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
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F
M
N
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 28
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 42
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 78
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 51
def subscribers
  @@subscribers ||= []
end
Class Private methods
add_event_subscriber(event)

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 63
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
Instance Public methods
finish(name, id, payload)

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 92
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
start(name, id, payload)

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 84
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