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

Class Public methods

attach_to(namespace, subscriber = new, notifier = ActiveSupport::Notifications, inherit_all: false)

Attach the subscriber to a namespace.

📝 Source code 
# File activesupport/lib/active_support/subscriber.rb, line 34
      def attach_to(namespace, subscriber = new, notifier = ActiveSupport::Notifications, inherit_all: false)
        @namespace  = namespace
        @subscriber = subscriber
        @notifier   = notifier
        @inherit_all = inherit_all

        subscribers << subscriber

        # Add event subscribers for all existing methods on the class.
        fetch_public_methods(subscriber, inherit_all).each do |event|
          add_event_subscriber(event)
        end
      end
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detach_from(namespace, notifier = ActiveSupport::Notifications)

Detach the subscriber from a namespace.

📝 Source code 
# File activesupport/lib/active_support/subscriber.rb, line 49
      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.
        fetch_public_methods(subscriber, true).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
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method_added(event)

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

📝 Source code 
# File activesupport/lib/active_support/subscriber.rb, line 68
      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
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new()

📝 Source code 
# File activesupport/lib/active_support/subscriber.rb, line 129
    def initialize
      @queue_key = [self.class.name, object_id].join "-"
      @patterns  = {}
      super
    end
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subscribers()

📝 Source code 
# File activesupport/lib/active_support/subscriber.rb, line 77
      def subscribers
        @@subscribers ||= []
      end
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Class Private methods

add_event_subscriber(event)

📝 Source code 
# File activesupport/lib/active_support/subscriber.rb, line 84
        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
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remove_event_subscriber(event)

📝 Source code 
# File activesupport/lib/active_support/subscriber.rb, line 95
        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
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Instance Public methods

finish(name, id, payload)

📝 Source code 
# File activesupport/lib/active_support/subscriber.rb, line 144
    def finish(name, id, payload)
      event = event_stack.pop
      event.finish!
      event.payload.merge!(payload)

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

📝 Source code 
# File activesupport/lib/active_support/subscriber.rb, line 135
    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
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