Blog: Simpler and more powerful components in Ember Octane with Glimmer Components

Introducing Glimmer components

The release of the Ember Octane edition back in December was one of Ember's biggest releases, bringing with it modern and streamlined APIs. At the core of the release are tracked properties and Glimmer components. While Octane has been out for quite some time, and subsequently Glimmer components, you are likely using Ember (or "classic") components in your application. To give some context as to the impact of Glimmer components in the Ember mental model, I'll be introducing Glimmer components from the viewpoint of classic Components.

The key points we will be addressing in this post are:

• Glimmer components use native class syntax and do not extend from EmberObject.
• Separation of internal state (properties) and external state (arguments).
• HTML-first approach makes for a much simpler API surface.

Native class

The first thing that catches the eye is that Glimmer components have a different base class with a radically different API (see Ember Component vs Glimmer component).

With the new Glimmer component implementation you are expected to use the native class syntax, but it also changes something more important: a Glimmer component no longer inherits from EmberObject. Now you should use native getters and setters, as well as tracked properties.

How you define the component actions also requires an adjustment. First, a bit of historical context. One of the first proposals for Ember components had actions defined as a member function of the component, like so:

export default Component.extend({
  myActionHandler() {
    console.log('myActionHandler triggered');
  },
});

Unfortunately, due to the API surface of components and the fact that one of the component lifecycle hooks was named destroy, that meant that users would unknowingly override component hooks and introduce strange bugs to their application. To address this, a system was conceived where you define component actions in the actions hash:

export default Component.extend({
  actions: {
    myActionHandler() {
      console.log('myActionHandler triggered');
    },
  },
});

But now that Glimmer components have quite a small API surface (roughly willDestroy, isDestroying and isDestroyed), we can go back to defining them as methods in the component, and then refer to them directly in the template.

So putting these things together, let's look at a component that receives a type of pasta and a type of sauce and displays it with a button to order. The component is invoked like so:

<OrderPasta @pasta="Spaghetti" @sauce="Carbonara" />

Now let us look at how both implementations differ. First, the Ember component:

/// app/components/order-pasta.js
import Component from '@ember/component';

export default Component.extend({
  dishName: computed('pasta', 'sauce', function () {
    return `${this.pasta} ${this.sauce}`;
  }),

  actions: {
    handleReservation() {
      console.log(`Ordered a plate of ${this.dishName}`);
    },
  },
});

<div>{{this.dishName}} <button {{action 'handleReservation'}}>Reserve</button></div>

And now the Glimmer component:

// app/components/order-pasta.js
import Component from '@ember/component';
import { tracked } from '@glimmer/tracking';
import { action } from '@ember/object';

export default class OrderPasta extends Component {
  get dishName() {
    return `${this.args.pasta} ${this.args.sauce}`;
  }

  @action
  handleReservation() {
    console.log(`Ordered a plate of ${this.dishName}`);
  }
}

<div>{{this.dishName}} <button {{on 'click' this.handleReservation}}>Reserve</button></div>

I won't go into details on the action to on modifier changes, but notice how in the Glimmer component we reference the callback directly, we used a native getter, and we defined the action handler as a decorated method. You might also have noticed that in the Glimmer component code I have referred to the passed-in values as this.args, which brings us to our next point.

Separation of properties and arguments

One behavior of classic components that can sneak up on even more experienced developers is the fact that classic components reflect passed-in state (arguments) onto internal state (properties). This has a big implication. Since properties and arguments are directly linked in classic components, you can effectively override any property from the outside by calling the component with an argument with the same name. You cannot refer to the argument's original value from within the component.

Glimmer components do away with this behavior by separating arguments into the args property of the component, and by making named argument bindings read-only, which gives developers a better guarantee that the data flow in a component tree isn't accidentally two-way bound.

Now developers have to be more explicit about certain patterns, for example default values for component properties.

Let's take the previous OrderPasta component and make the pasta type optional. I will only include the code necessary for this change to keep the code samples focused.

For the Ember component, we can see that adding a property with the proper name is enough. If the component is invoked with a different pasta argument, this will be overwritten.

/// app/components/order-pasta.js
import Component from '@ember/component';

export default Component.extend({
  pasta: 'Spaghetti',
});

For the Glimmer component we have to do a bit more legwork. We'll assume that pasta can change over time, so we will add a native getter so it gets automatically updated. We will also need to change our dishName getter to refer to the property instead.

// app/components/order-pasta.js
export default class OrderPasta extends Component {
  get pasta() {
    return this.args.pasta || 'Spaghetti';
  }

  get dishName() {
    return `${this.pasta} ${this.args.sauce}`;
  }
}

As you can see, we can now reason about the code locally instead of having to rely on the framework knowledge that pasta would be overwritten. We can use the name pasta for both the property and the argument because the syntax makes it clear which is which. If you need access to the argument you use this.args.pasta in the class or {{@pasta}} in the template. If you need to access the property, you use this.pasta in the class and {{this.pasta}} in the template.

