Backbone Radio

The Backbone Radio provides easy support for a number of messaging patterns for Backbone and Marionette. This is provided through two basic constructs:

Events - trigger events on a global object
Requests - a global request/reply implementation

Radio takes these two constructs and adds the channel implementation - providing namespaces for events and requests. In short, Radio is a global, namespaced, message bus system designed to allow two otherwise unrelated objects to communicate and share information.

Radio Concepts

The Radio message bus exposes some core concepts:

Channel - a namespace mechanism.
Event - alert other parts of your application that something happened.
Request - execute single functions in a different part of your application.

Channel

The channel is the biggest reason to use Radio as our event aggregator - it provides a clean point for dividing global events. To retrieve a channel, use Radio.channel(channelName):

var Radio = require('backbone.radio');

var myChannel = Radio.channel('basic');

myChannel.on('some:event', function() {
  // ...
});

The channel is accessible everywhere in your application. Simply import Radio and call channel() to add listeners, fire callbacks, or send requests.

var Radio = require('backbone.radio');

var someChannel = Radio.channel('basic');  // Exactly the same channel as above

someChannel.trigger('some:event');  // Will fire the function call above

Live example

Event

The Radio Event works exactly the same way as regular Backbone Events like model/collection events. In fact, it uses the Backbone.Events mixin internally, exposing its API:

channel.on('event', callback, [context]) - when event fires, call callback
channel.once('event', callback, [context]) - same as on, but triggered only once
channel.off('event') - stop listening to event
channel.trigger('event', ..args) - fires event and passes args into the resulting callback

Events are typically used to alert other parts of the system that something happened. For example, a user login expired or the user performed a specific action.

As the Radio can be imported anywhere, we can use it as a global event aggregator as such:

var Radio = require('backbone.radio');

var myChannel = Radio.channel('star');

myChannel.on('left:building', function(person) {
  console.log(person.get('name') + ' has left the building!');
});

var elvis = new Bb.Model({name: 'Elvis'});
myChannel.trigger('left:building', elvis);

myChannel.off('left:building');

Just like Backbone Events, the Radio respects the listenTo handler as well:

var Mn = require('backbone.marionette');
var Radio = require('backbone.radio');

var starChannel = Radio.channel('star');

var Star = Mn.Object.extend({

  initialize: function() {
    this.listenTo(starChannel, 'left:building', this.leftBuilding);
    this.listenTo(starChannel, 'enter:building', function(person) {
       console.log(person.get('name') + ' has entered the building!');
    });
  },

  leftBuilding: function(person) {
    console.log(person.get('name') + ' has left the building!');
  }
});

Note that the event handler can be defined as a method like used for 'left:building' event or inline like used in 'enter:building'.

Live example

As in Backbone, the event handler is called with this bound to the Star instance. See the Backbone documentation for the full list of Event handling methods.

When to use Events

The Event is a simple notification that something happened and you may or may not want other objects in your application to react to that. A few key principles to bear in mind are:

If you don't know what could act on the event, or don't care, use an Event
If you find yourself calling it an action that occurred, use an Event
If it's fine for many objects to perform an action, use an Event
If you don't mind that no objects react, use an Event

If your use case isn't covered here, consider whether you want to use a request instead.

Request

The Request API provides a uniform way for unrelated parts of the system to communicate with each other. For example, displaying notifications in response to system activity. To attach a listener to a request channel, use reply or replyOnce to attach a listener that immediately detaches after one call.

As with request, any arguments passed in channel.request will be passed into the callback.

var Mn = require('backbone.marionette');
var Radio = require('backbone.radio');

var channel = Radio.channel('notify');

var Notification = Mn.Object.extend({

  initialize: function() {
    channel.reply('show:success', this.showSuccessMessage);
    channel.reply('show:error', function(msg) {
       // ...
    });
  },

  showSuccessMessage: function(msg) {
    // ...
  }
});

So, for example, when a model sync fails:

var Mn = require('backbone.marionette');
var Radio = require('backbone.radio');

var channel = Radio.channel('notify');

var ModelView = Mn.View.extend({
  modelEvents: {
    error: 'showErrorMessage'
  },

  showErrorMessage: function() {
    channel.request('show:error', 'An error occurred contacting the server');
  }
});

Live example

Now, whenever the model attached to this View is unable to sync with the server, we can display an error message to the user.

