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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.

We can detach a subscriber as well:

ActiveRecord::StatsSubscriber.detach_from(:active_record)
Methods
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Class Public methods
attach_to(namespace, subscriber = new, notifier = ActiveSupport::Notifications)

Attach the subscriber to a namespace.

# File activesupport/lib/active_support/subscriber.rb, line 34
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
detach_from(namespace, notifier = ActiveSupport::Notifications)

Detach the subscriber from a namespace.

# File activesupport/lib/active_support/subscriber.rb, line 48
def detach_from(namespace, notifier = ActiveSupport::Notifications)
  @namespace  = namespace
  @subscriber = find_attached_subscriber
  @notifier   = notifier

  return unless subscriber

  subscribers.delete(subscriber)

  # Remove event subscribers of all existing methods on the class.
  subscriber.public_methods(false).each do |event|
    remove_event_subscriber(event)
  end

  # Reset notifier so that event subscribers will not add for new methods added to the class.
  @notifier = nil
end
method_added(event)

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

# File activesupport/lib/active_support/subscriber.rb, line 67
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()
# File activesupport/lib/active_support/subscriber.rb, line 124
def initialize
  @queue_key = [self.class.name, object_id].join "-"
  @patterns  = {}
  super
end
subscribers()
# File activesupport/lib/active_support/subscriber.rb, line 76
def subscribers
  @@subscribers ||= []
end
Class Private methods
add_event_subscriber(event)
# File activesupport/lib/active_support/subscriber.rb, line 83
def add_event_subscriber(event) # :doc:
  return if invalid_event?(event)

  pattern = prepare_pattern(event)

  # Don't add multiple subscribers (e.g. if methods are redefined).
  return if pattern_subscribed?(pattern)

  subscriber.patterns[pattern] = notifier.subscribe(pattern, subscriber)
end
remove_event_subscriber(event)
# File activesupport/lib/active_support/subscriber.rb, line 94
def remove_event_subscriber(event) # :doc:
  return if invalid_event?(event)

  pattern = prepare_pattern(event)

  return unless pattern_subscribed?(pattern)

  notifier.unsubscribe(subscriber.patterns[pattern])
  subscriber.patterns.delete(pattern)
end
Instance Public methods
finish(name, id, payload)
# File activesupport/lib/active_support/subscriber.rb, line 139
def finish(name, id, payload)
  event = event_stack.pop
  event.finish!
  event.payload.merge!(payload)

  method = name.split(".").first
  send(method, event)
end
start(name, id, payload)
# File activesupport/lib/active_support/subscriber.rb, line 130
def start(name, id, payload)
  event = ActiveSupport::Notifications::Event.new(name, nil, nil, id, payload)
  event.start!
  parent = event_stack.last
  parent << event if parent

  event_stack.push event
end