Classes and Instances

Classes and Instances

As you learn about Ember, you'll see code like Ember.Component.extend() and DS.Model.extend(). Here, you'll learn about this extend() method, as well as other major features of the Ember object model.

Defining Classes

To define a new Ember class, call the extend() method on Ember.Object:

const Person = Ember.Object.extend({
  say(thing) {
    alert(thing);
  }
});

This defines a new Person class with a say() method.

You can also create a subclass from any existing class by calling its extend() method. For example, you might want to create a subclass of Ember's built-in Ember.Component class:

export default Ember.Component.extend({
  classNameBindings: ['isUrgent'],
  isUrgent: true
});

Overriding Parent Class Methods

When defining a subclass, you can override methods but still access the implementation of your parent class by calling the special _super() method:

const Person = Ember.Object.extend({
  say(thing) {
    alert(`${this.get('name')} says: ${thing}`);
  }
});

const Soldier = Person.extend({
  say(thing) {
    // this will call the method in the parent class (Person#say), appending
    // the string ', sir!' to the variable `thing` passed in
    this._super(`${thing}, sir!`);
  }
});

let yehuda = Soldier.create({
  name: 'Yehuda Katz'
});

yehuda.say('Yes'); // alerts "Yehuda Katz says: Yes, sir!"

In certain cases, you will want to pass arguments to _super() before or after overriding.

This allows the original method to continue operating as it normally would.

One common example is when overriding the normalizeResponse() hook in one of Ember-Data's serializers.

A handy shortcut for this is to use a "spread operator", like ...arguments:

normalizeResponse(store, primaryModelClass, payload, id, requestType)  {
  // Customize my JSON payload for Ember-Data
  return this._super(...arguments);
}

The above example returns the original arguments (after your customizations) back to the parent class, so it can continue with its normal operations.

Creating Instances

Once you have defined a class, you can create new instances of that class by calling its create() method. Any methods, properties and computed properties you defined on the class will be available to instances:

const Person = Ember.Object.extend({
  say(thing) {
    alert(`${this.get('name')} says: ${thing}`);
  }
});

let person = Person.create();

person.say('Hello'); // alerts " says: Hello"

When creating an instance, you can initialize the values of its properties by passing an optional hash to the create() method:

const Person = Ember.Object.extend({
  helloWorld() {
    alert(`Hi, my name is ${this.get('name')}`);
  }
});

let tom = Person.create({
  name: 'Tom Dale'
});

tom.helloWorld(); // alerts "Hi, my name is Tom Dale"

Note that for performance reasons, while calling create() you cannot redefine an instance's computed properties and should not redefine existing or define new methods. You should only set simple properties when calling create(). If you need to define or redefine methods or computed properties, create a new subclass and instantiate that.

By convention, properties or variables that hold classes are PascalCased, while instances are not. So, for example, the variable Person would point to a class, while person would point to an instance (usually of the Person class). You should stick to these naming conventions in your Ember applications.

Initializing Instances

When a new instance is created, its init() method is invoked automatically. This is the ideal place to implement setup required on new instances:

const Person = Ember.Object.extend({
  init() {
    alert(`${this.get('name')}, reporting for duty!`);
  }
});

Person.create({
  name: 'Stefan Penner'
});

// alerts "Stefan Penner, reporting for duty!"

If you are subclassing a framework class, like Ember.Component, and you override the init() method, make sure you call this._super(...arguments)! If you don't, a parent class may not have an opportunity to do important setup work, and you'll see strange behavior in your application.

Arrays and objects defined directly on any Ember.Object are shared across all instances of that object.

const Person = Ember.Object.extend({
  shoppingList: ['eggs', 'cheese']
});

Person.create({
  name: 'Stefan Penner',
  addItem() {
    this.get('shoppingList').pushObject('bacon');
  }
});

Person.create({
  name: 'Robert Jackson',
  addItem() {
    this.get('shoppingList').pushObject('sausage');
  }
});

// Stefan and Robert both trigger their addItem.
// They both end up with: ['eggs', 'cheese', 'bacon', 'sausage']

To avoid this behavior, it is encouraged to initialize those arrays and object properties during init(). Doing so ensures each instance will be unique.

const Person = Ember.Object.extend({
  init() {
    this.set('shoppingList', ['eggs', 'cheese']);
  }
});

Person.create({
  name: 'Stefan Penner',
  addItem() {
    this.get('shoppingList').pushObject('bacon');
  }
});

Person.create({
  name: 'Robert Jackson',
  addItem() {
    this.get('shoppingList').pushObject('sausage');
  }
});

// Stefan ['eggs', 'cheese', 'bacon']
// Robert ['eggs', 'cheese', 'sausage']

Accessing Object Properties

When accessing the properties of an object, use the get() and set() accessor methods:

const Person = Ember.Object.extend({
  name: 'Robert Jackson'
});

let person = Person.create();

person.get('name'); // 'Robert Jackson'
person.set('name', 'Tobias Fünke');
person.get('name'); // 'Tobias Fünke'

Make sure to use these accessor methods; otherwise, computed properties won't recalculate, observers won't fire, and templates won't update.