Backbone Overview

Backbone is a JavaScript Model-View-Controller (MVC) library that can be used to create single page applications (SPAs). This article introduces the most important concepts in Backbone (Models, Collections, Views, Events, Router and Sync) and how they relate to building a client side JavaScript application.

What is Model-View-Controller (MVC)?

Model-View-Controller might be one of the most misunderstood software patterns ever. I'm not even sure I really understand what the original idea was (at least, what it meant to Smalltalk developers when first developed at Xerox PARC), and of course it has evolved over time into several different closely related ideas.

So this is far from authoritative, and I'll try to keep it simple. To me, MVC is a way to organize the implementation of a user interface in a way that separates concerns. Here, at a high level, are the various parts of a MVC framework:

• Model - this is the representation of data that models the domain of interest. For example, for a TODO application, a model might be a TODO item. But Models aren't just data structures, they also contain the domain logic. A TODO item might expose a complete method that knows how to mark a TODO item as completed and properly manage any additional details related to task completion.
• View - views are visual representations of models. A view reacts to changes in a model by updating the view to reflect that change.
• Controller - controllers handle user input, both displaying views that the user requests and handling keyboard and mouse events. Controllers typically translate user input into updates on Model instances.

The goal is separation of concerns. Models know nothing of views; in a web application, models would have nothing in them relating to HTML or CSS, for example. Views know only how to represent a model, but they contain no business logic. Views respond to changes in a model, but know nothing of what the user is doing. Controllers are the only place in the application to handle things like navigation through the application, display of multiple views at once, transitions between views and translating user actions into model updates. But controllers don't contain business logic.

How does Backbone relate to traditional MVC?

The Backbone API has the following types:

• Events
• Model
• Collection
• View
• Router
• Sync

Where do each of these concepts fit into the traditional MVC pattern?

Backbone.Events are the primary mechanism for loose coupling between the models and views. Backbone Views bind to changes on the Models or Collections.

Backbone.Model and Backbone.Collection are the M of MVC: these types can be extended to provide the data and the implementation of domain logic for the application.

Backbone.View classes are a little V and a little C when compare to classical MVC. Backbone.View contains a render method that is responsible for generating/updating the DOM that is the view of the model, and for doing so it is typical to have a template. The template is more the V of MVC than the View class. Backbone.View also acts as a Controller in that it binds user events to methods on the View class that will update Models.

Backbone.Router is also fairly controller-ish in the traditional MVC understanding. The Router responds to user requests for specific URLs and can handle them by showing the right type of Views and by making sure that Model instances are available.

Backbone.sync is the RESTful persistence mechanism for saving/updating models. In Backbone, the models (Model and Collection instances) know how to persist themselves (which endpoint to call, how to serialize/deserialize, etc.). In that sense, sync is more in the M of MVC than anything else, but I'm not sure that persistence is in the scope of traditional MVC. (Another way to think of it is that sync is a translation layer between a client side model and a server side model, which are just two representations of the same domain object.)

Events

Backbone.Events provides standard event listening and triggering behavior.

• Events.on(event, listener) adds a listener
• Events.off(event, listener) removes listener
• Events.trigger(event)

Backbone.Events can also be mixed in to other objects or classes via underscore's _.extend mechanism, and it is mixed in to Backbone.View, Backbone.Model etc. So these methods are available from most Backbone types.

By convention, Backbone Events are often namespaced. For example, you can listen to all changes on a model with model.on("change", listener), but you can also listen to just a change of the property "name" with model.on("change:name", listener).

But keep in mind there is no extra support in the implementation of Backbone.Events for namespaced events really, not like jQuery namespaced events. "change:name" works because Backbone.Model dispatches an event for each property that changes and also a "change" event. As another example, if you listen for "foo" and a "foo:bar" event is triggered, your event listener will not be called. You would have to trigger both "foo" and "foo:bar" in your code if you wanted to support namespacing.

