##Backbone Aura 0.8 Developer Preview

Backbone Aura is a decoupled, event-driven architecture on top of Backbone.js for developing widget-based applications. It takes advantage of patterns and best practices for developing maintainable applications and gives you greater control over widget-based development. Aura gives you complete control of a widget's lifecycle, allowing developers to dynamically start, stop, reload and clean-up parts of their application as needed.

Written by Addy Osmani and Dustin Boston, the project is based on concepts discussed by Nicholas Zakas in Scalable Application Architecture and by Addy in Large-scale Application Development.

Aura contains a multi-tiered architecture, consistening of:

• An Application Core
• A Sandbox
• Widgets
• Modules

Application Core

The Core has a number of responsibilities. Powered by the Mediator pattern, it:

• Provides the ability to manage a widget's lifecycle (start, stop, cleanup). This is powered by work we've done to expand on top of RequireJS 2.0's undef feature for unloading modules. Aura works around RequireJS's limitation of not being able to resolve a modules dependencies to allow the easy unloading of an entire widget. Unloading a widget equates to removing it from the RequireJS caches, deleting instance references to them (which can lower memory) and of course, cleaning up any DOM elements the widget was using.
• Implements aliases for DOM manipulation, templating and other lower-level utilities that pipe back to a library of choice. The idea here is that rather than interfacing with the libraries directly, accessing the Core aliases (through the Sandbox) allow developers to switch out the libraries they use at a later date with minimum impact to their application.
• Exposes Publish/Subscribe functionality that can be used for decoupled communication between parts of an application. Similar to the concept above, our Pub/Sub implementation can be easily replaced with that of another library and it should still work fine.

The Sandbox

Powered by the Facade pattern, the Sandbox:

• Provides a limited, lightweight API layer on top of the Core for the rest of an application to communicate through. Rather than exposing say, the entire API for a JavaScript library, we instead only expose those parts that developers in the project will need or are safe to use. This is particularly useful when working in teams.
• The Sandbox includes a permissions layer, allowing you to configure permissions for widgets such as whether a specific widget has the right to render to the page etc.

Widgets

• Widgets represent a complete unit of a page. They could be a calendar, a news block, a todo list or anything else.
• In Backbone.js terms, widgets are composed of Models, Views, Collections and Routers as well as any templates needed for the widget to rendered.
• Widgets should be developed such that any number of instances of them could exist on a page in harmony.
• Publish/Subscribe can be used to communicate between widgets. Alternatively, direct communication (as demonstrated by the controls widget in our examples) may be, however this is discouraged where Pub/Sub can be used instead.

Modules

• All of the files and demo widgets in Aura use AMD as their module format of choice
• These can of course be used with r.js for compilation and optimization if concerned about too many script files (compilation should always be used for production-level apps if using AMD in any situation)
• Whilst not an Aura feature, we also take advantage of RequireJS 2.0's shim capability to avoid the need to use patched versions of Backbone.js and Underscore.js (a concern with earlier versions of the project).

Sample Application

A demo application using Aura is included in the download featuring Calendar, Todo list and control widgets. Run aura/www on a local HTTP server to try it out.

We plan on writing up a more complex application using Aura as soon as a stable release is ready. We'll ensure it uses multiple views and handles some of the more challenging architectural issues developers commonly run into today.

Code Samples

Starting and stopping a Widget

    startCalendar: function(){
      sandbox.widgets.start('calendar', '#calendarapp');
    },

    stopCalendar: function(){
      sandbox.widgets.stop('calendar', '#calendarapp');
    }

Pulling together a Widget

define(['sandbox', './views/app', './collections/events', 'fullcalendar'], 
  function(sandbox, AppView, Events){
    return sandbox.subscribe('bootstrap', 'calendar', function (element) {
        var events = new Events();
        new AppView({el: sandbox.dom.find(element), collection: events}).render();
        events.fetch();
    });
});

Collections using the Sandbox

define(['sandbox', '../models/event'], function(sandbox, Event){

    var Events = sandbox.mvc.Collection({
        model: Event,
        // url: 'events'
        // Save all of the calendar items under the `"events"` namespace.
        localStorage: new sandbox.data.Store("events-backbone-require")
    }); 
    
    return Events;
});

