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 Active Record subscriber responsible for collecting statistics about queries:

module ActiveRecord
  class StatsSubscriber < ActiveSupport::Subscriber
    def sql(event)
      Statsd.timing("sql.#{event.payload[:name]}", event.duration)
    end
  end
end

And it's finally registered as:

ActiveRecord::StatsSubscriber.attach_to :active_record

Since we need to know all instance methods before attaching the log subscriber, the line above should be called after your subscriber definition.

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
F
M
N
S
Attributes
[R] namespace
[R] notifier
[R] subscriber
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 33
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 47
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
new()

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 79
def initialize
  @queue_key = [self.class.name, object_id].join "-"
  @patterns  = []
  super
end
subscribers()

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 56
def subscribers
  @@subscribers ||= []
end
Class Protected methods
add_event_subscriber(event)

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 64
def add_event_subscriber(event)
  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
Instance Public methods
finish(name, id, payload)

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 93
def finish(name, id, payload)
  finished  = Time.now
  event     = event_stack.pop
  event.end = finished
  event.payload.merge!(payload)

  method = name.split('.').first
  send(method, event)
end
start(name, id, payload)

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 85
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