1 files changed, 35 insertions, 3 deletions

diff --git a/activesupport/lib/active_support/subscriber.rb b/activesupport/lib/active_support/subscriber.rb
index 34c6f900c1..98be78b41b 100644
--- a/activesupport/lib/active_support/subscriber.rb
+++ b/activesupport/lib/active_support/subscriber.rb
@@ -31,22 +31,54 @@ module ActiveSupport
 
       # Attach the subscriber to a namespace.
       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|
-          next if %w{ start finish }.include?(event.to_s)
+          add_event_subscriber(event)
+        end
+      end
 
-          notifier.subscribe("#{event}.#{namespace}", subscriber)
+      # Adds event subscribers for all new methods added to the class.
+      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
 
       def subscribers
         @@subscribers ||= []
       end
+
+      protected
+
+      attr_reader :subscriber, :notifier, :namespace
+
+      def add_event_subscriber(event)
+        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
     end
 
+    attr_reader :patterns # :nodoc:
+
     def initialize
       @queue_key = [self.class.name, object_id].join "-"
+      @patterns  = []
       super
     end
 
@@ -71,7 +103,7 @@ module ActiveSupport
     private
 
       def event_stack
-        SubscriberQueueRegistry.get_queue(@queue_key)
+        SubscriberQueueRegistry.instance.get_queue(@queue_key)
       end
   end