Merge pull request #12285 from dasch/dasch/allow-attaching-up-front

Allow attaching to AS::Notifications namespace up front

1 files changed, 25 insertions, 2 deletions

diff --git a/activesupport/lib/active_support/subscriber.rb b/activesupport/lib/active_support/subscriber.rb
index 34c6f900c1..f2966f23fc 100644
--- a/activesupport/lib/active_support/subscriber.rb
+++ b/activesupport/lib/active_support/subscriber.rb
@@ -31,18 +31,41 @@ 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)
+
+        notifier.subscribe("#{event}.#{namespace}", subscriber)
+      end
     end
 
     def initialize