amineasli / event_dispatcher

event_dispatcher implements a lightweight version of the Observer design pattern

event_dispatcher

event_dispatcher gem provides a simple observer implementation, allowing you to subscribe and listen for events in your application with a simple and effective way.

It is very strongly inspired by the Symfony EventDispatcher component

Install

Install the gem :

gem install event_dispatcher

Accessing the gem :

require 'event_dispatcher'

Usage

Creating an Event

When an event is dispatched, it's identified by a unique name, which any number of listeners might be listening to. An event instance is also created and passed to all of the listeners :

class UserEvent
   attr_reader :user, :login_time
   
   def initialize(user, login_time)
      @user = user
      @login_time = login_time
   end
 
   def log
     "#{login_time} : User #{user} has just logged in."
   end
end

Creating an EventDispatcher

The Dispatcher is the commander of the event dispatcher system, it maintains a registry of listeners. It's responsible for notifying listeners when events are dispatched :

dispatcher = EventDispatcher::Dispatcher.new

Connecting Listeners

To take advantage of an existing event, a listener needs to be connected to the dispatcher so that it can be notified when the event is dispatched. There are two ways for attaching a listener to the dispatcher.

Using a block :

listener = lambda do |event|
   puts event.log
end

dispatcher.add_listener(:user_login, listener)

Or using a class instance with a handler method for the event :

class UserEventListener
   def handle(event)
      puts event.log
   end
end

listener = UserEventListener.new     
dispatcher.add_listener(:user_login, listener.method(:handle))

You may also specify a priority when subscribing to events. Listeners with higher priority will be run first :

dispatcher.add_listener(:user_login, listener1, 15) 
dispatcher.add_listener(:user_login, listener2, 10) 
dispatcher.add_listener(:user_login, listener3, 2) 

Dispatching an Event

Notifies all listeners of the given event, so the event instance is then passed to each listener :

event = UserEvent.new('Bobby', Time.now) 
dispatcher.dispatch(:user_login, event)

Stopping Event propagation

You may wish to stop the propagation of an event to other listeners. To do so you need to first mixing the module Event in your custom event class, then setting the event instance variable stop_propagation to true :

class UserEvent
   include EventDispatcher::Event
   # ... 
   # ... 
end

listener = lambda do |event|
   puts event.log
   event.stop_propagation = true
end

Removing Listeners

Removes an event listener from the specified event :

dispatcher.remove_listener!(:user_login, listener)

Removes a collection of listeners from the specified event :

dispatcher.remove_listeners!(:user_login) # calling remove_listeners! without argument will simply reset the entire listeners container

Tests

rake test

License

LGPL, see LICENSE.