Class StateMachine::Machine
In: lib/state_machine/machine.rb
Parent: Object

Represents a state machine for a particular attribute. State machines consist of states, events and a set of transitions that define how the state changes after a particular event is fired.

A state machine will not know all of the possible states for an object unless they are referenced somewhere in the state machine definition. As a result, any unused states should be defined with the other_states or state helper.

Actions

When an action is configured for a state machine, it is invoked when an object transitions via an event. The success of the event becomes dependent on the success of the action. If the action is successful, then the transitioned state remains persisted. However, if the action fails (by returning false), the transitioned state will be rolled back.

For example, 

  class Vehicle
    attr_accessor :fail, :saving_state

    state_machine :initial => :parked, :action => :save do
      event :ignite do
        transition :parked => :idling
      end

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

    def save
      @saving_state = state
      fail != true
    end
  end

  vehicle = Vehicle.new     # => #<Vehicle:0xb7c27024 @state="parked">
  vehicle.save              # => true
  vehicle.saving_state      # => "parked" # The state was "parked" was save was called

  # Successful event
  vehicle.ignite            # => true
  vehicle.saving_state      # => "idling" # The state was "idling" when save was called
  vehicle.state             # => "idling"

  # Failed event
  vehicle.fail = true
  vehicle.park              # => false
  vehicle.saving_state      # => "parked"
  vehicle.state             # => "idling"

As shown, even though the state is set prior to calling the save action on the object, it will be rolled back to the original state if the action fails. Note that this will also be the case if an exception is raised while calling the action.

Indirect transitions

In addition to the action being run as the result of an event, the action can also be used to run events itself. For example, using the above as an example: 

  vehicle = Vehicle.new           # => #<Vehicle:0xb7c27024 @state="parked">

  vehicle.state_event = 'ignite'
  vehicle.save                    # => true
  vehicle.state                   # => "idling"
  vehicle.state_event             # => nil

As can be seen, the save action automatically invokes the event stored in the state_event attribute (:ignite in this case).

One important note about using this technique for running transitions is that if the class in which the state machine is defined also defines the action being invoked (and not a superclass), then it must manually run the StateMachine hook that checks for event attributes.

For example, in ActiveRecord, DataMapper, Mongoid, MongoMapper, and Sequel, the default action (save) is already defined in a base class. As a result, when a state machine is defined in a model / resource, StateMachine can automatically hook into the save action.

On the other hand, the Vehicle class from above defined its own save method (and there is no save method in its superclass). As a result, it must be modified like so: 

    def save
      self.class.state_machines.transitions(self, :save).perform do
        @saving_state = state
        fail != true
      end
    end

This will add in the functionality for firing the event stored in the state_event attribute.

Callbacks

Callbacks are supported for hooking before and after every possible transition in the machine. Each callback is invoked in the order in which it was defined. See StateMachine::Machine#before_transition and StateMachine::Machine#after_transition for documentation on how to define new callbacks.

Note that callbacks only get executed within the context of an event. As a result, if a class has an initial state when it‘s created, any callbacks that would normally get executed when the object enters that state will not get triggered.

For example, 

  class Vehicle
    state_machine :initial => :parked do
      after_transition all => :parked do
        raise ArgumentError
      end
      ...
    end
  end

  vehicle = Vehicle.new   # => #<Vehicle id: 1, state: "parked">
  vehicle.save            # => true (no exception raised)

If you need callbacks to get triggered when an object is created, this should be done by one of the following techniques:

  • Use a before :create or equivalent hook: 
      class Vehicle
    before :create, :track_initial_transition

    state_machine do
      ...
    end
  end
  • Set an initial state and use the correct event to create the object with the proper state, resulting in callbacks being triggered and the object getting persisted (note that the :pending state is actually stored as nil): 
      class Vehicle
     state_machine :initial => :pending
      after_transition :pending => :parked, :do => :track_initial_transition

      event :park do
        transition :pending => :parked
      end

      state :pending, :value => nil
    end
  end

  vehicle = Vehicle.new
  vehicle.park
  • Use a default event attribute that will automatically trigger when the configured action gets run (note that the :pending state is actually stored as nil): 
      class Vehicle < ActiveRecord::Base
    state_machine :initial => :pending
      after_transition :pending => :parked, :do => :track_initial_transition

      event :park do
        transition :pending => :parked
      end

      state :pending, :value => nil
    end

    def initialize(*)
      super
      self.state_event = 'park'
    end
  end

  vehicle = Vehicle.new
  vehicle.save

Canceling callbacks

Callbacks can be canceled by throwing :halt at any point during the callback. For example, 

  ...
  throw :halt
  ...

