1 files changed, 100 insertions, 0 deletions
diff --git a/activemodel/lib/active_model/observing.rb b/activemodel/lib/active_model/observing.rb
new file mode 100644
index 0000000000..db758f5185
--- /dev/null
+++ b/activemodel/lib/active_model/observing.rb
@@ -0,0 +1,100 @@
+require 'observer'
+
+module ActiveModel
+  module Observing
+    module ClassMethods
+      def observers
+        @observers ||= []
+      end
+      
+      def observers=(*values)
+        @observers = values.flatten
+      end
+      
+      def instantiate_observers
+        observers.each { |o| instantiate_observer(o) }
+      end
+    
+    protected
+      def instantiate_observer(observer)
+        # string/symbol
+        if observer.respond_to?(:to_sym)
+          observer = observer.to_s.camelize.constantize.instance
+        elsif observer.respond_to?(:instance)
+          observer.instance
+        else
+          raise ArgumentError, "#{observer} must be a lowercase, underscored class name (or an instance of the class itself) responding to the instance method. Example: Person.observers = :big_brother # calls BigBrother.instance"
+        end
+      end
+      
+      # Notify observers when the observed class is subclassed.
+      def inherited(subclass)
+        super
+        changed
+        notify_observers :observed_class_inherited, subclass
+      end
+    end
+    
+    def self.included(receiver)
+      receiver.extend Observable, ClassMethods
+    end
+  end
+
+  class Observer
+    include Singleton
+    attr_writer :observed_classes
+
+    class << self
+      attr_accessor :models
+      # Attaches the observer to the supplied model classes.
+      def observe(*models)
+        @models = models.flatten
+        @models.collect! { |model| model.respond_to?(:to_sym) ? model.to_s.camelize.constantize : model }
+      end
+
+      def observed_class_name
+        @observed_class_name ||= 
+          if guessed_name = name.scan(/(.*)Observer/)[0]
+            @observed_class_name = guessed_name[0]
+          end
+      end
+
+      # The class observed by default is inferred from the observer's class name:
+      #   assert_equal [Person], PersonObserver.observed_class
+      def observed_class
+        if observed_class_name
+          observed_class_name.constantize
+        else
+          nil
+        end
+      end
+    end
+
+    # Start observing the declared classes and their subclasses.
+    def initialize
+      self.observed_classes = self.class.models if self.class.models
+      observed_classes.each { |klass| add_observer! klass }
+    end
+
+    # Send observed_method(object) if the method exists.
+    def update(observed_method, object) #:nodoc:
+      send(observed_method, object) if respond_to?(observed_method)
+    end
+
+    # Special method sent by the observed class when it is inherited.
+    # Passes the new subclass.
+    def observed_class_inherited(subclass) #:nodoc:
+      self.class.observe(observed_classes + [subclass])
+      add_observer!(subclass)
+    end
+
+    protected
+      def observed_classes
+        @observed_classes ||= [self.class.observed_class]
+      end
+
+      def add_observer!(klass)
+        klass.add_observer(self)
+      end
+  end
+end
+\ No newline at end of file