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Diving into CanJS


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If you haven't heard, there's a new kid in town: CanJS. What better way to dive into what this new framework offers than to build a contacts manager application? When you're done with this three-part series, you'll have all the tools you need to build your own JavaScript applications!

Choosing the Right Tool

Building a JavaScript application without the right tools is difficult. While jQuery is great at what it does, a DOM manipulation library doesn't provide any infrastructure for building applications. This is specifically why you need to use a library, like CanJS.

CanJS is a lightweight MVC library that gives you the tools you need to build JavaScript apps.

CanJS is a lightweight MVC library that gives you the tools you need to build JavaScript apps. It provides all the structure of the MVC (Model-View-Control) pattern, templates with live binding, routing support and is memory safe. It supports jQuery, Zepto, Mootools, YUI, Dojo and has a rich set of extensions and plugins.

In part one, you will:

  • Create a Control and View (client-side template) to display contacts
  • Represent data using Models
  • Simulate ajax responses using the fixtures plugin

Excited? You should be! Now let's get coding.

Setting Up Your Folders And HTML

You'll need to create a folder for your application. Inside this folder, you need four sub-folders: css, js, views and img. Your folder structure should look like this when you're done:

  • contacts_manager
    • css
    • js
    • views
    • img

Save this as index.html:

At the bottom of the page, we load jQuery, CanJS, the fixture plugin and your application code (contacts.js).

The CSS and images for this tutorial are in included in the source files, which can be downloaded above.

Building Your UI With Views

Views are client-side templates that are used to render parts of your app. CanJS supports multiple templating languages, but this tutorial will be using EJS (Embedded JavaScript), which is packaged with CanJS and supports live binding.

EJS templates look like HTML but with magic tags where you want dynamic behavior (using JavaScript). There are three types of magic tags in EJS:

  • runs JavaScript code,
  • runs a JavaScript statement, and writes the escaped result into the resulting HTML,
  • runs a JavaScript statement and writes the unescaped result into the resulting HTML (used for sub-templates).

Templates can be loaded from a file or script tag. In this tutorial templates will be loaded from EJS files.

Displaying Contacts

To render contacts, you'll need an EJS template. Save the following code as contactsList.ejs within your views folder:

contactLists.ejs will render a list of contacts. Let's examine the template code here in more detail:

The EJS list() helper invokes a callback function on each contact in the list. When used with an observable list, the list() helper will use live binding to re-run anytime the length of the list changes.

The code above uses an element callback to add the contact instance to the data of the <li>. Everything after the arrow is wrapped in a function that will be executed with el set to the current element.

This code renders the contactView.ejs sub-template for each contact. can.view.render() takes a template and data as its parameters and returns HTML.

Rendering A Single Contact

Sub-templates are an excellent way of organizing your views into manageable chunks. They also help simplify your templates and promote DRY (Don't Repeat Yourself). Later in this tutorial, you'll re-use this template to create contacts. Save this code as contactView.ejs in your views folder:

Each property of a contact is placed in an <input> tag. These will be used to add and update a contact's information.

Making Your Views Live

Any time EJS encounters attr() while processing a template, it knows that the surrounding code should be turned into an event handler bound to that property's changes. When the property is changed elsewhere in the app, the event handler is triggered and your UI will be updated. This is referred to as live binding. EJS Live binding is opt-in. It only turns on if you use attr() to access properties.

Let's look at one of the <input> tags from the contactView.ejs to see how this works:

The code in the magic tags will become an event handler bound to the contact's name property. When we update the name property, the event handler is run and the HTML will be updated.

Organizing Application Logic Using can.Control

can.Control creates an organized, memory-leak free, stateful control that can be used to create widgets or organize application logic. You create an instance of a Control on a DOM element and pass it data your control will need. You can define any number of functions in your Control and bind to events.

When the element your Control is bound to is removed from the DOM, the Control destroys itself, cleaning up any bound event handlers.

To create a new Control, extend can.Control() by passing it an object containing functions you want to define. In part two, event handlers will be passed in as well.

There are a few important variables and functions present in every Control instance:

  • this - A reference to the Control instance
  • this.element - The DOM element that you created the instance on
  • this.options - An object containing any data passed to the instance when it was created
  • init() - Called when an instance is created

Managing Contacts

Add the following snippet to your contacts.js file to create the Control that will manage contacts:

When an instance of Contacts is created, init() will do two things:

  1. Uses can.view() to render contacts. can.view() accepts two parameters: the file or id of the script tag containing our template code and data. It returns the rendered result as a documentFragment (a lightweight container that can hold DOM elements).
  2. Inserts the documentFragment from can.view() into the Control's element using jQuery's .html().

Representing Data Using Models

A Model abstracts the data layer of an application. Two models are needed in this application: one for contacts and one for categories. Add this code to contacts.js:

A model has five static methods that you can define to create, retrieve, update and delete data. They are findAll, findOne, create, update and destroy. You can overwrite these functions to work with any back-end, but the easiest way to define a Model is using REST service, as exemplified in the code above. You can safely omit any static methods that won't be used in an application.

It's important to point out here that the model instances in CanJS are actually what we call 'observables'. can.Observe provides the observable pattern for objects and can.Observe.List provides the observable pattern for arrays. This means you can get and set properties using attr() and bind to changes in those properties.

The findAll() method returns a Model.list, which is a can.Observe.List that triggers events when an element is added or removed from the list.

Simulating a Rest Service Using Fixtures

Fixtures intercept AJAX requests and simulate their response with a file or function. This is fantastic for testing, prototyping or when a back-end isn't ready yet. Fixtures are needed to simulate the REST service the models in this application are using.

But first, you'll need some sample data for the fixtures to use. Add the following code to contacts.js:

Now that you have some data, you need to wire it up to fixtures so you can simulate a REST service. can.fixture() takes two parameters. The first is the URL we want to intercept and the second is a file or function that is used to generate a response. Often URLs you want to intercept are dynamic and follow a pattern. In this case, you should use templated URLs. Simply add curly braces to the URL where you want to match wildcards.

Add the following to contacts.js:

The first four fixtures simulate the GET, POST, PUT and DELETE responses for the Contact model, and the fifth fixture simulates the GET response for the Category model.

Bootstrapping the Application

Your application has Models for your data, Views to render contacts, and a Control to hook everything up. Now you need to kickstart the application!

Add this to your contacts.js file:

Let's take a closer look at what is happening in this code:

Wait for the DOM to be ready using jQuery's document ready function.

Call findAll() on both models to retrieve all of the contacts and categories. Since findAll() returns a Deferred, $.when() is used to make both requests in parallel and execute a callback when they are finished.

Get the list of model instances from the response of the two findAll() calls. The responses are arrays, with the first index being the list of model instances retrieved.

Create an instance of the Contact Control on the #contacts element. The list of contacts and categories are passed into the Control.

When you run your application in a browser, you should see a list of contacts:

Wrapping Up

That does it for part one of this series! You've been introduced to the core of CanJS:

  • Models abstract the data layer in your application
  • Views are templates that turn data into HTML
  • Controls wire everything up.

In the next lesson, you'll create a Control and View to display categories and use routing to filter contacts. Hope to see you there!

Questions? Ask away below! Can't wait to learn more? Part two of the series has been posted here!