If a before callback halts the chain, the associated transition and all later callbacks are canceled. If an after callback halts the chain, the later callbacks are canceled, but the transition is still successful.

These same rules apply to around callbacks with the exception that any around callback that doesn‘t yield will essentially result in :halt being thrown. Any code executed after the yield will behave in the same way as after callbacks.

Note that if a before callback fails and the bang version of an event was invoked, an exception will be raised instead of returning false. For example, 

  class Vehicle
    state_machine :initial => :parked do
      before_transition any => :idling, :do => lambda {|vehicle| throw :halt}
      ...
    end
  end

  vehicle = Vehicle.new
  vehicle.park        # => false
  vehicle.park!       # => StateMachine::InvalidTransition: Cannot transition state via :park from "idling"

Observers

Observers, in the sense of external classes and not Ruby‘s Observable mechanism, can hook into state machines as well. Such observers use the same callback api that‘s used internally.

Below are examples of defining observers for the following state machine: 

  class Vehicle
    state_machine do
      event :park do
        transition :idling => :parked
      end
      ...
    end
    ...
  end

Event/Transition behaviors: 

  class VehicleObserver
    def self.before_park(vehicle, transition)
      logger.info "#{vehicle} instructed to park... state is: #{transition.from}, state will be: #{transition.to}"
    end

    def self.after_park(vehicle, transition, result)
      logger.info "#{vehicle} instructed to park... state was: #{transition.from}, state is: #{transition.to}"
    end

    def self.before_transition(vehicle, transition)
      logger.info "#{vehicle} instructed to #{transition.event}... #{transition.attribute} is: #{transition.from}, #{transition.attribute} will be: #{transition.to}"
    end

    def self.after_transition(vehicle, transition)
      logger.info "#{vehicle} instructed to #{transition.event}... #{transition.attribute} was: #{transition.from}, #{transition.attribute} is: #{transition.to}"
    end

    def self.around_transition(vehicle, transition)
      logger.info Benchmark.measure { yield }
    end
  end

  Vehicle.state_machine do
    before_transition :on => :park, :do => VehicleObserver.method(:before_park)
    before_transition VehicleObserver.method(:before_transition)

    after_transition :on => :park, :do => VehicleObserver.method(:after_park)
    after_transition VehicleObserver.method(:after_transition)

    around_transition VehicleObserver.method(:around_transition)
  end

One common callback is to record transitions for all models in the system for auditing/debugging purposes. Below is an example of an observer that can easily automate this process for all models: 

  class StateMachineObserver
    def self.before_transition(object, transition)
      Audit.log_transition(object.attributes)
    end
  end

  [Vehicle, Switch, Project].each do |klass|
    klass.state_machines.each do |attribute, machine|
      machine.before_transition StateMachineObserver.method(:before_transition)
    end
  end

Additional observer-like behavior may be exposed by the various integrations available. See below for more information on integrations.

Overriding instance / class methods

Hooking in behavior to the generated instance / class methods from the state machine, events, and states is very simple because of the way these methods are generated on the class. Using the class‘s ancestors, the original generated method can be referred to via super. For example, 

  class Vehicle
    state_machine do
      event :park do
        ...
      end
    end

    def park(*args)
      logger.info "..."
      super
    end
  end

In the above example, the park instance method that‘s generated on the Vehicle class (by the associated event) is overridden with custom behavior. Once this behavior is complete, the original method from the state machine is invoked by simply calling super.

The same technique can be used for state, state_name, and all other instance and class methods on the Vehicle class.

Method conflicts

By default state_machine does not redefine methods that exist on superclasses (including Object) or any modules (including Kernel) that were included before it was defined. This is in order to ensure that existing behavior on the class is not broken by the inclusion of state_machine.

If a conflicting method is detected, state_machine will generate a warning. For example, consider the following class: 

  class Vehicle
    state_machine do
      event :open do
        ...
      end
    end
  end

In the above class, an event named "open" is defined for its state machine. However, "open" is already defined as an instance method in Ruby‘s Kernel module that gets included in every Object. As a result, state_machine will generate the following warning: 

  Instance method "open" is already defined in Object, use generic helper instead or set StateMachine::Machine.ignore_method_conflicts = true.

Even though you may not be using Kernel‘s implementation of the "open" instance method, state_machine isn‘t aware of this and, as a result, stays safe and just skips redefining the method.

