Property change listener

Mickael (Mickael) — Thu, 08 Feb 2007 16:00:35 GMT

How to implement a Property change listener on ruby class
Origin : http://rawblock.blogspot.com/2007/02/transparent-property-change-listeners.html



class ListenerSupport

 

  def self.listen_to(klass, *properties)

    setup(klass)

    properties.each do |p|

      klass.instance_eval do

        #rename original method

        alias_method "#{p}_orig=", "#{p}="

        #create proxy method in it's place that will fire

        #changes, and delegate to the original implementation

        define_method "#{p}=" do |value|

          old_value = send "#{p}"

          send "#{p}_orig=",value

          fire_property_changed p, old_value, value

        end

      end

    end

  end

 

  def self.setup(klass)

    klass.instance_eval do

     

      define_method :fire_property_changed do |prop, pre, post|

        return if pre == post

        eval %{

          @listeners[prop].each do |l|

              l.property_changed(self, prop, pre, post)

          end unless @listeners.nil?

        }

      end unless instance_methods.include? :fire_property_changed

     

      define_method :add_property_listener do |prop, listener|

        eval %{

          @listeners = {} if @listeners.nil?

          prop_listeners = @listeners.include?(prop) ? @listeners[prop] : []

          @listeners[prop] = prop_listeners

          prop_listeners << listener unless prop_listeners.include? listener

        }

      end unless instance_methods.include? :add_property_listener

     

    end

  end

 

end

The general idea is that the original class is modified so that all properties that are listened to have their accessor write methods intercepted, and any property change listeners are notified when that property changes.

Let's walk through how it works:

* A standard Ruby class with property accessors is created. e.g.

class RubyBean
attr_accessor :foo, :bar
end

* A call to ListenerSupport.listen_to is made to modify the class to allow properties to be listened to.

ListenerSupport.listen_to RubyBean, :foo, :bar

* listen_to adds methods add_property_listener and fire_property_changed to allow listeners to be added to RubyBean for specific properties.
* listen_to then aliases the original methods foo= and bar= to foo_orig= and bar_orig=. New methods for foo= and bar= are defined that delegate the work to the original method, then call fire_property_changed to notify the listeners.
* Listeners are called, and any custom action (such as GUI component binding) can be carried out.

DZone Snippets: listener code

Property change listener