Views using the Sandbox

define(['sandbox', './event', '../models/event', 'text!../templates/base.html'],
        function(sandbox, EventView, Event, baseTemplate) {

    var AppView = sandbox.mvc.View({

        baseTemplate: sandbox.template.parse(baseTemplate),

        initialize: function(){

            // $el and $() are actually proxying 
            // through to sandbox.dom.find()
            this.$el.html(baseTemplate);
            
            this.calendar = this.$(".content");
            
            sandbox.events.bindAll(this); 

Sandbox Extension For Backbone

define(["aura_sandbox", "core", "perms", 'jquery_ui'],
    function (sandbox, core, perms) {

        var facade = Object.create(sandbox);
        facade.data.Store = core.data.Store;
        facade.mvc = {};
        facade.widgets = {};

        facade.mvc.View = function (view) {
            return core.mvc.View.extend(view);
        };
        facade.mvc.Model = function (model) {
            return core.mvc.Model.extend(model);
        };
        facade.mvc.Collection = function (collection) {
            return core.mvc.Collection.extend(collection);
        };

        facade.widgets.start = function(channel, el){
            return sandbox.start.apply(this, arguments);
        };

        facade.widgets.stop = function(channel, el){
            return sandbox.stop.apply(this, arguments);
        };

        return facade;
});

Aura Directory Structure

-- js/aura

Contains the core implementation of the Application Core (mediator.js), Sandbox (facade.js) and base for widget Permissions validation (permissions.js).

-- js/ext

Extensions to the Application Core, Sandbox and Permissions can be found here. These contain example specific extensions such as support for Backbone.js and bootstrap/load permissions for the example's widgets.

-- js/widgets

The three sample widgets for the example: Calendar, Todos and Controls. Both the Calendar and Todos persist using localStorage whilst the Controls widget is there to just demonstrate how one could control the start and stop of widgets through the UI. Normally this process would be handled by modules.

app.js

RequireJS 2.0 configuration, including shim config to allow the loading of non AMD-patched versions of libraries such as Underscore.js and Backbone.js. This is the initial point of starting up the widgets for an application.

##API

Core

• mediator.start(channel, el) e.g mediator.start('calendar', '#calendarapp')
• mediator.stop(channel, el) e.g mediator.stop('calendar', #calendarapp')
• mediator.unload(channel) e.g mediator.unload('calendar')
• mediator.publish(channel)
• mediator.subscribe(channel, callback, context)
• mediator.util.each() => _.each()
• mediator.util.extend() => _.extend()
• mediator.util.method(fn, context)
• mediator.util.parseJSON()
• mediator.util.rest(arr, index)
• mediator.util.delay()
• mediator.dom.find(selector, context) => $(..)
• mediator.dom.data(selector, attribute) => $(..).data()
• mediator.events.listen(context, events, selector, callback)
• mediator.events.bindAll()
• mediator.data.deferred() => $.Deferred
• mediator.template.parse() => _.template() (can be switched out)

Sandbox

• facade.start(channel, el)
• facade.stop(channel, el)
• facade.publish(channel)
• facade.subscribe(subscriber, channel, callback)
• facade.dom.find(selector, context)
• facade.events.listen(context,events,selector,callback)
• facade.events.bindAll()
• facade.util.each(..)
• facade.util.rest(..)
• facade.util.delay(..)
• facade.util.extend(..)
• facade.template(..)

Permissions

• permissions.extend(extension)
• permissions.validate(subscriber, channel)

Backbone Extensions For Aura

Core

• mediator.data.Store => Backbone localStorage adapter
• mediator.mvc => Backbone

Sandbox

• facade.mvc.View
• facade.mvc.Model
• facade.mvc.Collection
• facade.widgets.start(channel, el)
• facade.widgets.stop(channel, el)

Permissions

• Bootstrap permissions to display/render for specific widgets (e.g permissions.todos: {bootstrap: true})

Why A Developer Preview?

Aura is currently missing two important items needed to help us get out a stable release. These are good unit tests and stronger documentation. When the project has these and we've confirmed everything works as expected, we'll announce it for others to check out. The developer preview is our way of letting developers play with some new toys early on and get community feedback on whether the project is useful or not.

At minimum it offers a reference application for some of the ideas Nicholas and Addy have spoken and written about in the past. We welcome your thoughts and any feedback on the project. Thanks!