As with almost all helpers methods defined by state_machine in your class, there are generic methods available for working around this method conflict. In the example above, you can invoke the "open" event like so: 

  vehicle = Vehicle.new       # => #<Vehicle:0xb72686b4 @state=nil>
  vehicle.fire_events(:open)  # => true

  # This will not work
  vehicle.open                # => NoMethodError: private method `open' called for #<Vehicle:0xb72686b4 @state=nil>

If you want to take on the risk of overriding existing methods and just ignore method conflicts altogether, you can do so by setting the following configuration: 

  StateMachine::Machine.ignore_method_conflicts = true

This will allow you to define events like "open" as described above and still generate the "open" instance helper method. For example: 

  StateMachine::Machine.ignore_method_conflicts = true

  class Vehicle
    state_machine do
      event :open do
        ...
    end
  end

  vehicle = Vehicle.new   # => #<Vehicle:0xb72686b4 @state=nil>
  vehicle.open            # => true

By default, state_machine helps prevent you from making mistakes and accidentally overriding methods that you didn‘t intend to. Once you understand this and what the consequences are, setting the ignore_method_conflicts option is a perfectly reasonable workaround.

Integrations

By default, state machines are library-agnostic, meaning that they work on any Ruby class and have no external dependencies. However, there are certain libraries which expose additional behavior that can be taken advantage of by state machines.

This library is built to work out of the box with a few popular Ruby libraries that allow for additional behavior to provide a cleaner and smoother experience. This is especially the case for objects backed by a database that may allow for transactions, persistent storage, search/filters, callbacks, etc.

When a state machine is defined for classes using any of the above libraries, it will try to automatically determine the integration to use (Agnostic, ActiveModel, ActiveRecord, DataMapper, Mongoid, MongoMapper, or Sequel) based on the class definition. To see how each integration affects the machine‘s behavior, refer to all constants defined under the StateMachine::Integrations namespace.

Methods

action_hook   action_hook?   add_callback   add_events   add_sibling_machine_configs   add_states   after_failure   after_initialize   after_transition   around_transition   attribute   before_transition   create_with_scope   create_without_scope   define_action_helpers   define_action_helpers?   define_action_hook   define_event_helpers   define_helper   define_helpers   define_name_helpers   define_path_helpers   define_scopes   define_state_accessor   define_state_initializer   define_state_predicate   draw   draw   dynamic_initial_state?   errors_for   event   find_or_create   generate_message   initial_state   initial_state=   initialize_state   initialize_state?   invalidate   new   on   other_states   owner_class=   owner_class_ancestor_has_method?   paths_for   pluralize   read   reset   run_scope   sibling_machines   state   transaction   transition   within_transaction   write  

Included Modules

Assertions EvalHelpers MatcherHelpers StateMachine::InstanceMethods

Attributes

action  [R]  The action to invoke when an object transitions
callbacks  [R]  The callbacks to invoke before/after a transition is performed

Maps :before => callbacks and :after => callbacks

default_messages  [RW] 
events  [R]  The events that trigger transitions. These are sorted, by default, in the order in which they were defined.
ignore_method_conflicts  [RW] 
name  [R]  The name of the machine, used for scoping methods generated for the machine as a whole (not states or events)
namespace  [R]  An identifier that forces all methods (including state predicates and event methods) to be generated with the value prefixed or suffixed, depending on the context.
owner_class  [RW]  The class that the machine is defined in
states  [R]  A list of all of the states known to this state machine. This will pull states from the following sources:
  • Initial state
  • State behaviors
  • Event transitions (:to, :from, and :except_from options)
  • Transition callbacks (:to, :from, :except_to, and :except_from options)
  • Unreferenced states (using other_states helper)

These are sorted, by default, in the order in which they were referenced.

use_transactions  [R]  Whether the machine will use transactions when firing events

Public Class methods

draw(class_names, options = {})

Draws the state machines defined in the given classes using GraphViz. The given classes must be a comma-delimited string of class names.

Configuration options:

  • :file - A comma-delimited string of files to load that contain the state machine definitions to draw
  • :path - The path to write the graph file to
  • :format - The image format to generate the graph in
  • :font - The name of the font to draw state names in

[Source]

     # File lib/state_machine/machine.rb, line 464
464:       def draw(class_names, options = {})
465:         raise ArgumentError, 'At least one class must be specified' unless class_names && class_names.split(',').any?
466:         
467:         # Load any files
468:         if files = options.delete(:file)
469:           files.split(',').each {|file| require file}
470:         end
471:         
472:         class_names.split(',').each do |class_name|
473:           # Navigate through the namespace structure to get to the class
474:           klass = Object
475:           class_name.split('::').each do |name|
476:             klass = klass.const_defined?(name) ? klass.const_get(name) : klass.const_missing(name)
477:           end
478:           
479:           # Draw each of the class's state machines
480:           klass.state_machines.each_value do |machine|
481:             machine.draw(options)
482:           end
483:         end
484:       end
find_or_create(owner_class, *args, &block)