One problem with setting up event listeners, especially with a View listening to changes on a Model, is that the object being listened to has a reference to the listener. For example, let's say you have a ContactView that is listening for changes to a Contact Model instance. If you dispose of the ContactView but forget to remove all of the event listeners, then the Contact Model still has a reference to your ContactView. This leads to a couple problems. First, your ContactView instance won't get garbage collected, so your app is now leaking memory. Second, whenever the Contact Model updates it will continue to call the event listener on the zombie ContactView and update it, so you have a lot of unnecessary code execution (and debugging this can get really confusing when you see code executed several times instead of just once as expected).

To solve this, Backbone.Events has two helpful methods, listenTo and stopListening. listenTo has this method signature

object.listenTo(other, event, callback)

Whereas you call on on the object you want to listen to, listenTo is called on the listening object. The advantage here is that the listening object can keep track of all of the listeners and can remove them all at once, which is what stopListening does.

One other note about events. In a Backbone app, you typically have two types of events: Backbone events and DOM events. DOM events are typically set up for a View in the events hash, which we'll get to later. When using the events hash, this refers to the View, pretty much what you expect. However, keep in mind, if you programmatically use the jQuery API to bind to DOM events then this refers to the DOM element.

Model

To create a Backbone.Model, extend the Backbone.Model class. You can specify default values for various properties.

app.Todo = Backbone.Model.extend({

    // Default attributes ensure that each todo created has `title` and `completed` keys.
    defaults: {
        title: '',
        completed: false
    }
});

To instantiate a Model instance, call the constructor and pass in the attributes for the instance. Any defaulted attributes that aren't specified will receive their default values.

Models have get and set functions for reading and writing attribute values. As mentioned above, calling set("myattr", newvalue) causes two events to be dispatched: "change:myattr" and "change".

Models also have a special property called id which uniquely identifies a Model instance. This can be set to an integer or UUID string. You pass it in the attributes hash when creating a Model instance. You can also set the idAttribute property on the Model class if you have a property on your Models that can be used to uniquely identify instances.

Collection

Create a new Collection by extending Backbone.Collection. You typically set the model property to the Model class of this collection.

var TodoList = Backbone.Collection.extend({

    // Reference to this collection's model
    model: app.Todo,

});

Collections redispatch Model events, so you can simply listen, for example, to "change" events on the Collection and you'll be notified whenever a Model instance changes.

You can add and remove Models to and from a Collection. These methods will dispatch "add" and "remove" events. Removing a model occurs when you call remove and pass in a Model instance that has the same id as a Model instance already in the Collection. Collections also have a reset method which updates the Collection all at once and dispatches just a single "reset" event, which is more efficient for example when initially loading a Collection.

Collections have several Underscore methods, like _.each, that you can call directly.

Collections are typically where you will set up the RESTful URL by setting the url property. You can then call fetch to retrieve all of the model instances in the collection and add them to the collection instance. We'll look at this more in the Sync section below.

View

Backbone View's always have a DOM element that they can render into. By default Backbone.View will create the DOM element for you, but if the DOM element you want to attach this View instance to is already in the page you can set el to a selector for that DOM element

var MyView = Backbone.View.extend({
    el: '#myview',
    //...
});

If you let Backbone create your DOM element, you can specify tagName, className, id and attributes to be set on that DOM element and Backbone will apply them. That is, if you specify a tagName of li and a className of todo-item, Backbone will create a <li class="todo-item" /> element in the DOM for you.

As a convenience, View's also have a $el property which is the jQuery wrapped reference to the View's DOM element.

A typical thing to do when you initialize a View is to pass it the model or collection instance you want it to bind to. In the View's initialize method, you can then bind to changes to that Model or Collection:

var TodoView = Backbone.View.extend({
    tagName: "li",
    className: "todo-item",
    initialize: function(){
        this.listenTo( this.model, "change", this.render );
        //...
    }
});

var todo = new TodoView({model: aTodo});

Besides binding to events on the Model, a View typically binds to DOM events. This is one of the controller-ish things a Backbone View does. To bind to DOM events, specify the events hash when creating the view. The keys in the events hash are in the form of 'eventName selector' and the value of each is the name of a function (or a function reference instead if you want).

var TodoView = Backbone.View.extend({
    events: {
        'click .toggle': 'toggleCompleted',
        // ...
    }
});

Inside of a DOM event handler, this refers to the View.

