little event mechanism for changed positions

lib/risc/position/object_position.rb

@@ -6,6 +6,7 @@ module Risc
       def initialize( object, at)
         @at = at
         @object = object
+        @listeners = []
         raise "not int #{self}-#{at}" unless @at.is_a?(Integer)
       end
 
@@ -32,6 +33,25 @@ module Risc
         @at = pos
         true
       end
+
+      # Register a handler position change event.
+      # The object calls position_changed on the handler object
+      #
+      #   obj.position_changed( changed_position )
+      #
+      # @param [Object] object handling position_changed
+      def register_listener( handler)
+        @listeners << handler
+      end
+
+      def unregister_listener(handler)
+        @listeners.delete handler
+      end
+
+      # Trigger position change  and pass self to position_changed
+      def trigger()
+        @listeners.each { |handler| handler.position_changed( self ) }
+      end
     end
   end
 end

lib/util/eventable.rb

@@ -3,7 +3,7 @@ module Util
   # Events are stored in the `@events` ivar.
   module Eventable
 
-    # Risc a handler for the given event name.
+    # Register a handler for the given event name.
     # The event name is the method name called on the handler object
     #
     #   obj.on(:foo , some_object_that_implements foo( whateverargs)

test/risc/position/test_object_position.rb

@@ -48,5 +48,31 @@ module Risc
         assert_equal 5 , pos.at
       end
     end
+    class TestPositionEvents < MiniTest::Test
+      def setup
+        @position = ObjectPosition.new(self,0)
+      end
+      def test_has_register
+        assert @position.register_listener(self)
+      end
+      def test_can_unregister
+        assert @position.register_listener(self)
+        assert @position.unregister_listener(self)
+      end
+      def test_fires
+        @position.register_listener(self)
+        @position.trigger
+        assert_equal @position , @trigger
+      end
+      def test_no_fire_after_unregister
+        assert @position.register_listener(self)
+        assert @position.unregister_listener(self)
+        @position.trigger
+        assert_nil @trigger
+      end
+      def position_changed(pos)
+        @trigger = pos
+      end
+    end
   end
 end