fsm_with_conditions.rb

require 'state_machine'

class LightSwitch
  attr_accessor :dark_outside
  alias :dark_outside? :dark_outside

  state_machine :state, :initial => :_off_ do
    event :turn_on do
      transition :_off_ => :_on_, :if => lambda {|ls| ls.dark_outside? }
    end
    event :turn_off do
      transition :_on_ => :_off_
    end
  end
end

ls = LightSwitch.new
ls.state # => "_off_"
ls.dark_outside = false
ls.turn_on # => false
ls.state # => "_off_"
ls.dark_outside = true
ls.turn_on # => true
ls.state # => "_on_"