ActiveSupport::Subscriber is an object set to consume ActiveSupport::Notifications. The subscriber dispatches notifications to a registered object based on its given namespace.

An example would be an Active Record subscriber responsible for collecting statistics about queries:

module ActiveRecord
  class StatsSubscriber < ActiveSupport::Subscriber
    attach_to :active_record

    def sql(event)
      Statsd.timing("sql.#{event.payload[:name]}", event.duration)
    end
  end
end

After configured, whenever a “sql.active_record” notification is published, it will properly dispatch the event (ActiveSupport::Notifications::Event) to the sql method.

Methods
A
M
S
Attributes
[R] namespace

TODO Change this to private once we've dropped Ruby 2.2 support. Workaround for Ruby 2.2 “private attribute?” warning.
[R] notifier

TODO Change this to private once we've dropped Ruby 2.2 support. Workaround for Ruby 2.2 “private attribute?” warning.
[R] subscriber

TODO Change this to private once we've dropped Ruby 2.2 support. Workaround for Ruby 2.2 “private attribute?” warning.
Class Public methods
attach_to(namespace, subscriber = new, notifier = ActiveSupport::Notifications)

Attach the subscriber to a namespace.

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 30
def attach_to(namespace, subscriber = new, notifier = ActiveSupport::Notifications)
  @namespace  = namespace
  @subscriber = subscriber
  @notifier   = notifier

  subscribers << subscriber

  # Add event subscribers for all existing methods on the class.
  subscriber.public_methods(false).each do |event|
    add_event_subscriber(event)
  end
end
method_added(event)

Adds event subscribers for all new methods added to the class.

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 44
def method_added(event)
  # Only public methods are added as subscribers, and only if a notifier
  # has been set up. This means that subscribers will only be set up for
  # classes that call #attach_to.
  if public_method_defined?(event) && notifier
    add_event_subscriber(event)
  end
end
subscribers()

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 53
def subscribers
  @@subscribers ||= []
end
Class Private methods
add_event_subscriber(event)

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 65
    def add_event_subscriber(event) # :doc:
      return if %w{ start finish }.include?(event.to_s)

      pattern = "#{event}.#{namespace}"

      # Don't add multiple subscribers (eg. if methods are redefined).
      return if subscriber.patterns.include?(pattern)

      subscriber.patterns << pattern
      notifier.subscribe(pattern, subscriber)
    end
  end

  attr_reader :patterns # :nodoc:

  def initialize
    @queue_key = [self.class.name, object_id].join "-"
    @patterns  = []
    super
  end

  def start(name, id, payload)
    e = ActiveSupport::Notifications::Event.new(name, Time.now, nil, id, payload)
    parent = event_stack.last
    parent << e if parent

    event_stack.push e
  end

  def finish(name, id, payload)
    finished  = Time.now
    event     = event_stack.pop
    event.end = finished
    event.payload.merge!(payload)

    method = name.split(".".freeze).first
    send(method, event)
  end

  private

    def event_stack
      SubscriberQueueRegistry.instance.get_queue(@queue_key)
    end
end