Trivial state machine example

stateexample.rb

module StateMachine
  def self.included(base) = base.extend(ClassMethods)

  def transition_to(to)
    before = self.class.do_before_transition(state)
    send(before) if before
    return @state = to
  end
        
  def transition(transitions)
    curstate = state
    transitions.each do |from, to|
      transition_to(to) if Array(from).member?(curstate)
    end
    return nil
  end

  def unless_state(*states) = states.member?(state) || yield
  def if_state(*states) = states.member?(state) && yield

  def state = @state || (self.respond_to?(:initial) ? initial : nil)

  module ClassMethods
    def before(from, method) = (@@before ||= {})[from] = method
    def do_before_transition(state) = (@@before||={})[state]
    def event(name, &block)
      define_method(name, block)
      # Do other stuff for introspection here.
    end
  end
end

class Vehicle
  include StateMachine

  def initialize
    @auto_shop_busy = false
  end
  
  def initial = :parked
    
  before(:parked, :put_on_seatbelt)
  
  def park     = transition([:idling, :first_gear] => :parked)
  def ignite   = transition(:parked => :idling)
  def idle     = transition(:first_gear => :idling)
  def shift_up = transition(
    :idling => :first_gear,
    :first_gear => :second,
    :third_gear => :third
  )
  
  def crash = unless_state(:parked, :stalled) do
    transition_to(:stalled) if !passed_inspection?
  end

  def moving? = unless_state(:parked, :stalled, :idling)
  def speed   = if_state(:idling, :first_gear) ? 10 : nil
  def repair  = if_state(:stalled) do
    transition_to(:parked) unless auto_shop_busy
  end
    
  def passed_inspection? = false

  def put_on_seatbelt = puts "putting on seatelt"
end


v = Vehicle.new
p v.state
v.ignite
p v.state
puts "Crashing"
v.crash
p v.state
v.park
p v.state