Extract a base class from ActiveSupport::LogSubscriber

Adds a ActiveSupport::Subscriber base class that LogSubscriber inherits
from. By inheriting from Subscriber, other kinds of subscribers can take
advantage of the event attachment system.

1 files changed, 76 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/subscriber.rb b/activesupport/lib/active_support/subscriber.rb
new file mode 100644
index 0000000000..a6ad473b59
--- /dev/null
+++ b/activesupport/lib/active_support/subscriber.rb
@@ -0,0 +1,76 @@
+module ActiveSupport
+  # 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::Subscriber.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
+  # <tt>ActiveRecord::Subscriber</tt> definition.
+  #
+  # After configured, whenever a "sql.active_record" notification is published,
+  # it will properly dispatch the event (ActiveSupport::Notifications::Event) to
+  # the `sql` method.
+  class Subscriber
+    class << self
+
+      # Attach the subscriber to a namespace.
+      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
+
+      def subscribers
+        @@subscribers ||= []
+      end
+    end
+
+    def initialize
+      @queue_key = [self.class.name, object_id].join  "-"
+      super
+    end
+
+    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
+
+    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
+
+    private
+
+    def event_stack
+      Thread.current[@queue_key] ||= []
+    end
+  end
+end