If you checked the API documentation linked at the beginning of the blog post, you might still be wondering how Glimmer components managed to reduce the API surface so much. We will not be covering lifecycle hooks in this post (which account for the removal of didInsertElement, didReceiveAttrs, didRender, didUpdate, didUpdateAttrs, willRender, willUpdate), instead we will be focusing on the APIs that allows one to customize the wrapper element of Ember Components.

HTML-first APIs

Ember components have been around since before the 1.0 release, where they were introduced as isolated views. Views are long gone from everyday applications, but their legacy still lives on in Ember components. Ember components have a very distinct characteristic, the component's template is wrapped by a hidden element, <div> by default. To customize this element, you have to use certain APIs in the component's class, like tagName, classNameBindings and attributeBindings.

Let us go with a practical example to make it clearer. We are going to make a button component that receives a primary CSS class and a disabled HTML attribute. The component template itself will be the content for the button.

// app/components/my-button.js
import Component from '@ember/component';

export default Component.extend({
  tagName: 'button',

  attributeBindings: ['isDisabled:disabled'],
  isDisabled: false,

  classNameBindings: ['buttonType'],
  buttonType: 'primary',
});

// app/components/my-button.hbs
{{yield}}

Now you can call MyButton and dynamically change the class and the attribute:

<MyButton>Click me</MyButton>
{{!-- renders --}} <button class="primary">Click me</button>

<MyButton @isDisabled={{true}}>Click me</MyButton>
{{!-- renders --}} <button class="primary" disabled>Click me</button>

<MyButton @buttonType={{'secondary'}}>Click me</MyButton>
{{!-- renders --}} <button class="secondary">Click me</button>

<MyButton @isDisabled={{true}} @buttonType={{'secondary'}}>Click me</MyButton>
{{!-- renders --}} <button class="secondary" disabled>Click me</button>

You might look at this example and think, why are we using JavaScript to specify how we want the HTML to look when we have templates and I would agree with you. More than that, the Ember team agrees with you, that's why Glimmer components no longer have an implicit wrapper element so now you can configure this in the component's template itself. This, coupled with being able to specify HTML attributes and the new ...attributes make template-only components much more feasible.

Let's implement MyButton in Glimmer. As mentioned, we are moving the code from the component class to the template, so we don't need the JavaScript file.

// app/components/my-button.hbs
<button
  class={{if @buttonType @buttonType 'primary'}}
  ...attributes
>
  {{yield}}
</button>

Much more straightforward! In {{if @buttonType @buttonType 'primary'}} we are saying that the value should be primary if @buttonType is not defined, and in ...attribute we are telling Ember where to put any HTML attributes that are passed in when calling the component.

In this case we do not want to pass class as an HTML attribute when calling the component because class is special in that it merged together the existing classes in the component with whatever you pass to the component:

// app/components/my-button.hbs
<button
  class='primary'
  ...attributes
>
  {{yield}}
</button>```

```handlebars
<MyButton class="two">Multiple classes</MyButton>

Renders:

<button class="primary two">Multiple classes</button>

For the sake of exemplifying attributeBindings I glossed over the fact that Ember will apply any HTML attributes you pass to an Ember component to the implicit wrapper element. Now that we are explicitly using ...attributes in our Glimmer component, we need to update how we're calling the component:

<MyButton>Click me</MyButton>
{{!-- renders --}} <button class="primary">Click me</button>

<MyButton disabled>Click me</MyButton>
{{!-- renders --}} <button class="primary" disabled>Click me</button>

<MyButton @buttonType={{'secondary'}}>Click me</MyButton>
{{!-- renders --}} <button class="secondary">Click me</button>

<MyButton @buttonType={{'secondary'}} disabled={{true}}>Click me</MyButton>
{{!-- renders --}} <button class="secondary" disabled>Click me</button>

A common point of dissatisfaction with frameworks, including Ember, is the so called "magic". This is when the framework does something for you that you have no insight into, so you may not understand what is happening or why. For example, Ember components having an implicit wrapper element, and Ember automatically applying ...attributes to it.

With the introduction of Glimmer components in the Octane edition plus the template improvements already mentioned in a previous post, Ember has made it much easier to reason locally about your component. By keeping layout logic in the layout, instead of the component class you only have to look in one place to know what the component will render. By moving towards native JavaScript syntax and functionality, Ember has diminished the uncertainty of whether a certain functionality is provided by Ember or by JavaScript.

I hope this exploration of Glimmer components and Ember components was useful for you and at the very least gave you a renewed appreciation for the Octane Edition design effort.

If you are looking for help migrating your codebase to these new idioms, or you want to level up your engineering team, contact us so we can work together towards achieving your goals.