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?

Solution 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

Solution 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.

Solution 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!'

Solution 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

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Comments

[Boris B.]

This answer feels the most ruby-ish. It relies on dynamic nature of the language rather than emulating a strongly-typed one, it's simple to use and understand, and it supports multicast.