Mediador

Event venue built as a mixin.

Installation

npm install mediador --save

Browser and RequireJS

Mediador is also available for the browser and RequireJS. You can install it with bower.

bower install mediador --save

Usage

Instance

emit maps each element in the second argument array as an argument to send to the listener function.

var Mediador = require("mediador")
 
var mediador = new Mediador
 
mediador.on("event", function (text) {
  console.log("event emitted with arg: " + text)
})
 
mediador.emit("event", ["hello"])

Mixin: Inheriting on/emit

var Mediador = require("mediador")
 
var YourClass = function () {}
YourClass.prototype.on      = Mediador.prototype.on
YourClass.prototype.emit = Mediador.prototype.emit
 
var yourInstance = new YourClass()
 
yourInstance.on("event", function (you) {
  console.log(you + " already firing events!")
})
 
yourInstance.emit("event", ["Me"])

The simplest way to use Mediador is to make instances, but it is not the recommended way.

Mediador is a mixin. The idea is that you can add its methods to the prototypes or objects that you want to amplify with events, without having to make them inherit directly from Mediador.

The events are stored in the listeners property within the emitter object, in this case yourInstance.

Bear this in mind so to not accidentally override the property.

You can find these examples in the examples folder in this repo. You can run them without installing mediador by running npm link on the repo folder (may require sudo).

Off with the listener

var Mediador = require("mediador")
 
var YourClass = function () {}
YourClass.prototype.on      = Mediador.prototype.on
YourClass.prototype.off     = Mediador.prototype.off
YourClass.prototype.emit = Mediador.prototype.emit
 
var yourInstance = new YourClass()
var listener     = function (you) {
  console.log(you + " already firing events!")
}
 
yourInstance.on("event", listener)
 
yourInstance.emit("event", ["Me"])
 
yourInstance.off("event", listener)
 
// Will do nothing
yourInstance.emit("event", ["Nothing"])

You can add the methods directly in an object

var Mediador     = require("mediador")
var emitter      = {}
 
emitter.on       = Mediator.prototype.on
emitter.emit  = Mediator.prototype.emit
 
emitter.on("event", function (text) {
  console.log(text)
})
 
emitter.emit("event", ["Hello World"])

The emitter is sent as argument

The event emitter is always sent to the listener functions as the last argument.

This is crucial, because otherwise in the contexts of many listeners there would be no available references to the emitter–the emitter would be unreachable. The emitter is needed for listeners to be able to emit further events in it–in fact, that might be the only way for a listener to pass information forward.

Another strategy to make the emitter available for the listeners would be to bind the listener to this, this being the emitter object. I really don't like when libraries do that.

var Mediador = require("mediador")
 
var YourClass = function () {}
YourClass.prototype.on      = Mediador.prototype.on
YourClass.prototype.off     = Mediador.prototype.off
YourClass.prototype.emit = Mediador.prototype.emit
 
var yourInstance = new YourClass()
 
yourInstance.on("event", function (irrelevant, emitter) {
  emitter.emit("completed")
})
 
yourInstance.on("completed", function () {
  console.log("The 'event' was successfully emited and 'completed' too")
})
 
yourInstance.emit("event", ["something irrelevant"])

It also works in bulk

var Mediador = require("mediador")
 
var YourClass = function () {}
YourClass.prototype.on      = Mediador.prototype.on
YourClass.prototype.off     = Mediador.prototype.off
YourClass.prototype.emit    = Mediador.prototype.emit
 
var yourInstance = new YourClass()
 
var subscriptionSet = {
  event: function () {
    console.log("Called the event")
  }
}
 
yourInstance.on(subscriptionSet)
 
yourInstance.emit("event")
 
yourInstance.off(subscriptionSet)

What is the subscriptionSet?

Mediador introduces the concept of an subscriptionSet, that is just an object where every property method will be added as a listener in the emitter. For example, if the set has a read method, its function will be added as a listener for the read event in the emitter.

This is very useful both for adding events in bulk and for removing them. Mediador provides support for both operations in the on and off methods.

Why does Mediador inserts the subscriptions property?

This is a question about namespace pollution.

When you add a subscription using the on method, Mediador adds the subscription to the subscriptions property of the object, which will be created if not present and used as it is if present. Mediador uses the subscription property without checking for the property's type. The property may well have been written by another function not related to Mediador. Is this OK?

The thing is, there are several strategies to avoid name collision within an object, and most of them involve encapsulating the library specific data in a private data object or using some kind of namespace (such a as naming the property mediator_subscriptions or _subscriptions). I don't favor this approach because:

1. Are you really planning on using several event libraries on the same object?
2. Will you gladly use a library that extends your object blindfolded and risk name collision anyways?
3. Why closing the door to interacting with the properties set by the library? You may very well wish to modify or query the subscriptions set.

In other words, I consider that using lightly a library that extends your objects is a poor design choice. Mediador and other libraries of its kind should be considered part of your design and taken for what they are: tested, encapsulated and standardized methods to achieve certain behaviors that are useful only because they save you time.

Why is there no once method?

Remember:

// You can name an anonymous function to call it from within
// You can totally do this:
mediador.on("fire", function notAnonymousAnymore() {
  mediador.off("fire", notAnonymousAnymore)
  console.log("The 'fire' event was called")
})

Corollary: EventEmitter's once method is not required. Keep your APIs simple.

