Allow attaching to AS::Notifications namespace up front

Before, you were required to attach *after* adding the methods to the
class, since the attachment process needed the methods to be present.

With this change, any new method will also be attached to the configured
namespace.

activesupport/CHANGELOG.md

+*   Allow attaching event subscribers to ActiveSupport::Notifications namespaces
+    before they're defined. Essentially, this means instead of this:
+
+        class JokeSubscriber < ActiveSupport::Subscriber
+          def sql(event)
+            puts "A rabbi and a priest walk into a bar..."
+          end
+
+          # This call needs to happen *after* defining the methods.
+          attach_to "active_record"
+        end
+
+    You can do this:
+
+      class JokeSubscriber < ActiveSupport::Subscriber
+        # This is much easier to read!
+        attach_to "active_record"
+
+        def sql(event)
+          puts "A rabbi and a priest walk into a bar..."
+        end
+      end
+
+    This should make it easier to read and understand these subscribers.
+
+    *Daniel Schierbeck*
+
 *   Add `Date#middle_of_day`, `DateTime#middle_of_day` and `Time#middle_of_day` methods.
 
     Also added `midday`, `noon`, `at_midday`, `at_noon` and `at_middle_of_day` as aliases.

activesupport/lib/active_support/subscriber.rb

@@ -31,18 +31,41 @@ class << self
 
       # 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

activesupport/test/subscriber_test.rb

+require 'abstract_unit'
+require 'active_support/subscriber'
+
+class TestSubscriber < ActiveSupport::Subscriber
+  attach_to :doodle
+
+  cattr_reader :event
+
+  def self.clear
+    @@event = nil
+  end
+
+  def open_party(event)
+    @@event = event
+  end
+
+  private
+
+  def private_party(event)
+    @@event = event
+  end
+end
+
+class SubscriberTest < ActiveSupport::TestCase
+  def setup
+    TestSubscriber.clear
+  end
+
+  def test_attaches_subscribers
+    ActiveSupport::Notifications.instrument("open_party.doodle")
+
+    assert_equal "open_party.doodle", TestSubscriber.event.name
+  end
+
+  def test_does_not_attach_private_methods
+    ActiveSupport::Notifications.instrument("private_party.doodle")
+
+    assert_nil TestSubscriber.event
+  end
+end