ActiveModel::Observing: stop using Observable Ruby module, re-implement notify_observers

`Observable#notify_observers` from Ruby always returns false (which halts ActiveRecord
callback chains) and has extra features (like `changed`) that were never used.

Signed-off-by: Jeremy Kemper <jeremy@bitsweat.net>

activemodel/lib/active_model/observing.rb

@@ -1,4 +1,3 @@
-require 'observer'
 require 'singleton'
 require 'active_support/core_ext/array/wrap'
 require 'active_support/core_ext/module/aliasing'
@@ -8,10 +7,6 @@ module ActiveModel
   module Observing
     extend ActiveSupport::Concern
 
-    included do
-      extend Observable
-    end
-
     module ClassMethods
       # Activates the observers assigned. Examples:
       #
@@ -41,6 +36,22 @@ module ActiveModel
         observers.each { |o| instantiate_observer(o) }
       end
 
+      def add_observer(observer)
+        unless observer.respond_to? :update
+          raise ArgumentError, "observer needs to respond to `update'"
+        end
+        @observer_instances ||= []
+        @observer_instances << observer
+      end
+
+      def notify_observers(*arg)
+        if defined? @observer_instances
+          for observer in @observer_instances
+            observer.update(*arg)
+          end
+        end
+      end
+
       protected
         def instantiate_observer(observer) #:nodoc:
           # string/symbol
@@ -56,7 +67,6 @@ module ActiveModel
         # Notify observers when the observed class is subclassed.
         def inherited(subclass)
           super
-          changed
           notify_observers :observed_class_inherited, subclass
         end
     end
@@ -70,7 +80,6 @@ module ActiveModel
       #   notify_observers(:after_save)
       # end
       def notify_observers(method)
-        self.class.changed
         self.class.notify_observers(method, self)
       end
   end

activerecord/test/cases/lifecycle_test.rb

@@ -7,6 +7,14 @@ require 'models/comment'
 
 class SpecialDeveloper < Developer; end
 
+class SalaryChecker < ActiveRecord::Observer
+  observe :special_developer
+
+  def before_save(developer)
+    return developer.salary > 80000
+  end
+end
+
 class TopicaAuditor < ActiveRecord::Observer
   observe :topic
 
@@ -159,4 +167,16 @@ class LifecycleTest < ActiveRecord::TestCase
     assert_equal [ValidatedComment, ValidatedComment, ValidatedCommentObserver], callers,
       "model callbacks did not fire before observers were notified"
   end
+
+  test "able to save developer" do
+    SalaryChecker.instance # activate
+    developer = SpecialDeveloper.new :name => 'Roger', :salary => 100000
+    assert developer.save, "developer with normal salary failed to save"
+  end
+
+  test "unable to save developer with low salary" do
+    SalaryChecker.instance # activate
+    developer = SpecialDeveloper.new :name => 'Rookie', :salary => 50000
+    assert !developer.save, "allowed to save a developer with too low salary"
+  end
 end