Returning Values from Reply

The Request API is also able to return values, making it extremely useful for accessing objects that would be otherwise difficult to access. As an example, let's assume we attach the currently logged-in user to the Application object and we want to know if they're still logged-in.

var Mn = require('backbone.marionette');
var Radio = require('backbone.radio');

var channel = Radio.channel('user');

var App = Mn.Application.extend({
  initialize: function() {
    channel.reply('user:logged:in', this.isLoggedIn);
  },

  isLoggedIn: function() {
    return this.model.getLoggedIn();
  }
});

Then, from another view, instead of trying to find the User model. we simply request it:

var Radio = require('backbone.radio');

var channel = Radio.channel('user');

var loggedIn = channel.request('user:logged:in');  // App.model.getLoggedIn()

Live example

When to use Requests

A Request is, as you might guess, a request for information or for something to happen. You will probably want to use requests when:

You call the request an action to perform e.g. show:notification
You want to get the return value of the request
You want to call exactly one function

In addition to this documentation, the Radio documentation can be found on Github.

Marionette Integration

The Marionette.Object class provides bindings to provide automatic event listeners and / or request replies on your object instances. This works with a bound channelName to let us provide listeners using the radioEvents and radioRequets properties. Anything that extends from Mn.Object has access to this API.

API

channelName - defines the Radio channel that will be used for the requests and/or events
getChannel() - returns a Radio.Channel instance using channelName
radioEvents - defines an events hash with the events to be listened and its respective handlers
radioRequets - defines an events hash with the requests to be replied and its respective handlers

Examples

Listening to events in an object

var Mn = require('backbone.marionette');

var Star = Mn.Object.extend({
  channelName: 'star',

  radioEvents: {
    'left:building': 'leftBuilding'
  },

  leftBuilding: function(person) {
    console.log(person.get('name') + ' has left the building!');
  }
});

Live example

This gives us a clear definition of how this object interacts with the star radio channel.

Replying to requests in a object

var Mn = require('backbone.marionette');

var Notification = Mn.Object.extend({
  channelName: 'notify',

  radioRequests: {
    'show:success': 'showSuccessMessage',
    'show:error': 'showErrorMessage'
  },

  showSuccessMessage: function(msg) {
    // ...
  },

  showErrorMessage: function(msg) {
    // ...
  }
});

Live example

We now have a clear API for communicating with the Notification across the application. Don't forget to define the channelName on your Object definition.

As with a normal request/reply, we can return values from these bound handlers:

var Mn = require('backbone.marionette');

var App = Mn.Application.extend({
  channelName: 'user',

  radioRequests: {
    'user:logged:in': 'isLoggedIn'
  },

  isLoggedIn: function() {
    return this.model.getLoggedIn();
  }
});

Live example

Events and requests in same object

var Mn = require('backbone.marionette');

var NotificationHandler = Mn.Object.extend({
  channelName: 'notify',

  radioRequests: {
    'show:success': 'showSuccessMessage',
    'show:error': 'showErrorMessage'
  },

  radioEvents: {
    'user:logged:in': 'showProfileButton',
    'user:logged:out': 'hideProfileButton'
  },

  showSuccessMessage: function(message) {
    // ...
  },

  showErrorMessage: function(message) {
    // ...
  },

  showProfileButton: function(user) {
    // ...
  },

  hideProfileButton: function(user) {
    // ...
  }
});

In an unrelated module:

var Radio = require('backbone.radio');
var User = require('./models/user');

var notifyChannel = Radio.channel('notify');
var userModel = new User();

// The following will call Notification.showErrorMessage(message)
notifyChannel.request('show:error', 'A generic error occurred!');

// The following will call Notification.showProfileButton(user)
notifyChannel.trigger('user:logged:in', userModel);

Live example