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Component templates can leave a placeholder that users of the component can fill with their own HTML.

To make that more concrete, let's take a look at two similar components representing different user's messages.

"app/components/received-message.hbs"
<aside>
  <div class="avatar is-active" title="Tomster's avatar">T</div>
</aside>
<section>
  <h4 class="username">
    Tomster
    <span class="local-time">their local time is 4:56pm</span>
  </h4>

  <p>
    Hey Zoey, have you had a chance to look at the EmberConf
    brainstorming doc I sent you?
  </p>
</section>
"app/components/sent-message.hbs"
<aside class="current-user">
  <div class="avatar" title="Zoey's avatar">Z</div>
</aside>
<section>
  <h4 class="username">Zoey</h4>

  <p>Hey!</p>

  <p>
    I love the ideas! I'm really excited about where this year's
    EmberConf is going, I'm sure it's going to be the best one yet.
    Some quick notes:
  </p>

  <ul>
    <li>
      Definitely agree that we should double the coffee budget this
      year (it really is impressive how much we go through!)
    </li>
    <li>
      A blimp would definitely make the venue very easy to find, but
      I think it might be a bit out of our budget. Maybe we could
      rent some spotlights instead?
    </li>
    <li>
      We absolutely will need more hamster wheels, last year's line
      was <em>way</em> too long. Will get on that now before rental
      season hits its peak.
    </li>
  </ul>

  <p>Let me know when you've nailed down the dates!</p>
</section>

Instead of having two different components, one for sent messages and one for received messages, we could instead try to create a single message component. Inside of this message component, we could substitute the avatar and username with generic components, too.

Their structure is pretty straightforward and similar, so we can use arguments and conditionals to handle the differences in content between them (see the previous chapters for details on how to do this).

"app/components/message.hbs"
<Message::Avatar
  @title={{@avatarTitle}}
  @initial={{@avatarInitial}}
  @isActive={{@userIsActive}}
  class="{{if @isCurrentUser "current-user"}}"
/>
<section>
  <Message::Username
    @name={{@username}}
    @localTime={{@userLocalTime}}
  />

  ...
</section>

This works pretty well, but the message content is very different. It's also pretty long, so it wouldn't really be easy to pass the message content through as an argument. What we really want is to leave a placeholder for any content supplied by the <Message> tag.

The way to do this in Ember is by using the {{yield}} syntax.

"app/components/message.hbs"
<Message::Avatar
  @title={{@avatarTitle}}
  @initial={{@avatarInitial}}
  @isActive={{@userIsActive}}
  class="{{if @isCurrentUser "current-user"}}"
/>
<section>
  <Message::Username
    @name={{@username}}
    @localTime={{@userLocalTime}}
  />

  {{yield}}
</section>
Zoey says...
{{yield}} is named after a similar concept in scripting languages, including Ruby, JavaScript and Python. You don't need to understand the connection in order to use it, but if you're in the mood for a tangent, check out the yield operator in JavaScript

You can think of using {{yield}} as leaving a placeholder for the content of the <Message> tag.

"app/components/received-message.hbs"
<Message
  @username="Tomster"
  @userIsActive={{true}}
  @userLocalTime="4:56pm"

  @avatarTitle="Tomster's avatar"
  @avatarInitial="T"
>
  <p>
    Hey Zoey, have you had a chance to look at the EmberConf
    brainstorming doc I sent you?
  </p>
</Message>
"app/components/sent-message.hbs"
<Message
  @username="Zoey"
  @isCurrentUser={{true}}

  @avatarTitle="Zoey's avatar"
  @avatarInitial="Z"
>
  <p>Hey!</p>

  <p>
    I love the ideas! I'm really excited about where this year's
    EmberConf is going, I'm sure it's going to be the best one yet.
    Some quick notes:
  </p>

  <ul>
    <li>
      Definitely agree that we should double the coffee budget this
      year (it really is impressive how much we go through!)
    </li>
    <li>
      A blimp would definitely make the venue very easy to find, but
      I think it might be a bit out of our budget. Maybe we could
      rent some spotlights instead?
    </li>
    <li>
      We absolutely will need more hamster wheels, last year's line
      was <em>way</em> too long. Will get on that now before rental
      season hits its peak.
    </li>
  </ul>

  <p>Let me know when you've nailed down the dates!</p>
</Message>

As shown here, we can pass different content into the tag. The content of the tag is also referred to as the block. The {{yield}} syntax yields to the block once the block is passed into the component.

Zoey says...
You can think of the Message component like a function, and the block as a callback that you're passing to the component. From this perspective, the {{yield}} syntax calls the callback.

Conditional Blocks

Sometimes, we may want to provide some default content if the user of a component hasn't provided a block. For instance, consider an error message dialog that has a default message in cases where we don't know what error occurred. We could show the default message using the (has-block) syntax in an ErrorDialog component.

app/components/error-dialog.hbs
<dialog>
  {{#if (has-block)}}
    {{yield}}
  {{else}}
    An unknown error occurred!
  {{/if}}
</dialog>

Now, if we use our ErrorDialog component without a block, we'll get the default message.

<ErrorDialog/>

<!-- rendered -->
<dialog>
  An unknown error occurred!
</dialog>

If we had a more detailed message, though, we could use the block to pass it to the dialog.

<ErrorDialog>
  <Icon type="no-internet" />
  <p>You are not connected to the internet!</p>
</ErrorDialog>

Block Parameters

Blocks can also pass values back into the template, similar to a callback function in JavaScript. Consider for instance a simple BlogPost component.

app/components/blog-post.hbs
<h1>{{@post.title}}</h1>
<h2>{{@post.author}}</h2>

{{@post.body}}

<!-- usage -->
<BlogPost @post={{@blogPost}} />

We may want to give the user the ability to put extra content before or after the post, such as an image or a profile. Since we don't know what the user wants to do with the body of the post, we can instead pass the body back to them.

app/components/blog-post.hbs
<h1>{{@post.title}}</h1>
<h2>{{@post.author}}</h2>

{{yield @post.body}}

<!-- usage -->
<BlogPost @post={{@blogPost}} as |postBody|>
  <img alt="" role="presentation" src="./blog-logo.png">

  {{postBody}}

  <AuthorBio @author={{@blogPost.author}} />
</BlogPost>

We can yield back multiple values as well, separated by spaces.

app/components/blog-post.hbs
{{yield @post.title @post.author @post.body }}

<!-- usage -->
<BlogPost @post={{@blogPost}} as |postTitle postAuthor postBody|>
  <img alt="" role="presentation" src="./blog-logo.png">

  {{postTitle}}

  {{postBody}}

  <AuthorBio @author={{postAuthor}} />
</BlogPost>
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