fix arity check to use "parameters" for backwards compat

1 files changed, 9 insertions, 4 deletions

diff --git a/activesupport/lib/active_support/notifications/fanout.rb b/activesupport/lib/active_support/notifications/fanout.rb
index 2fd683a1de..4e4ca70942 100644
--- a/activesupport/lib/active_support/notifications/fanout.rb
+++ b/activesupport/lib/active_support/notifications/fanout.rb
@@ -70,13 +70,18 @@ module ActiveSupport
 
       module Subscribers # :nodoc:
         def self.new(pattern, listener)
+          subscriber_class = Timed
+
           if listener.respond_to?(:start) && listener.respond_to?(:finish)
             subscriber_class = Evented
           else
-            if listener.respond_to?(:arity) && listener.arity == 1
-              subscriber_class = EventObject
-            else
-              subscriber_class = Timed
+            # Doing all this to detect a block like `proc { |x| }` vs
+            # `proc { |*x| }` or `proc { |**x| }`
+            if listener.respond_to?(:parameters)
+              params = listener.parameters
+              if params.length == 1 && params.first.first == :opt
+                subscriber_class = EventObject
+              end
             end
           end