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state_machine

state_machine adds support for creating state machines for attributes on any Ruby class.

Resources

API

Bugs

Development

Source

  • git://github.com/pluginaweek/state_machine.git

Description

State machines make it dead-simple to manage the behavior of a class. Too often, the state of an object is kept by creating multiple boolean attributes and deciding how to behave based on the values. This can become cumbersome and difficult to maintain when the complexity of your class starts to increase.

state_machine simplifies this design by introducing the various parts of a real state machine, including states, events, transitions, and callbacks. However, the api is designed to be so simple you don't even need to know what a state machine is :)

Some brief, high-level features include:

  • Defining state machines on any Ruby class

  • Multiple state machines on a single class

  • Namespaced state machines

  • before/after/around/failure transition hooks with explicit transition requirements

  • Integration with ActiveModel, ActiveRecord, DataMapper, Mongoid, MongoMapper, and Sequel

  • State predicates

  • State-driven instance / class behavior

  • State values of any data type

  • Dynamically-generated state values

  • Event parallelization

  • Attribute-based event transitions

  • Path analysis

  • Inheritance

  • Internationalization

  • GraphViz visualization creator

Examples of the usage patterns for some of the above features are shown below. You can find much more detailed documentation in the actual API.

Usage

Example

Below is an example of many of the features offered by this plugin, including:

  • Initial states

  • Namespaced states

  • Transition callbacks

  • Conditional transitions

  • State-driven instance behavior

  • Customized state values

  • Parallel events

  • Path analysis

Class definition:

class Vehicle
  attr_accessor :seatbelt_on, :time_used

  state_machine :state, :initial => :parked do
    before_transition :parked => any - :parked, :do => :put_on_seatbelt

    after_transition :on => :crash, :do => :tow
    after_transition :on => :repair, :do => :fix
    after_transition any => :parked do |vehicle, transition|
      vehicle.seatbelt_on = false
    end

    after_failure :on => :ignite, :do => :log_start_failure

    around_transition do |vehicle, transition, block|
      start = Time.now
      block.call
      vehicle.time_used += Time.now - start
    end

    event :park do
      transition [:idling, :first_gear] => :parked
    end

    event :ignite do
      transition :stalled => same, :parked => :idling
    end

    event :idle do
      transition :first_gear => :idling
    end

    event :shift_up do
      transition :idling => :first_gear, :first_gear => :second_gear, :second_gear => :third_gear
    end

    event :shift_down do
      transition :third_gear => :second_gear, :second_gear => :first_gear
    end

    event :crash do
      transition all - [:parked, :stalled] => :stalled, :unless => :auto_shop_busy?
    end

    event :repair do
      # The first transition that matches the state and passes its conditions
      # will be used
      transition :stalled => :parked, :if => :auto_shop_busy?
      transition :stalled => same
    end

    state :parked do
      def speed
        0
      end
    end

    state :idling, :first_gear do
      def speed
        10
      end
    end

    state :second_gear do
      def speed
        20
      end
    end
  end

  state_machine :alarm_state, :initial => :active, :namespace => 'alarm' do
    event :enable do
      transition all => :active
    end

    event :disable do
      transition all => :off
    end

    state :active, :value => 1
    state :off, :value => 0
  end

  def initialize
    @seatbelt_on = false
    @time_used = 0
    super() # NOTE: This *must* be called, otherwise states won't get initialized
  end

  def put_on_seatbelt
    @seatbelt_on = true
  end

  def auto_shop_busy?
    false
  end

  def tow
    # tow the vehicle
  end

  def fix
    # get the vehicle fixed by a mechanic
  end

  def log_start_failure
    # log a failed attempt to start the vehicle
  end
end

Note the comment made on the initialize method in the class. In order for state machine attributes to be properly initialized, super() must be called. See StateMachine::MacroMethods for more information about this.

