only be called once and the result will be cached. You can specify various properties that your computed property depends on. This will force the cached result to be recomputed if the dependencies are modified.

In the following example we declare a computed property - fullName - by calling .Ember.computed() with property dependencies (firstName and lastName) as leading arguments and getter accessor function. The fullName getter function will be called once (regardless of how many times it is accessed) as long as its dependencies have not changed. Once firstName or lastName are updated any future calls (or anything bound) to fullName will incorporate the new values.

let Person = Ember.Object.extend({
  // these will be supplied by `create`
  firstName: null,
  lastName: null,

  fullName: Ember.computed('firstName', 'lastName', function() {
    let firstName = this.get('firstName'),
        lastName  = this.get('lastName');

    return firstName + ' ' + lastName;
  })
});

let tom = Person.create({
  firstName: 'Tom',
  lastName: 'Dale'
});

tom.get('fullName') // 'Tom Dale'

You can also define what Ember should do when setting a computed property by providing additional function (set) in hash argument. If you try to set a computed property, it will try to invoke setter accessor function with the key and value you want to set it to as arguments.

let Person = Ember.Object.extend({
  // these will be supplied by `create`
  firstName: null,
  lastName: null,

  fullName: Ember.computed('firstName', 'lastName', {
    get(key) {
      let firstName = this.get('firstName'),
          lastName  = this.get('lastName');

      return firstName + ' ' + lastName;
    },
    set(key, value) {
      let [firstName, lastName] = value.split(' ');

      this.set('firstName', firstName);
      this.set('lastName', lastName);

      return value;
    }
  })
});

let person = Person.create();

person.set('fullName', 'Peter Wagenet');
person.get('firstName'); // 'Peter'
person.get('lastName');  // 'Wagenet'

You can overwrite computed property with normal property (no longer computed), that won't change if dependencies change, if you set computed property and it won't have setter accessor function defined.

You can also mark computed property as .readOnly() and block all attempts to set it.

let Person = Ember.Object.extend({
  // these will be supplied by `create`
  firstName: null,
  lastName: null,

  fullName: Ember.computed('firstName', 'lastName', {
    get(key) {
      let firstName = this.get('firstName');
      let lastName  = this.get('lastName');

      return firstName + ' ' + lastName;
    }
  }).readOnly()
});

let person = Person.create();
person.set('fullName', 'Peter Wagenet'); // Uncaught Error: Cannot set read-only property "fullName" on object: <(...):emberXXX>

Additional resources: - New CP syntax RFC - New computed syntax explained in "Ember 1.12 released"