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Observers

Note: Observers are often over-used by new Ember developers. Observers are used heavily within the Ember framework itself, but for most problems Ember app developers face, computed properties are the appropriate solution.

Ember supports observing any property, including computed properties.

Observers should contain behavior that reacts to changes in another property. Observers are especially useful when you need to perform some behavior after a binding has finished synchronizing.

You can set up an observer on an object by using observer:

import EmberObject, {
  computed,
  observer
} from '@ember/object';

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

  fullName: computed('firstName', 'lastName', function() {
    return `${this.firstName} ${this.lastName}`;
  }),

  fullNameChanged: observer('fullName', function() {
    // deal with the change
    console.log(`fullName changed to: ${this.fullName}`);
  })
});

let person = Person.create({
  firstName: 'Yehuda',
  lastName: 'Katz'
});

// observer won't fire until `fullName` is consumed first
person.get('fullName'); // "Yehuda Katz"
person.set('firstName', 'Brohuda'); // fullName changed to: Brohuda Katz

Because the fullName computed property depends on firstName, updating firstName will fire observers on fullName as well.

Observers and asynchrony

Observers in Ember are currently synchronous. This means that they will fire as soon as one of the properties they observe changes. Because of this, it is easy to introduce bugs where properties are not yet synchronized:

import { observer } from '@ember/object';

Person.reopen({
  lastNameChanged: observer('lastName', function() {
    // The observer depends on lastName and so does fullName. Because observers
    // are synchronous, when this function is called the value of fullName is
    // not updated yet so this will log the old value of fullName
    console.log(this.fullName);
  })
});

This synchronous behavior can also lead to observers being fired multiple times when observing multiple properties:

import { observer } from '@ember/object';

Person.reopen({
  partOfNameChanged: observer('firstName', 'lastName', function() {
    // Because both firstName and lastName were set, this observer will fire twice.
  })
});

person.set('firstName', 'John');
person.set('lastName', 'Smith');

To get around these problems, you should make use of Ember.run.once(). This will ensure that any processing you need to do only happens once, and happens in the next run loop once all bindings are synchronized:

import { observer } from '@ember/object';
import { once } from '@ember/runloop';

Person.reopen({
  partOfNameChanged: observer('firstName', 'lastName', function() {
    once(this, 'processFullName');
  }),

  processFullName() {
    // This will only fire once if you set two properties at the same time, and
    // will also happen in the next run loop once all properties are synchronized
    console.log(this.fullName);
  }
});

person.set('firstName', 'John');
person.set('lastName', 'Smith');

Observers and object initialization

Observers never fire until after the initialization of an object is complete.

If you need an observer to fire as part of the initialization process, you cannot rely on the side effect of set. Instead, specify that the observer should also run after init by using Ember.on():

import EmberObject, { observer } from '@ember/object';
import { on } from '@ember/object/evented';

Person = EmberObject.extend({
  init() {
    this.set('salutation', 'Mr/Ms');
  },

  salutationDidChange: on('init', observer('salutation', function() {
    // some side effect of salutation changing
  }))
});

Unconsumed Computed Properties Do Not Trigger Observers

If you never get() a computed property, its observers will not fire even if its dependent keys change. You can think of the value changing from one unknown value to another.

This doesn't usually affect application code because computed properties are almost always observed at the same time as they are fetched. For example, you get the value of a computed property, put it in DOM (or draw it with D3), and then observe it so you can update the DOM once the property changes.

If you need to observe a computed property but aren't currently retrieving it, get it in your init() method.

Outside of class definitions

You can also add observers to an object outside of a class definition using addObserver():

person.addObserver('fullName', function() {
  // deal with the change
});