Ruby Observable uses idiom similar to smalltalk circa 1980. This gist creates MotoObservable with style similar to smalltalk circa 1999.

gistfile1.rb

# Example program at the bottom. This gist can be copy/pasted into a file and run

#================= MotoObservable

require 'set'

# 1. Subscribe using 'when, send, to' terminology to register any method on any object as an event callback
# 2. Use 'trigger_event' to trigger an event along with arbitrary number of arguments
# 3. Simple way of unregistering

module MotoObservable
  def _handlers
    @_handlers ||= {}
  end
  def when(opts={})
    if opts[:send].nil?
      (self._handlers[opts[:event]] ||= {}).delete(opts[:to])
    else
      (self._handlers[opts[:event]] ||= {})[opts[:to]] = opts[:send]
    end
  end
  def trigger_event evt, *args
    self._handlers[evt].each do |observer, callback|
      observer.send(callback, *args)
    end
  end
end

#================= Example Program

# first the tv
class TV
  include MotoObservable
end

# Now the tv
class CouchPotato
  def wake_up_and_repeat_line_from_tv(message, time)
    sleep Kernel.rand(2.0) # (waking up)
    puts "CouchPotato says: 'Hey, the TV said \"#{message}\" at #{time}"
  end
end

tv = TV.new
potato = CouchPotato.new
tv.when(event: :dialog, send: :wake_up_and_repeat_line_from_tv, to: potato)
tv.trigger_event(:dialog, Time.now, "Real housewives from OC up next") #=> CouchPotato says:...
tv.when(event: :dialog, send: nil, to: potato) # Unregister
tv.trigger_event(:dialog, Time.now, "Real housewives from OC up next") #=> nothing happens since we unregistered