A simple DSL to decorate existing methods with state transition guards.

Instead of using a DSL to define events, SimpleStateMachine decorates methods to help you encapsulate state and guard state transitions.

Write a method, arguments are allowed:

def activate_account(activation_code)
  # call other methods, no need to add these in callbacks
  ...
  log.debug "Try to activate account with #{activation_code}"
end

Now mark the method as an event and specify how the state should transition when the method is called. In this example, the activate_account method will set the state to :active if the initial state is :pending.

event :activate_account, :pending => :active

That’s it! You can now call the method and the state will automatically change. If the state change is not allowed, a SimpleStateMachine::Error is raised.

When using ActiveRecord / ActiveModel you can add an error to the errors object. This will prevent the state from being changed.

def activate_account(activation_code)
  if activation_code_invalid?(activation_code)
    errors.add(:activation_code, 'Invalid')
  end
end

You can rescue exceptions and specify the failure state

def download_data
  Service.download_data
end
event :download_data, :pending => :downloaded,
      Service::ConnectionError => :download_failed

To add a state machine:

• extend SimpleStateMachine

• set the initial state

• turn methods into events

  class LampWithHotelSwitch
  
    extend SimpleStateMachine
  
    def initialize
      self.state = :off
    end
  
    def push_switch_1
      puts 'pushed switch 1 #{state}'
    end
    event :push_switch_1, :off => :on,
                          :on  => :off
  
    # define another event
    # note that implementation of :push_switch_2 is optional
    event :push_switch_2, :off => :on,
                          :on  => :off
  
  end
  

To add a state machine with ActiveRecord persistence:

• extend SimpleStateMachine::ActiveRecord,

• set the initial state in after_initialize,

• turn methods into events

  class User < ActiveRecord::Base
  
    extend SimpleStateMachine::ActiveRecord
    # define a custum state_method (state is default)
    state_machine_definition.state_method = :ssm_state
  
    def after_initialize
      self.ssm_state ||= 'new'
      # if you get an ActiveRecord::MissingAttributeError
      # you'll probably need to do (http://bit.ly/35q23b):
      #   write_attribute(:ssm_state, "new") unless read_attribute(:ssm_state)
    end
  
    def invite
      self.activation_code = Digest::SHA1.hexdigest("salt #{Time.now.to_f}")
      #send_activation_email
    end
    event :invite, :new => :invited
  
    def confirm_invitation activation_code
      if self.activation_code != activation_code
        errors.add 'activation_code', 'is invalid'
      end
    end
    event :confirm_invitation, :invited => :active
  
    # :all can be used to catch all from states
    event :suspend, :all => :suspended
  end
  

This generates the following methods

• {event}_and_save works like save

• {event}_and_save! works like save!

• {event}! works the same as {event}_and_save!

• {state}? whether or not the current state is {state}

This code was just released, we do not claim it to be stable.

• Fork the project.

• Make your feature addition or bug fix.

• Add tests for it. This is important so I don’t break it in a future version unintentionally.

• Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)

• Send me a pull request. Bonus points for topic branches.

Copyright © 2010 Marek & Petrik. See LICENSE for details.