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
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F
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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)
  subscribers << subscriber

  subscriber.public_methods(false).each do |event|
    next if %w{ start finish }.include?(event.to_s)

    notifier.subscribe("#{event}.#{namespace}", subscriber)
  end
end
new()

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 48
def initialize
  @queue_key = [self.class.name, object_id].join "-"
  super
end
subscribers()

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 43
def subscribers
  @@subscribers ||= []
end
Instance Public methods
finish(name, id, payload)

Source: show | on GitHub 

# File activesupport/lib/active_support/subscriber.rb, line 61
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 53
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