Using the above class as an example, you can interact with the state machine like so:

vehicle = Vehicle.new           # => #<Vehicle:0xb7cf4eac @state="parked", @seatbelt_on=false>
vehicle.state                   # => "parked"
vehicle.state_name              # => :parked
vehicle.human_state_name        # => "parked"
vehicle.parked?                 # => true
vehicle.can_ignite?             # => true
vehicle.ignite_transition       # => #<StateMachine::Transition attribute=:state event=:ignite from="parked" from_name=:parked to="idling" to_name=:idling>
vehicle.state_events            # => [:ignite]
vehicle.state_transitions       # => [#<StateMachine::Transition attribute=:state event=:ignite from="parked" from_name=:parked to="idling" to_name=:idling>]
vehicle.speed                   # => 0

vehicle.ignite                  # => true
vehicle.parked?                 # => false
vehicle.idling?                 # => true
vehicle.speed                   # => 10
vehicle                         # => #<Vehicle:0xb7cf4eac @state="idling", @seatbelt_on=true>

vehicle.shift_up                # => true
vehicle.speed                   # => 10
vehicle                         # => #<Vehicle:0xb7cf4eac @state="first_gear", @seatbelt_on=true>

vehicle.shift_up                # => true
vehicle.speed                   # => 20
vehicle                         # => #<Vehicle:0xb7cf4eac @state="second_gear", @seatbelt_on=true>

# The bang (!) operator can raise exceptions if the event fails
vehicle.park!                   # => StateMachine::InvalidTransition: Cannot transition state via :park from :second_gear

# Generic state predicates can raise exceptions if the value does not exist
vehicle.state?(:parked)         # => false
vehicle.state?(:invalid)        # => IndexError: :invalid is an invalid name

# Namespaced machines have uniquely-generated methods
vehicle.alarm_state             # => 1
vehicle.alarm_state_name        # => :active

vehicle.can_disable_alarm?      # => true
vehicle.disable_alarm           # => true
vehicle.alarm_state             # => 0
vehicle.alarm_state_name        # => :off
vehicle.can_enable_alarm?       # => true

vehicle.alarm_off?              # => true
vehicle.alarm_active?           # => false

# Events can be fired in parallel
vehicle.fire_events(:shift_down, :enable_alarm) # => true
vehicle.state_name                              # => :first_gear
vehicle.alarm_state_name                        # => :active

vehicle.fire_events!(:ignite, :enable_alarm)    # => StateMachine::InvalidTransition: Cannot run events in parallel: ignite, enable_alarm

# Human-friendly names can be accessed for states/events
Vehicle.human_state_name(:first_gear)               # => "first gear"
Vehicle.human_alarm_state_name(:active)             # => "active"

Vehicle.human_state_event_name(:shift_down)         # => "shift down"
Vehicle.human_alarm_state_event_name(:enable)       # => "enable"

# Available transition paths can be analyzed for an object
vehicle.state_paths                                       # => [[#<StateMachine::Transition ...], [#<StateMachine::Transition ...], ...]
vehicle.state_paths.to_states                             # => [:parked, :idling, :first_gear, :stalled, :second_gear, :third_gear]
vehicle.state_paths.events                                # => [:park, :ignite, :shift_up, :idle, :crash, :repair, :shift_down]

# Find all paths that start and end on certain states
vehicle.state_paths(:from => :parked, :to => :first_gear) # => [[
                                                          #       #<StateMachine::Transition attribute=:state event=:ignite from="parked" ...>,
                                                          #       #<StateMachine::Transition attribute=:state event=:shift_up from="idling" ...>
                                                          #    ]]

Integrations

In addition to being able to define state machines on all Ruby classes, a set of out-of-the-box integrations are available for some of the more popular Ruby libraries. These integrations add library-specific behavior, allowing for state machines to work more tightly with the conventions defined by those libraries.

The integrations currently available include:

  • ActiveModel classes

  • ActiveRecord models

  • DataMapper resources

  • Mongoid models

  • MongoMapper models

  • Sequel models

A brief overview of these integrations is described below.

ActiveModel

The ActiveModel integration is useful for both standalone usage and for providing the base implementation for ORMs which implement the ActiveModel API. This integration adds support for validation errors, dirty attribute tracking, and observers. For example,

class Vehicle
  include ActiveModel::Dirty
  include ActiveModel::Validations
  include ActiveModel::Observing

  attr_accessor :state
  define_attribute_methods [:state]

  state_machine :initial => :parked do
    before_transition :parked => any - :parked, :do => :put_on_seatbelt
    after_transition any => :parked do |vehicle, transition|
      vehicle.seatbelt = 'off'
    end
    around_transition :benchmark

    event :ignite do
      transition :parked => :idling
    end

    state :first_gear, :second_gear do
      validates_presence_of :seatbelt_on
    end
  end

  def put_on_seatbelt
    ...
  end

  def benchmark
    ...
    yield
    ...
  end
end

class VehicleObserver < ActiveModel::Observer
  # Callback for :ignite event *before* the transition is performed
  def before_ignite(vehicle, transition)
    # log message
  end

  # Generic transition callback *after* the transition is performed
  def after_transition(vehicle, transition)
    Audit.log(vehicle, transition)
  end

  # Generic callback after the transition fails to perform
  def after_failure_to_transition(vehicle, transition)
    Audit.error(vehicle, transition)
  end
end

For more information about the various behaviors added for ActiveModel state machines and how to build new integrations that use ActiveModel, see StateMachine::Integrations::ActiveModel.

