Brian Genisio's House of Bilz

More Adventures in MVVM

My Experiment:

Can I move over to Ruby as my primary programming language when developing WPF and Silverlight applications?  I have been playing around with what it would mean to use Ruby to write my ViewModels, which would also make it easier to incorporate models using Ruby.

My next few blog posts will revolve around some things I have learned with this experiment.  As described in my disclaimer, I am a bit of a hack when it comes to Ruby.  There are probably better ways to do what I am suggesting.  If so, feel free to call me out.  I won’t mind.  Seriously.

INotifyPropertyChanged

So, if I am going to write ViewModels in Ruby, the first thing is to implement INotifyPropertyChanged, which is an interface that includes one event.  It is used by WPF and SIlverlight to update the UI when something changes:

public interface INotifyPropertyChanged
{
    event PropertyChangedEventHandler PropertyChanged;
}

But… how do I define events from Ruby?  It’s not like Ruby has events as a language feature.  As it turns out, in C# for every event (Foo), two methods are created: add_Foo() and remove_Foo() which are mapped from Foo += handler and Foo –= handler.  So, to get the PropertyChanged event methods implemented in Ruby, I just implement the add_ and remove_ methods.  Following is the full implementation of a Ruby class that implements INotifyPropertyChanged:

  class TestClass

    include System::ComponentModel::INotifyPropertyChanged

    def initialize

        @handlers = []

    end

    def add_PropertyChanged(handler)

        @handlers << handler

    end

    def remove_PropertyChanged(handler)

        @handlers.delete(handler)

    end

    def PropertyChanged(sender, args)

        @handlers.each { |h| h.invoke(sender, args) }

    end

  end

So, with that, the Ruby code can send an event to .Net code by calling PropertyChanged(). 

But… Seriously… Lets Metaprogram, Shall We?

The previous code is kind of awkward.  Every time you want to declare an event, you have to write this code?

Of course not.  This is Ruby.  We have metaprogramming!  Lets create some code to generate this code.  I talked with a ruby developer friend of mine, Toby Tripp, and he helped me get to this mixin code that will generate the methods for you:

module DotNetEvents

  module ClassMethods

    def declare_event( *events )

      events.each do |event|

        define_handler_methods( event )

      end

    end

    def define_handler_methods( event )

      event_name = event.to_s.gsub(/(?:^|_)(.)/) { $1.upcase }

      class_eval <<-EOS

        def handlers( event_name )

          @event_handlers ||= Hash.new

          @event_handlers[ event_name ] ||= []

        end

        def add_#{event_name}( handler )

          handlers( :#{event_name} ) << handler

        end

        def remove_#{event_name}( handler )

          handlers( :#{event_name} ).delete handler

        end

        def #{event_name}( sender, args )

          handlers( :#{event_name} ).each do |handler|

            handler.invoke sender, args

          end

      end

      EOS

    end

  end

  def self.included(receiver)

    receiver.extend ClassMethods

  end

end

Now that we have this declare_event method, we can use it to define events.  Just implement the interface, and declare the event:

class TestClass

  include DotNetEvents

  include System::ComponentModel::INotifyPropertyChanged

  declare_event :PropertyChanged

end

Note that I have, indeed, tested this with WPF.  The WPF system properly hooks up the event and when the Ruby code fires it, the UI gets updated.

From here, I can build my ViewModel base class in Ruby.  Some things to solve still: making notification of PropertyChanged events easy, commands, data binding, ViewModel hookups, etc.  But, at least, I am on my way :)

Tags: MVVM, Ruby, Silverlight, WPF

This entry was posted on Monday, June 14th, 2010 at 1:34 pm and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.