To actually render a view to the page, you provide an implementation of render. The job of render is to update this.el however necessary. The typical way to do this is to have a String based template, by default, an Underscore template. You'll set template in a view to a template function that takes an object with the properties of your model and returns a String of HTML with the model properties applied.

There are several approaches to how to organize your templates, but the simplest way is to put them in the web page inside a <script> tag with type set to text/template.

<script type="text/template" id="template-id">
    <input type="checkbox" <%= completed ? 'checked' : '' %> />
    <label><%- description></label>
</script>

In your View class, you can read in the template once with

//... inside View class
    template: _.template( this.$('#template-id').html() );

Then you can implement render like so:

//... inside View class
    render: function() {
        this.$el.html( this.template( this.model.toJSON() ) );

        // Anything else you might want to do...

        return this;
    }

Note that this.$ is a reference to the jQuery object. Also, render returns this so that parent View components can render child View components and then include them in themselves. For example, a container of these TodoView instances might do something like:

    render: function() {

        todos.each( function(todo) {
            var todoView = new TodoView({model: todo});
            this.$el.append( todoView.render().el );
        }
    }

When you are done with a View, you can call remove on it. This removes this.el from the DOM and also calls stopListening to remove all of its event listeners.

Router

Backbone.Router allows mapping URLs to actions and events. You create a router instance by extending Backbone.Router. You can specify a routes hash that maps url patterns to function names (or function references). This is somewhat similar to the events hash in a View.

Route url patterns can use the :param syntax of the *param splat syntax. :param is used to match a part of a URL. A splat matches the rest of the URL.

var Router = Backbone.Router.extend({

    routes: {
        '/search/:query': 'search',
        //....
    },

    search: function(query){
    }
});

As an example of how you would use a Router, consider an email application. When the user navigates to an email folder, the app will display a list of the email messages in that folder. You would set up a route for an email folder, maybe like so:

routes: {
    '/folder/:folderId': 'goToFolder'
}

Then when a user clicks on the email folder, you can call Router.navigate('/folder/123', {trigger: true}). If you want to handle updating the view to show the folder and just have the router update the URL, you would not pass {trigger: true}.

So now you need to kick things off for your app and have your Router handle whatever the current URL is (for example, if the user bookmarked that email folder link and navigated to it directly). To do that you call start on Backbone.history.

var appRouter = new Router();
Backbone.history.start({pushState: true});

Backbone.history is a kind of global router. It can make use of the History API, calling pushState if available, falling back to updating the URL fragment if not available. You have to pass {pushState: true} to start to make use of pushState; it is something you opt into in Backbone.

Sync

Backbone.sync handles persistence. By default, it supports RESTfully retrieving and saving models to a server, assuming a JSON API. In the simplest cases, you just define the base url on your Collections.

app.Library = Backbone.Collection.extend({
    model: app.Book,
    url: '/api/books'
});

When you call the following methods, the following URLs are invoked via AJAX:

• Collection.create maps to a POST to the URL /api/books
  • Collection.create is a convenience function that saves the new model to the server and adds it to the Collection.
• Collection.fetch maps to a GET to the URL /api/books
• Collection.update maps to a PUT to the URL /api/books/{id}
• Collection.delete maps to a DELETE to the URL /api/books/{id}

There's a lot you can customize however. Models can implement Model.parse to have custom response parsing logic. You can override Backbone.ajax to customize the AJAX logic. And you can go whole hog and override Backbone.sync to completely customize how to load/persist data, for example, using LocalStorage as a backing store for your app.

Additional Resources

• Backbone Fundamentals
  • A lot of the code examples in this blog post come from Backbone Fundamentals. Highly recommended.
• History of MVC
• C2's MVC page
• Namespacing events is just a convention @ StackOverflow
• Caniuse.com pushState