ActiveRecord

The ActiveRecord integration adds support for database transactions, automatically saving the record, named scopes, validation errors, and observers. For example,

class Vehicle < ActiveRecord::Base
  state_machine :initial => :parked do
    before_transition :parked => any - :parked, :do => :put_on_seatbelt
    after_transition any => :parked do |vehicle, transition|
      vehicle.seatbelt = 'off'
    end
    around_transition :benchmark

    event :ignite do
      transition :parked => :idling
    end

    state :first_gear, :second_gear do
      validates_presence_of :seatbelt_on
    end
  end

  def put_on_seatbelt
    ...
  end

  def benchmark
    ...
    yield
    ...
  end
end

class VehicleObserver < ActiveRecord::Observer
  # Callback for :ignite event *before* the transition is performed
  def before_ignite(vehicle, transition)
    # log message
  end

  # Generic transition callback *after* the transition is performed
  def after_transition(vehicle, transition)
    Audit.log(vehicle, transition)
  end
end

For more information about the various behaviors added for ActiveRecord state machines, see StateMachine::Integrations::ActiveRecord.

DataMapper

Like the ActiveRecord integration, the DataMapper integration adds support for database transactions, automatically saving the record, named scopes, Extlib-like callbacks, validation errors, and observers. For example,

class Vehicle
  include DataMapper::Resource

  property :id, Serial
  property :state, String

  state_machine :initial => :parked do
    before_transition :parked => any - :parked, :do => :put_on_seatbelt
    after_transition any => :parked do |transition|
      self.seatbelt = 'off' # self is the record
    end
    around_transition :benchmark

    event :ignite do
      transition :parked => :idling
    end

    state :first_gear, :second_gear do
      validates_presence_of :seatbelt_on
    end
  end

  def put_on_seatbelt
    ...
  end

  def benchmark
    ...
    yield
    ...
  end
end

class VehicleObserver
  include DataMapper::Observer

  observe Vehicle

  # Callback for :ignite event *before* the transition is performed
  before_transition :on => :ignite do |transition|
    # log message (self is the record)
  end

  # Generic transition callback *after* the transition is performed
  after_transition do |transition|
    Audit.log(self, transition) # self is the record
  end

  around_transition do |transition, block|
    # mark start time
    block.call
    # mark stop time
  end

  # Generic callback after the transition fails to perform
  after_transition_failure do |transition|
    Audit.log(self, transition) # self is the record
  end
end

Note that the DataMapper::Observer integration is optional and only available when the dm-observer library is installed.

For more information about the various behaviors added for DataMapper state machines, see StateMachine::Integrations::DataMapper.

Mongoid

The Mongoid integration adds support for automatically saving the record, basic scopes, validation errors and callbacks. For example,

class Vehicle
  include Mongoid::Document

  state_machine :initial => :parked do
    before_transition :parked => any - :parked, :do => :put_on_seatbelt
    after_transition any => :parked do |vehicle, transition|
      vehicle.seatbelt = 'off' # self is the record
    end
    around_transition :benchmark

    event :ignite do
      transition :parked => :idling
    end

    state :first_gear, :second_gear do
      validates_presence_of :seatbelt_on
    end
  end

  def put_on_seatbelt
    ...
  end

  def benchmark
    ...
    yield
    ...
  end
end

For more information about the various behaviors added for Mongoid state machines, see StateMachine::Integrations::Mongoid.

MongoMapper

The MongoMapper integration adds support for automatically saving the record, basic scopes, validation errors and callbacks. For example,

class Vehicle
  include MongoMapper::Document

  state_machine :initial => :parked do
    before_transition :parked => any - :parked, :do => :put_on_seatbelt
    after_transition any => :parked do |vehicle, transition|
      vehicle.seatbelt = 'off' # self is the record
    end
    around_transition :benchmark

    event :ignite do
      transition :parked => :idling
    end

    state :first_gear, :second_gear do
      validates_presence_of :seatbelt_on
    end
  end

  def put_on_seatbelt
    ...
  end

  def benchmark
    ...
    yield
    ...
  end
end

For more information about the various behaviors added for MongoMapper state machines, see StateMachine::Integrations::MongoMapper.

