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Adds support for creating state machines for attributes on any Ruby class

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README.rdoc

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 status 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 transition hooks with explicit transition requirements

  • ActiveRecord integration

  • DataMapper integration

  • Sequel integration

  • States of any data type

  • State predicates

  • State-driven behavior

  • GraphViz visualization creator

Examples of the usage patterns for some of the above features are shown below. You can find 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 behavior

Class definition:

class Vehicle
  attr_accessor :seatbelt_on

  state_machine :state, :initial => 'parked' do
    before_transition :from => %w(parked idling), :do => :put_on_seatbelt
    after_transition :on => 'crash', :do => :tow
    after_transition :on => 'repair', :do => :fix
    after_transition :to => 'parked' do |vehicle, transition|
      vehicle.seatbelt_on = false
    end

    event :park do
      transition :to => 'parked', :from => %w(idling first_gear)
    end

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

    event :idle do
      transition :to => 'idling', :from => 'first_gear'
    end

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

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

    event :crash do
      transition :to => 'stalled', :from => %w(first_gear second_gear third_gear), :unless => :auto_shop_busy?
    end

    event :repair do
      transition :to => 'parked', :from => 'stalled', :if => :auto_shop_busy?
    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 :hood_state, :initial => 'closed', :namespace => 'hood' do
    event :open do
      transition :to => 'opened', :from => 'closed'
    end

    event :close do
      transition :to => 'closed', :from => 'opened'
    end
  end

  def initialize
    @seatbelt_on = false
    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
end

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.parked?                 # => true
vehicle.can_ignite?             # => true
vehicle.next_ignite_transition  # => #<StateMachine::Transition:0xb7c34cec ...>
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 via :park from "second_gear"

# Generic state predicates can raise exceptions if the value does not exist
vehicle.state?('parked')        # => true
vehicle.state?('invalid')       # => ArgumentError: "parked" is not a known state value

# Namespaced machines have uniquely-generated methods
vehicle.can_open_hood?          # => true
vehicle.open_hood               # => true
vehicle.can_close_hood?         # => true

vehicle.hood_opened?            # => true
vehicle.hood_closed?            # => false

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.

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:

  • ActiveRecord models

  • DataMapper resources

  • Sequel models

A brief overview of these integrations is described below.

ActiveRecord

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

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

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

  def put_on_seatbelt
    ...
  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 *before* 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.

With enumerations

Using the acts_as_enumeration plugin with an ActiveRecord integration, states can be transparently stored using record ids in the database like so:

class VehicleState < ActiveRecord::Base
  acts_as_enumeration

  create :id => 1, :name => 'parked'
  create :id => 2, :name => 'idling'
  ...
end

class Vehicle < ActiveRecord::Base
  belongs_to :state, :class_name => 'VehicleState'

  state_machine :state, :initial => 'parked' do
    ...

    event :park do
      transition :to => 'parked', :from => %w(idling first_gear)
    end
  end

  ...
end

Notice that the state machine definition remains exactly the same. However, when interacting with the records, the actual state will be stored using the identifiers defined for the enumeration:

vehicle = Vehicle.create  # => #<Vehicle id: 1, seatbelt_on: false, state_id: 1>
vehicle.ignite            # => true
vehicle                   # => #<Vehicle id: 1, seatbelt_on: true, state_id: 2>

This allows states to take on more complex functionality other than just being a string value.

DataMapper

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

class Vehicle
  include DataMapper::Resource

  property :id, Serial
  property :state, String

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

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

  def put_on_seatbelt
    ...
  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 *before* the transition is performed
  after_transition do |transition, saved|
    Audit.log(self, transition) if saved # 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.

Sequel

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

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

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

  def put_on_seatbelt
    ...
  end
end

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

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:

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

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}_#{attribute}.#{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:rails CLASS=Vehicle

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:merb CLASS=Vehicle

Interactive graphs

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

Testing

To run the entire test suite (will test ActiveRecord, DataMapper, and Sequel integrations if the proper dependencies are available):

rake test

Dependencies

By default, there are no dependencies. If using specific integrations, those dependencies are listed below.

References

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