How to do Events in Ruby?

Question

I come from a C# background, and have just started programming in Ruby. The thing is, that I need to know how I can raise events in my classes so that various observers can be triggered when things need to happen.

The problem is the books I have on Ruby don't even mention events, let alone provide examples. Is anyone able to help me?

Answer 1

I tried writing a GUI library in Ruby with a little C and primarily Ruby. It ended up being so slow I gave up and never released it. But I wrote an event system for it that I tried to make easier than C#'s. I rewrote it a couple times to make it easier to use. I hope it is somewhat helpful.

class EventHandlerArray < Array
  def add_handler(code=nil, &block)
    if(code)
      push(code)
    else
      push(block)
    end
  end
  def add
    raise "error"
  end
  def remove_handler(code)
    delete(code)
  end
  def fire(e)
    reverse_each { |handler| handler.call(e) }
  end
end

# with this, you can do:
#  event.add_handler
#  event.remove_handler
#  event.fire (usually never used)
#  fire_event
#  when_event
# You just need to call the events method and call super to initialize the events:
#  class MyControl
#    events :mouse_down, :mouse_up,
#           :mouse_enter, :mouse_leave
#    def initialize
#      super
#    end
#    def when_mouse_up(e)
#      # do something
#    end
#  end
#  control = MyControl.new
#  control.mouse_down.add_handler {
#    puts "Mouse down"
#  }
# As you can see, you can redefine when_event in a class to handle the event.
# The handlers are called first, and then the when_event method if a handler didn't
# set e.handled to true. If you need when_event to be called before the handlers,
# override fire_event and call when_event before event.fire. This is what painting
# does, for handlers should paint after the control.
#  class SubControl < MyControl
#    def when_mouse_down(e)
#      super
#      # do something
#    end
#  end
def events(*symbols)
  # NOTE: Module#method_added

  # create a module and 'include' it
  modName = name+"Events"
  initStr = Array.new
  readerStr = Array.new
  methodsStr = Array.new
  symbols.each { |sym|
    name = sym.to_s
    initStr << %Q{
      @#{name} = EventHandlerArray.new
    }
    readerStr << ":#{name}"
    methodsStr << %Q{
      def fire_#{name}(e)
        @#{name}.fire(e)
        when_#{name}(e) if(!e.handled?)
      end
      def when_#{name}(e)
      end
    }
  }
  eval %Q{
    module #{modName}
      def initialize(*args)
        begin
          super(*args)
        rescue NoMethodError; end
        #{initStr.join}
      end
      #{"attr_reader "+readerStr.join(', ')}
      #{methodsStr.join}
    end
    include #{modName}
  }
end

class Event
  attr_writer :handled
  def initialize(sender)
    @sender = @sender
    @handled = false
  end
  def handled?; @handled; end
end

Answer 2

The question has already been answered, but there's an observer built right into the standard library if you want to give that a look. I've used it in the past for a small game project, and it works very well.

Answer 3

Extremely simple Ruby listener. This is not exactly a replacement for .NET events, but this one is an extremely simple example of a very simple listener.

module Listenable

    def listeners() @listeners ||= [] end

    def add_listener(listener)
        listeners << listener
    end

    def remove_listener(listener)
        listeners.delete listener
    end

    def notify_listeners(event_name, *args)
        listeners.each do |listener|
            if listener.respond_to? event_name
                listener.__send__ event_name, *args
            end
        end
    end

end

To use:

class CowListenable
    include Listenable

    def speak
        notify_listeners :spoken, 'moooo!'
    end

end

class CowListener

    def initialize(cow_listenable)
        cow_listenable.add_listener self
    end

    def spoken(message)
        puts "The cow said '#{message}'"
    end

end

cow_listenable = CowListenable.new
CowListener.new(cow_listenable)
cow_listenable.speak

Output:

The cow said 'moooo!'

Answer 4

Disclosure: I am the maintainer of the event_aggregator gem

Depending on how you want to approach the problem you could potentially use an event aggregator. This way you can publish messages of a certain type and then have your objects listen to the types you want them to receive. This can in certain cases be better than normal events because you get a very loose coupling between your objects. The event producer and listener does not need to share a reference to the other.

There is a gem that helps you with this called event_aggregator. With it you can do the following:

#!/usr/bin/ruby

require "rubygems"
require "event_aggregator"

class Foo
    include EventAggregator::Listener
    def initialize()
        message_type_register( "MessageType1", lambda{|data| puts data } )

        message_type_register( "MessageType2", method(:handle_message) )
    end

    def handle_message(data)
        puts data
    end

    def foo_unregister(*args)
        message_type_unregister(*args)
    end
end

class Bar
    def cause_event
        EventAggregator::Message.new("MessageType1", ["Some Stuff",2,3]).publish
    end
    def cause_another_event
        EventAggregator::Message.new("MessageType2", ["Some More Stuff",2,3]).publish
    end
end

f = Foo.new

b = Bar.new
b.cause_event
b.cause_another_event
# => Some Stuff
     2
     3
# => Some More Stuff
     2
     3

Be aware that it is async by default, so if you execute just this script the script might exit before the events are passed. To disable async behaviour use:

EventAggregator::Message.new("MessageType1", ["Some Stuff",2,3], false).publish
#The third parameter indicates async

Hopefully this can be helpful in your case

Answer 5

I'm not sure of exactly what you mean but you could probably use exceptions in your classes and raise them on certain "events". If you need event for GUI development then most GUI frameworks define their own event handling style.

Hope this somewhat answers you're question.