Sequel

Like the ActiveRecord integration, the Sequel integration adds support for database transactions, automatically saving the record, named scopes, validation errors and callbacks. For example,

class Vehicle < Sequel::Model
  state_machine :initial => :parked do
    before_transition :parked => any - :parked, :do => :put_on_seatbelt
    after_transition any => :parked do |transition|
      self.seatbelt = 'off' # self is the record
    end
    around_transition :benchmark

    event :ignite do
      transition :parked => :idling
    end

    state :first_gear, :second_gear do
      validates_presence_of :seatbelt_on
    end
  end

  def put_on_seatbelt
    ...
  end

  def benchmark
    ...
    yield
    ...
  end
end

For more information about the various behaviors added for Sequel state machines, see StateMachine::Integrations::Sequel.

Compatibility

Although state_machine introduces a simplified syntax, it still remains backwards compatible with previous versions and other state-related libraries. For example, transitions and callbacks can continue to be defined like so:

class Vehicle
  state_machine :initial => :parked do
    before_transition :from => :parked, :except_to => :parked, :do => :put_on_seatbelt
    after_transition :to => :parked do |transition|
      self.seatbelt = 'off' # self is the record
    end

    event :ignite do
      transition :from => :parked, :to => :idling
    end
  end
end

Although this verbose syntax will most likely always be supported, it is recommended that any state machines eventually migrate to the syntax introduced in version 0.6.0.

Tools

Generating graphs

This library comes with built-in support for generating di-graphs based on the events, states, and transitions defined for a state machine using GraphViz. This requires that both the ruby-graphviz gem and graphviz library be installed on the system.

Examples

To generate a graph for a specific file / class:

rake state_machine:draw FILE=vehicle.rb CLASS=Vehicle

To save files to a specific path:

rake state_machine:draw FILE=vehicle.rb CLASS=Vehicle TARGET=files

To customize the image format / orientation:

rake state_machine:draw FILE=vehicle.rb CLASS=Vehicle FORMAT=jpg ORIENTATION=landscape

To generate multiple state machine graphs:

rake state_machine:draw FILE=vehicle.rb,car.rb CLASS=Vehicle,Car

Note that this will generate a different file for every state machine defined in the class. The generated files will use an output filename of the format #{class_name}_#{machine_name}.#{format}.

For examples of actual images generated using this task, see those under the examples folder.

Ruby on Rails Integration

There is a special integration Rake task for generating state machines for classes used in a Ruby on Rails application. This task will load the application environment, meaning that it's unnecessary to specify the actual file to load.

For example,

rake state_machine:draw CLASS=Vehicle

If you are using this library as a gem in Rails 2.x, the following must be added to the end of your application's Rakefile in order for the above task to work:

require 'tasks/state_machine'

If you are using Rails 3.0+, you must also add the following to your application's Gemfile:

gem 'ruby-graphviz', :require => 'graphviz'

Merb Integration

Like Ruby on Rails, there is a special integration Rake task for generating state machines for classes used in a Merb application. This task will load the application environment, meaning that it's unnecessary to specify the actual files to load.

For example,

rake state_machine:draw CLASS=Vehicle

Interactive graphs

Jean Bovet's Visual Automata Simulator is a great tool for “simulating, visualizing and transforming finite state automata and Turing Machines”. It can help in the creation of states and events for your models. It is cross-platform, written in Java.

Testing

To run the core test suite (does not test any of the integrations):

rake test

Test specific versions of integrations like so:

rake test INTEGRATION=active_model VERSION=3.0.0
rake test INTEGRATION=active_record VERSION=2.0.0
rake test INTEGRATION=data_mapper VERSION=0.9.4
rake test INTEGRATION=mongoid VERSION=2.0.0
rake test INTEGRATION=mongo_mapper VERSION=0.5.5
rake test INTEGRATION=sequel VERSION=2.8.0

Caveats

The following caveats should be noted when using state_machine:

  • DataMapper: Attribute-based event transitions are disabled when dm-validations 0.9.4 - 0.9.6 is in use

  • Overridden event methods won't get invoked when using attribute-based event transitions

  • around_transition callbacks in ORM integrations won't work on JRuby since it doesn't support continuations

Dependencies

  • Ruby 1.8.6 or later

If using specific integrations:

If graphing state machine:

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