Simple, global ActiveRecord event observers, using a middleware architecture, that can easily be turned on and off. Designed for audit trails, activity feeds and other application-level event handlers.

gem install active_model_listener

First, require Active Model Listener above your rails initializer:

# environment.rb
require 'active_model_listener'
Rails::Initializer.run do |config|
  # ...
end

Add the listeners to the ActiveModelListener in an initializer:

# config/initializers/active_model_listener.rb
ActiveModelListener.listeners << ActivityFeedListener

NOTE: you can also use a string or symbol version of the class. The following are all equivalent:

ActiveModelListener.listeners << ActivityFeedListener
ActiveModelListener.listeners << "ActivityFeedListener"
ActiveModelListener.listeners << "activity_feed_listener"
ActiveModelListener.listeners << :activity_feed_listener

Then, create a listener class that defines methods for after_create, after_update and/or after_destroy:

class ActivityFeedListener
  class << self
    def after_create(record)
      # do stuff
    end

    def after_update(record)
      # do stuff
    end

    def after_destroy(record)
      # do stuff
    end
  end
end

Inside the after_create, after_destroy and after_update methods all calls will be called without listeners, to prevent recursion. If you need listeners to be turned on during those blocks, you'll have to set them yourself within the blocks.

When unit testing if your listeners are all firing your unit tests become integration tests. To avoid this, you can easily turn off listeners for all specs all the time:

Spec::Runner.configure do |config|
  config.before(:each) do
    ActiveModelListener.listeners.clear
  end
end

Then, when you want them back on again, you can either turn them back on for a spec:

describe "Integrating with listeners" do
  before do
    ActiveModelListener.listeners << FooListener
  end
end

When doing data imports, migrations or certain actions that need to only use certain listeners, you can easily specify which ones you'd like to use:

ActiveModelListener.with_listeners AuditListener, ActivityListener do
  Article.create! :title => "foo"
end

After the block runs, the original listeners are restored.

If you want to run some code with no listeners, you can do so with:

ActiveModelListener.without_listeners do
  Article.create! :title => "foo"
end

ActiveRecord Observers are:

• Hard to apply to large numbers of models (you have to explicitly declare every one)
• Hard to turn off in tests
• Hard to selectively enable / disable

ActiveModelListener applies to all ActiveRecord models anywhere in your app, all the time. ActiveModelListener listeners are very easy to turn off during unit tests as well.

Copyright (c) 2009 Jeff Dean. See LICENSE for details.