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Defining a Component

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To define a component, run:

ember generate component my-component-name

Ember components are used to turn markup text and styles into reusable content. Components consist of two parts: a JavaScript component file that defines behavior, and its accompanying Handlebars template that defines the markup for the component's UI.

Components must have at least one dash in their name. So blog-post is an acceptable name, and so is audio-player-controls, but post is not. This prevents clashes with current or future HTML element names, aligns Ember components with the W3C Custom Elements spec, and ensures Ember detects the components automatically.

A sample component template could look like this:

app/templates/components/blog-post.hbs
<article class="blog-post">
  <h1>{{title}}</h1>
  <p>{{yield}}</p>
  <p>Edit title: {{input type="text" value=title}}</p>
</article>

Given the above template, you can now use the {{blog-post}} component:

app/templates/index.hbs
{{#each model as |post|}}
  {{#blog-post title=post.title}}
    {{post.body}}
  {{/blog-post}}
{{/each}}

Its model is populated in model hook in the route handler:

app/routes/index.js
import Route from '@ember/routing/route';

export default Route.extend({
  model() {
    return this.get('store').findAll('post');
  }
});

Each component is backed by an element under the hood. By default, Ember will use a <div> element to contain your component's template. To learn how to change the element Ember uses for your component, see Customizing a Component's Element.

Defining a Component Subclass

Often times, your components will just encapsulate certain snippets of Handlebars templates that you find yourself using over and over. In those cases, you do not need to write any JavaScript at all. Define the Handlebars template as described above and use the component that is created.

If you need to customize the behavior of the component you'll need to define a subclass of Component. For example, you would need a custom subclass if you wanted to change a component's element, respond to actions from the component's template, or manually make changes to the component's element using JavaScript.

Ember knows which subclass powers a component based on its filename. For example, if you have a component called blog-post, you would create a file at app/components/blog-post.js. If your component was called audio-player-controls, the file name would be at app/components/audio-player-controls.js.

Dynamically rendering a component

The {{component}} helper can be used to defer the selection of a component to run time. The {{my-component}} syntax always renders the same component, while using the {{component}} helper allows choosing a component to render on the fly. This is useful in cases where you want to interact with different external libraries depending on the data. Using the {{component}} helper would allow you to keep different logic well separated.

The first parameter of the helper is the name of a component to render, as a string. So {{component 'blog-post'}} is the same as using {{blog-post}}.

The real value of {{component}} comes from being able to dynamically pick the component being rendered. Below is an example of using the helper as a means of choosing different components for displaying different kinds of posts:

app/templates/components/foo-component.hbs
<h3>Hello from foo!</h3>
<p>{{post.body}}</p>
app/templates/components/bar-component.hbs
<h3>Hello from bar!</h3>
<div>{{post.author}}</div>
app/routes/index.js
import Route from '@ember/routing/route';

export default Route.extend({
  model() {
    return this.get('store').findAll('post');
  }
});
app/templates/index.hbs
{{#each model as |post|}}
  {{!-- either foo-component or bar-component --}}
  {{component post.componentName post=post}}
{{/each}}

When the parameter passed to {{component}} evaluates to null or undefined, the helper renders nothing. When the parameter changes, the currently rendered component is destroyed and the new component is created and brought in.

Picking different components to render in response to the data allows you to have different template and behavior for each case. The {{component}} helper is a powerful tool for improving code modularity.

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