Ember Application Structure

On a high-level, you structure an Ember application by designing a series of nested routes that correspond to nested application state. This guide will first cover the high-level concepts, and then walk you through an example.

Routing

A user navigates through your application by making choices about what to view. For example, if you had a blog, your user might first choose between your Posts and your About page. In general, you want to have a default for this first choice (in this case, probably Posts).

Once the user has made their first choice, they're usually not done. In the context of Posts, the user will eventually view an individual post and its comments. Inside of an individual post, they can choose between viewing a list of comments and a list of trackbacks.

Importantly, in all of these cases, the user is choosing what to display on the page. As you descend deeper into your application state, those choices affect smaller areas of the page.

In the next section, we'll cover how you control these areas of the page. For now, let's look at how to structure your templates.

When the user first enters the application, the application is on the screen, and it has an empty outlet that the router will control. In Ember, an outlet is an area of a template that has its child template determined at runtime based on user interaction.

The template for the Application (application.handlebars) will look something like this:

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<h1>My Application</h1>
{{outlet}}

By default, the router will initially enter the list of posts state, and fill in the outlet with posts.handlebars. We will see later how this works exactly.

As expected, the list of posts template will render a list of posts. Clicking on the link for an individual post will replace the contents of the application's outlet with the template for an individual post.

The template will look like this:

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{{#each post in controller}}
<h1><a {{action showPost context="post" href=true}}>{{post.title}}</a></h1>
<div>{{post.intro}}</div>
{{/post}}

When clicking on a link for an individual post, the application will move into the individual post state, and replace posts.handlebars in the application's outlet with post.handlebars.

In this case, the individual post also has an outlet. In this case, the outlet will allow the user to choose between viewing comments or trackbacks.

The template for an individual post looks like this:

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<h1>{{title}}</h1>
<div class="body">
  {{body}}
</div>
{{outlet}}

Again, the {{outlet}} simply specifies that the router will make the decision about what to put in that area of the template.

Because {{outlet}} is a feature of all templates, as you go deeper into the route hierarchy, each route will naturally control a smaller part of the page.

How it Works

Now that you understand the basic theory, let's take a look at how the router controls your outlets.

Templates, Controllers, and Views

First, for every high-level handlebars template, you will also have a view and a controller with the same name. For example:

  • application.handlebars: the template for the main application view
  • App.ApplicationController: the controller for the template. The initial variable context of application.handlebars is an instance of this controller.
  • App.ApplicationView: the view object for the template.

In general, you will use view objects to handle events and controller objects to provide data to your templates.

Ember provides two primary kinds of controllers, ObjectController and ArrayController. These controllers serve as proxies for model objects and lists of model objects.

We start with controllers rather than exposing the model objects directly to your templates so that you have someplace to put view-related computed properties and don't end up polluting your models with view concerns.

You also connect {{outlet}}s using the template's associated controller.

The Router

Your application's router is responsible for moving your application through its states in response to user action.

Let's start with a simple router:

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App.Router = Ember.Router.extend({
  root: Ember.State.extend({
    index: Ember.State.extend({
      route: '/',
      redirectsTo: 'posts'
    }),
    posts: Ember.State.extend({
      route: '/posts'
    }),
    post: Ember.State.extend({
      route: '/posts/:post_id'
    })
  })
});

This router sets up three top-level states: an index state, a state that shows a list of posts, and a state that shows an individual post.

In our case, we'll simply redirect the index route to the posts state. In other applications, you may want to have a dedicated home page.

So far, we have a list of states, and our app will dutifully enter the posts state, but it doesn't do anything. When the application enters the posts state, we want it to connect the {{outlet}} in the application template. We accomplish this using the connectOutlets callback.

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App.Router = Ember.Router.extend({
  root: Ember.State.extend({
    index: Ember.State.extend({
      route: '/',
      redirectsTo: 'posts'
    }),
    posts: Ember.State.extend({
      route: '/posts',
      connectOutlets: function(router) {
        router.get('applicationController').connectOutlet(App.PostsView, App.Post.find());
      }
    }),
    post: Ember.State.extend({
      route: '/posts/:post_id'
    })
  })
});

This connectOutlet call does a few things for us:

  • It creates a new instance of App.PostsView, using the posts.handlebars template.
  • It sets the content property of postsController to a list of all of the available posts (App.Post.find()) and makes postController the controller for the new App.PostsView.
  • It connects the new view to the outlet in application.handlebars.

In general, you should just think of these objects as operating in tandem. You will always provide the content for a view's controller when you create a view.

Transitions and URLs

Next, we will want to provide a way for an application in the posts state to move into the post state. We accomplish this by specifying a transition.

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posts: Ember.State.extend({
  route: '/posts',
  showPost: Ember.State.transitionTo('post'),
  connectOutlets: function(router) {
    router.get('applicationController').connectOutlet(App.PostsView, App.Post.find());
  }
})

You invoke this transition by using the {{action}} helper in the current template.

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{{#each post in controller}}
  <h1><a {{action showPost context="post" href=true}}>{{post.title}}</a></h1>
{{/each}}

When a user clicks on a link with an {{action}} helper, Ember will dispatch an event to the current state with the specified name. In this case, the event is a transition.

