From: John Small <jsmall@...>
Date: 2001-01-04T01:56:48+09:00
Subject: [ruby-talk:8571] If Observable#notify_observers returned true/false then false could imply veto of change

Hi,

If Observable#notify_observers were changed from

      def notify_observers(*arg)
         if defined? @observer_state and @observer_state
           if defined? @observer_peers
              for i in @observer_peers.dup
                 i.update(*arg)
              end
           end
           @observer_state = false
        end
     end

to

      def notify_observers(*arg)
         veto = false
         if defined? @observer_state and @observer_state
           if defined? @observer_peers
              for i in @observer_peers.dup
                 veto ||= !i.update(*arg)
              end
           end
           @observer_state = false
        end
        !veto
     end

then an updated observer could have the option of vetoing the
proposed change:

      require 'observer'

      class Foo
         include Observable

         def constrainedProperty
            @constrainedProperty
         end

         def constrainedProperty=(newValue)
            changed
            if notify_observers(self,
:constrainedProperty,@constrainedProperty,newValue)
               @constrainedProperty = newValue
               true
           else
              false
           end
        end

        def boundProperty
           @boundProperty
        end

        def boundProperty=(newValue)
           oldValue = @boundProperty
           @boundProperty = newValue
           changed
           notify_observers(self,:boundProperty,oldValue,newValue)
        end

        def initialize
           @constrainedProperty = @boundProperty = 1
        end

      end

      class FooObserver
         def update(object,property,oldValue,newValue)
             if object.kind_of?(Foo)
                case property
                   when :constrainedProperty
                       return false if oldValue == newValue
                       return true
                end
             end
         end
     end

     f = Foo.new
     fo = FooObserver.new
     f.add_observer(fo)
     f.constrainedProperty = 1   # => false: useless change
     f.constrainedProperty = 2   # => true:  I approve
     f.boundProperty = 3           # => thanks for letting me know!


John