State Pattern

module TestState
  InvalidStateError = Class.new(RuntimeError)

  def get_state stately
    Object.const_get("TestState::#{stately.state}").new stately
  rescue NameError
    raise InvalidStateError, "Invalid State '#{stately.state}'"
  end
  
  ACTIONS = [:start, :stop]

  ACTIONS.each do |action|
    define_method action do
      get_state(self).public_send action
    end
  end

  class Base
    def initialize stately
      @stately = stately
    end

    TestState::ACTIONS.each do |action|
      define_method action do
         raise InvalidStateError, "Can't #{action} from '#{@stately.state}'"
      end
    end
  end
  
  class New < Base
    def start
      @stately.state = 'Start'
    end
  end

  class Start < Base
    def stop
      @stately.state = 'Stop'
    end
  end

  class Stop < Base

  end

end


class Foo
  attr_accessor :state

  include TestState

  def initialize
    @state = "New"
  end
end


foo = Foo.new
p foo.state

foo.start
p foo.state

begin
  foo.start
rescue
  p "State Exception"
end
p foo.state

foo.stop
p foo.state