Because we used a transition, Ember was also able to generate a URL for this link. Ember uses the id property of the context to fill in the :post_id dynamic segment of the post state.

Next, we will need to implement connectOutlets on the post state. This time, the connectOutlets method will receive the post object specified as the context to the {{action}} helper.

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post: Ember.State.extend({
  route: '/posts/:post_id',
  connectOutlets: function(router, post) {
    router.get('applicationController').connectOutlet(App.PostView, post);
  }
})

To recap, the connectOutlet call performs a number of steps:

  • It creates a new instance of App.PostView, using the post.handlebars template.
  • It sets the content property of postController to the post that the user clicked on.
  • It connects the new view to the outlet in application.handlebars.

You don't have to do anything else to get the link (/posts/1) to work if the user saves it as a bookmark and comes back to it later.

If the user enters the page for the first time with the URL /posts/1, the router will perform a few steps:

  • Figure out what state the URL corresponds with (in this case, post)
  • Extract the dynamic segment (in this case :post_id) from the URL and call App.Post.find(post_id). This works using a naming convention: the :post_id dynamic segment corresponds to App.Post.
  • Call connectOutlets with the return value of App.Post.find.

This means that regardless of whether the user enters the post state from another part of the page or through a URL, the router will invoke the connectOutlets method with the same object.

Nesting

Finally, let's implement the comments and trackbacks functionality.

Because the post state uses the same pattern as the root state, it will look very similar.

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post: Ember.State.extend({
  route: '/posts/:post_id',
  connectOutlets: function(router, post) {
    router.get('applicationController').connectOutlet(App.PostView, post);
  },
  index: Ember.State.extend({
    route: '/',
    redirectsTo: 'comments'
  }),
  comments: Ember.State.extend({
    route: '/comments',
    showTrackbacks: Ember.State.transitionTo('trackbacks'),
    connectOutlets: function(router) {
      var postController = router.get('postController');
      postController.connectOutlet(App.CommentsView, postController.get('comments'));
    }
  }),
  trackbacks: Ember.State.extend({
    route: '/trackbacks',
    showComments: Ember.State.transitionTo('comments'),
    connectOutlets: function(router) {
      var postController = router.get('postController');
      postController.connectOutlet(App.TrackbacksView, postController.get('trackbacks'));
    }
  })
})

There are only a few changes here:

  • We specify the showTrackbacks and showComments transitions only in the states where transitioning makes sense.
  • Since we are setting the view for the outlet in post.handlebars, we call connectOutlet on postController
  • In this case, we get the content for the commentsController and trackbacksController from the current post. The postController is a proxy for the underlying Post, so we can retrieve the associations directly from the postController.

Here's the template for an individual post.

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<h1>{{title}}</h1>
<div class="body">
  {{body}}
</div>
<p>
  <a {{action showComments href=true}}>Comments</a> |
  <a {{action showTrackbacks href=true}}>Trackbacks</a>
</p>
{{outlet}}

And finally, coming back from a bookmarked link will work fine with this nested setup. Let's take a look at what happens when the user enters the site at /posts/1/trackbacks.

  • The router determines what state the URL corresponds with (post.trackbacks), and enters the state.
  • For each state along the way, the router extracts any dynamic segments and calls connectOutlets. This mirrors the path a user would take as they move through the application. As before, the router will call the connectOutlet method on the post with App.Post.find(1).
  • When the router gets to the trackbacks state, it will invoke connectOutlets. Because the connectOutlets method for post has set the content of the postController, the trackbacks state will retrieve the association.

Again, because of the way the connectOutlets callback works with dynamic URL segments, the URL generated by an {{action}} helper is guaranteed to work later.

Asynchrony

One final point: you might be asking yourself how this system can work if the app has not yet loaded Post 1 by the time App.Post.find(1) is called.

The reason this works is that ember-data always returns an object immediately, even if it needs to kick off a query. That object starts off with an empty data hash. When the server returns the data, ember-data updates the object's data, which also triggers bindings on all defined attributes (properties defined using DS.attr).

When you ask this object for its trackbacks, it will likewise return an empty ManyArray. When the server returns the associated content along with the post, ember-data will also automatically update the trackbacks array.

In your trackbacks.handlebars template, you will have done something like:

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<ul>
{{#each trackback in controller}}
  <li><a {{bindAttr href="trackback.url"}}>{{trackback.title}}</a></li>
{{/each}}
</ul>

When ember-data updates the trackbacks array, the change will propagate through the trackbacksController and into the DOM.

You may also want to avoid showing partial data that is not yet loaded. In that case, you could do something like:

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<ul>
{{#if controller.isLoaded}}
  {{#each trackback in controller}}
    <li><a {{bindAttr href="trackback.url"}}>{{trackback.title}}</a></li>
  {{/each}}
{{else}}
  <li><img src="/spinner.gif"> Loading trackbacks...</li>
{{/if}}
</ul>

When ember-data populates the ManyArray for the trackbacks from the server-provided data, it also sets the isLoaded property. Because all template constructs, including #if automatically update the DOM if the underlying property changes, this will "just work".