Mediador.prototype.on

on( endpoint, callback [, context [, subscriptionClass ] ] )

Creates an stores a subscription object, passing the endpoint, callback, context and the current venue (this) to the subscription's constructor.

By default Mediador uses it's own Subscription type, which takes any kind of endpoint as argument and makes an equality comparison with the event to decide whether to emit itself or not. The most common use case of this Subscription type is to use Strings as endpoints and events: used in that way, Mediador has close compatibility with Node's EventEmitter.

The Subscription type can be overriden however, by calling registerSubscription in the Mediador instance or in the prototype of a Mediador descendant. Once overriden, the behaviour depends on the implementation of the registered type.

If provided, the subscriptionClass function will be used to create the instances instead of searching for one for the current object or the parent.

Chainable.

Arguments

• String endpoint
• Function callback
• optional Object context
• optional Function subscriptionClass

Returns

• Mediador this

on( subscriptionSet )

If on is called with only one argument that is an object, the argument will be assumed to be a subscriptionSet. A subscriptionSet is a way to create subscriptions in bulk: each property of the set that has a Function as value will be used to create a subscription:

• The property name will be used as the endpoint
• The Function will be used as the callback
• The object itself will be used as context

The subscription type in usage should be capable of dealing with String endpoints, so this feature is not useful if the Subscription requires endpoints of a different type.

Chainable.

Arguments

• Object subscriptionSet

Returns

• Mediador this

Mediador.prototype.emit

emit( args )

Notifies all the subscriptions about the emission. Each subscription may decide to fire the callback or not.

To notify the subscriptions, is fires the notify method of the subscription passing all the arguments plus this, the venue, as the last argument.

Chainable.

Arguments

• Object default arguments object

Returns

• Mediador this

Mediador.prototype.off

off( endpoint, callback [, context [, subscriptionClass ] ] )

Removes the subscription that matches the provided endpoint, callback and, if provided, context and subscriptionClass. By default (using the default Mediador.Subscription) it removes the subscription which endpoint is equal to the provided one and has the same context and callback. The endpoint is typically a String, such as "event", and consequently the behavior will be the same as Node EventEmitter's. If the subscription class has been overrided, the behavior may vary.

The off method relies on the subscription object's match method to decide whether or not there is a match between the subscription and the provided arguments.

If subscriptionClass is provided, it checks the type of the subscription and only if it matches, it passes all the other arguments to the match function. If no subscriptionClass is provided, it passes all arguments to match. The venue itself (this) will also be passed to the match method, as the last argument.

If match returns true, the subscription will be removed from the subcriptions Array. Any amount of subscriptions may be removed.

Chainable.

Arguments

• Object endpoint
• Function callback
• optional Object context
• optional Function subscriptionClass

Returns

• Mediador this

off( subscriptionSet )

If off is called with only one argument that is an object, the argument will be assumed to be a subscriptionSet. A subscriptionSet is a way to manage subscriptions in bulk: each property of the set that has a Function as value will be used by off to remove a subscription:

• The property name will be used as the endpoint
• The Function will be used as the callback
• The object itself will be used as context

The subscription type in usage should be capable of dealing with String endpoints, so this feature is not useful if the Subscription requires endpoints of a different type.

Chainable.

Arguments

• Object subscriptionSet

Returns

• Mediador this

Mediador.prototype.registerSubscription

registerSubscription( subscriptionClass )

Registers the given subscription class as the one to be used with this object. Chainable

Arguments

• Function subscriptionClass

Returns

• Mediador this

Mediador.getSubscriptionClassFor

getSubscriptionClassFor( target )

Returns the subscription class (Function) to be instantiated for the provided target.

Arguments

• Object target

Returns

• Function function

Mediador.registerSubscriptionClassFor

registerSubscriptionClassFor( target, subscriptionClass )

Registers the passed subscriptionClass as to be used for the target.

If the target is null, it overrides the default.

Arguments

• Object target
• Function subscriptionClass

Mediador.createSubscriptionFor

createSubscriptionFor( target, endpoint, callback, context )

Creates and returns a subscription from the class corresponding to the target, forwarding the endpoint, callback and context to the constructor.

Arguments

• Object target
• Object endpoint
• Function callback
• Object object

Returns

• Object subscription

Mediador.Subscription

Mediador.Subscription is the default subscription object for Mediador. Mediador con be configured to use different subcription objects as long as they expose the same interface:

• new <subscription>(endpoint, callback, context):
  • the endpoint is the object that represents the type of events to which this subscription is bound. In the default subscription the endpoint is always of type String, but it can be anything in other implementations.
  • the callback is a Function that will be executed then the subscription is invoked.
  • the context is an object that will be used as this when running the callback.
• .notify(event, arguments [, venue]) : Boolean:
  • the subscription object should contain a method called notify that receives an event of the appropiate type as the first argument (type String in the default Subscription) and an Array of arguments as the second argument. The Subscription object is responsible of matching the event to it's own endpoint to find out whether it should invoke the callback or not. The notify event should return a Boolean with the result of the matching: true if the callback was fired, false if not. The venue is an optional argument that in the default Subscription object is forwarded as the last argument to the callback.
• .match(event, callback) : Boolean:
  • the subscription object should also contain a method called match that returns whether or not the subscription matches the event/callback pair sent. It returns a Boolean. This method is used by the venue to be able to detach subscriptions while delegating the matching procedure to the subscription object. Testing

---

Then clone the repo and run:

> npm install
> make test

License

Copyright 2014 Xavier Via

ISC license.

See LICENSE attached.