What are Computed Properties?
In a nutshell, computed properties let you declare functions as properties. You create one by defining a computed property as a function, which Ember will automatically call when you ask for the property. You can then use it the same way you would any normal, static property.
It's super handy for taking one or more normal properties and transforming or manipulating their data to create a new value.
Computed properties in action
We'll start with a simple example.
We have a Person
object with firstName
and lastName
properties, but we also want a fullName
property that joins the two names when either of them changes:
import EmberObject, { computed } from '@ember/object';
Person = EmberObject.extend({
// these will be supplied by `create`
firstName: null,
lastName: null,
fullName: computed('firstName', 'lastName', function() {
return `${this.firstName} ${this.lastName}`;
})
});
let ironMan = Person.create({
firstName: 'Tony',
lastName: 'Stark'
});
ironMan.fullName; // "Tony Stark"
This declares fullName
to be a computed property, with firstName
and lastName
as the properties it depends on.
The first time you access the fullName
property, the function will be called and the results will be cached.
Subsequent access of fullName
will read from the cache without calling the function.
Changing any of the dependent properties causes the cache to invalidate, so that the computed function runs again on the next access.
Computed properties only recompute when they are consumed
A computed property will only recompute its value when it is consumed. Properties are consumed in two ways:
- By being accessed, for example
ironMan.fullName
- By being referenced in a handlebars template that is currently being rendered, for example
{{ironMan.fullName}}
Outside of those two circumstances the code in the property will not run, even if one of the property's dependencies are changed.
We'll modify the fullName
property from the previous example to log to the console:
import Ember from 'ember';
…
fullName: computed('firstName', 'lastName', function() {
console.log('compute fullName'); // track when the property recomputes
return `${this.firstName} ${this.lastName}`;
})
…
Using the new property, it will only log after a fullName
is accessed, and then only if either the firstName
or lastName
has been previously changed:
let ironMan = Person.create({
firstName: 'Tony',
lastName: 'Stark'
});
ironMan.fullName; // 'compute fullName'
ironMan.set('firstName', 'Bruce') // no console output
ironMan.fullName; // 'compute fullName'
ironMan.fullName; // no console output since dependencies have not changed
Multiple dependents on the same object
In the previous example, the fullName
computed property depends on two other properties of the same object.
However, you may find that you have to observe the properties of a different object.
For example, look at this computed property:
import EmberObject, { computed } from '@ember/object';
const Home = EmberObject.extend({
location: {
streetName: 'Evergreen Terrace',
streetNumber: 742
},
address: computed('location.streetName', 'location.streetNumber', function() {
return `${this.location.streetNumber} ${this.location.streetName}`;
})
});
let home = Home.create()
home.address // 742 Evergreen Terrace
home.set('location.streetNumber', 744)
home.address // 744 Evergreen Terrace
It is important to observe an object's properties, not the object itself that has properties nested inside. If the object reference location
is used as a dependent key, the computed property will not recalculate when the streetName
or streetNumber
properties change.
import EmberObject, { computed } from '@ember/object';
const Home = EmberObject.extend({
location: {
streetName: 'Evergreen Terrace',
streetNumber: 742
},
address: computed('location', function() {
return `${this.location.streetNumber} ${this.location.streetName}`;
})
});
let home = Home.create()
home.address // 742 Evergreen Terrace
home.set('location.streetNumber', 744)
home.address // 742 Evergreen Terrace
home.set('location', {
streetName: 'Evergreen Terrace',
streetNumber: 744
})
home.address // 744 Evergreen Terrace
Since both streetName
and streetNumber
are properties on the location
object, we can use a short-hand syntax called brace expansion to declare the dependents keys.
You surround the dependent properties with braces ({}
), and separate with commas, like so:
import EmberObject, { computed } from '@ember/object';
const Home = EmberObject.extend({
location: {
streetName: 'Evergreen Terrace',
streetNumber: 742
},
address: computed('location.{streetName,streetNumber}', function() {
return `${this.location.streetNumber} ${this.location.streetName}`;
})
});
Chaining computed properties
You can use computed properties as values to create new computed properties.
Let's add a description
computed property to the previous example,
and use the existing fullName
property and add in some other properties:
import EmberObject, { computed } from '@ember/object';
Person = EmberObject.extend({
firstName: null,
lastName: null,
age: null,
country: null,
fullName: computed('firstName', 'lastName', function() {
return `${this.firstName} ${this.lastName}`;
}),
description: computed('fullName', 'age', 'country', function() {
return `${this.fullName}; Age: ${this.age}; Country: ${this.country}`;
})
});
let captainAmerica = Person.create({
firstName: 'Steve',
lastName: 'Rogers',
age: 80,
country: 'USA'
});
captainAmerica.get('description'); // "Steve Rogers; Age: 80; Country: USA"
Dynamic updating
Computed properties, by default, observe any changes made to the properties they depend on and are dynamically updated when they're called. Let's use computed properties to dynamically update.
captainAmerica.set('firstName', 'William');
captainAmerica.description; // "William Rogers; Age: 80; Country: USA"
So this change to firstName
was observed by fullName
computed property, which was itself observed by the description
property.
Setting any dependent property will propagate changes through any computed properties that depend on them, all the way down the chain of computed properties you've created.
Setting Computed Properties
You can also define what Ember should do when setting a computed property. If you try to set a computed property, it will be invoked with the key (property name), and the value you want to set it to. You must return the new intended value of the computed property from the setter function.
import EmberObject, { computed } from '@ember/object';
Person = EmberObject.extend({
firstName: null,
lastName: null,
fullName: computed('firstName', 'lastName', {
get(key) {
return `${this.firstName} ${this.lastName}`;
},
set(key, value) {
let [firstName, lastName] = value.split(/\s+/);
this.set('firstName', firstName);
this.set('lastName', lastName);
return value;
}
})
});
let captainAmerica = Person.create();
captainAmerica.set('fullName', 'William Burnside');
captainAmerica.firstName; // William
captainAmerica.lastName; // Burnside
Computed property macros
Some types of computed properties are very common. Ember provides a number of computed property macros, which are shorter ways of expressing certain types of computed property.
In this example, the two computed properties are equivalent:
import EmberObject, { computed } from '@ember/object';
import { equal } from '@ember/object/computed';
Person = EmberObject.extend({
fullName: 'Tony Stark',
isIronManLongWay: computed('fullName', function() {
return this.fullName === 'Tony Stark';
}),
isIronManShortWay: equal('fullName', 'Tony Stark')
});
To see the full list of computed property macros, have a look at the API documentation