There are several small satellite Islands that surround the main Island that I live on, and believe it or not seem to be fairly reasonably populated. The one depicted in the photo above reputedly has a retired Doctor living on it, who owns a small cannon which he fires off while dressed in full sea-captain regalia. I’ve definitely heard the cannon, but never seen the Doctor with my own eyes…yet. I am curious as to how these folks receive their mail, though. What would their address be? Well, this led me to think about how Ember finds the things it needs.

Following on from my previous article about Beginning to understand the Ember.js Container, I used a metaphor of a ‘concierge’ that was able to look things up when needed. Today I’m going to look at that Concierge and see how it is that they do their job.

The Return of the Concierge (metaphor)

In Ember, the Concierge is the person responsible for taking the requested name that we want to look up, in this case let’s say ‘controller:index’, and translating that an actual object that we can use. In Ember land, this person is called the DefaultResolver and its only purpose is to provide that necessary lookup and translation service of string based names to real things.

It’s worth noting that there are actually two completely different Resolvers kicking around at the moment: one that ships with stock Ember, the DefaultResolver, and the other is Stefan Penner’s ember-resolver project which is the foundation of how Ember-CLI performs its module based magic. We’re only going to look at Ember’s stock Resolver today.

Previously we saw that we could register an object to be found at a particular ‘address’ using the following syntax inside an Initializer:

App.register('services:session', App.Session);

This creates a link between the string ‘services:session’ and our App.Session object. When we or Ember needs to perform a lookup on the Container for an object, the Container is internally going to use the DefaultResolver to do the reverse lookup of the string name we supply to get the actual object.

Finding things not in the Container

The Container isn’t used to store everything in Ember. Templates, for example, live in the Ember.TEMPLATES variable, which isn’t stored inside the Container. This is where the DefaultResolver steps in again to help locate what Ember needs.

The DefaultResolver has several methods for looking up Routes, Models, Helpers, Templates, Controllers and Views, and all of them can be overridden. It is through the DefaultResolver that the Container is able to provide a unified lookup service to the rest of the system.

Let’s say you wanted to have all of your Modal dialog templates located under a /modals subdirectory of /templates. If you try to do that without overriding the DefaultResolver‘s basic implementation of how Templates are looked up in Ember.TEMPLATES, Ember won’t be able to find the modal templates. Here’s how you could provide your own implementation that looks to see if a requested template ends in the word ‘modal’, and if it does, add the /modals prefix to the lookup path:

 1 window.App = Ember.Application.create(
 2   Resolver: Ember.DefaultResolver.extend({
 3     resolveTemplate: function(parsedName) {
 4       var modalSuffix = 'modal';
 5       var templateName = parsedName.name;
 6 
 7       // If the template ends in 'modal' then we alter the lookup path.
 8       if( templateName.indexOf(modalSuffix, templateName.length - modalSuffix.length) !== -1 ){
 9         return Ember.TEMPLATES['modals/'+templateName];
10       }
11 
12       // Call the default implementation for all the other templates.
13       return this._super(parsedName);
14   })
15 
16 )

In the above example, we’ve replaced the stock Resolver that ships with Ember without our own, which extends the basic DefaultResolver and overrides the resolveTemplate hook to implement the Modal template lookup functionality we need. Maybe you’d also like to override how a Modal’s Controllers are also looked up to match the same folder convention. It’s possible using this technique.

Summary

The DefaultResolver does a good job and is a main workhorse in Ember. Knowing what it does and how to tweak it as necessary can be very useful for establishing some of your own conventions, but use it with care.