Attempts to find or create a state machine for the given class. For example, 

  StateMachine::Machine.find_or_create(Vehicle)
  StateMachine::Machine.find_or_create(Vehicle, :initial => :parked)
  StateMachine::Machine.find_or_create(Vehicle, :status)
  StateMachine::Machine.find_or_create(Vehicle, :status, :initial => :parked)

If a machine of the given name already exists in one of the class‘s superclasses, then a copy of that machine will be created and stored in the new owner class (the original will remain unchanged).

[Source]

     # File lib/state_machine/machine.rb, line 431
431:       def find_or_create(owner_class, *args, &block)
432:         options = args.last.is_a?(Hash) ? args.pop : {}
433:         name = args.first || :state
434:         
435:         # Find an existing machine
436:         if owner_class.respond_to?(:state_machines) && machine = owner_class.state_machines[name]
437:           # Only create a new copy if changes are being made to the machine in
438:           # a subclass
439:           if machine.owner_class != owner_class && (options.any? || block_given?)
440:             machine = machine.clone
441:             machine.initial_state = options[:initial] if options.include?(:initial)
442:             machine.owner_class = owner_class
443:           end
444:           
445:           # Evaluate DSL
446:           machine.instance_eval(&block) if block_given?
447:         else
448:           # No existing machine: create a new one
449:           machine = new(owner_class, name, options, &block)
450:         end
451:         
452:         machine
453:       end
new(owner_class, *args, &block)

Creates a new state machine for the given attribute

[Source]

     # File lib/state_machine/machine.rb, line 539
539:     def initialize(owner_class, *args, &block)
540:       options = args.last.is_a?(Hash) ? args.pop : {}
541:       assert_valid_keys(options, :attribute, :initial, :initialize, :action, :plural, :namespace, :integration, :messages, :use_transactions)
542:       
543:       # Find an integration that matches this machine's owner class
544:       if options.include?(:integration)
545:         @integration = StateMachine::Integrations.find_by_name(options[:integration]) if options[:integration]
546:       else
547:         @integration = StateMachine::Integrations.match(owner_class)
548:       end
549:       
550:       if @integration
551:         extend @integration
552:         options = (@integration.defaults || {}).merge(options)
553:       end
554:       
555:       # Add machine-wide defaults
556:       options = {:use_transactions => true, :initialize => true}.merge(options)
557:       
558:       # Set machine configuration
559:       @name = args.first || :state
560:       @attribute = options[:attribute] || @name
561:       @events = EventCollection.new(self)
562:       @states = StateCollection.new(self)
563:       @callbacks = {:before => [], :after => [], :failure => []}
564:       @namespace = options[:namespace]
565:       @messages = options[:messages] || {}
566:       @action = options[:action]
567:       @use_transactions = options[:use_transactions]
568:       @initialize_state = options[:initialize]
569:       self.owner_class = owner_class
570:       self.initial_state = options[:initial] unless sibling_machines.any?
571:       
572:       # Merge with sibling machine configurations
573:       add_sibling_machine_configs
574:       
575:       # Define class integration
576:       define_helpers
577:       define_scopes(options[:plural])
578:       after_initialize
579:       
580:       # Evaluate DSL
581:       instance_eval(&block) if block_given?
582:     end

Public Instance methods

action_hook?(self_only = false)

Determines whether an action hook was defined for firing attribute-based event transitions when the configured action gets called.

[Source]

      # File lib/state_machine/machine.rb, line 1917
1917:     def action_hook?(self_only = false)
1918:       @action_hook_defined || !self_only && owner_class.state_machines.any? {|name, machine| machine.action == action && machine != self && machine.action_hook?(true)}
1919:     end
after_failure(*args, &block)

Creates a callback that will be invoked after a transition failures to be performed so long as the given requirements match the transition.

See before_transition for a description of the possible configurations for defining callbacks. Note however that you cannot define the state requirements in these callbacks. You may only define event requirements.

The callback

Failure callbacks get invoked whenever an event fails to execute. This can happen when no transition is available, a before callback halts execution, or the action associated with this machine fails to succeed. In any of these cases, any failure callback that matches the attempted transition will be run.

For example, 

  class Vehicle
    state_machine do
      after_failure do |vehicle, transition|
        logger.error "vehicle #{vehicle} failed to transition on #{transition.event}"
      end

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

      ...
    end
  end

[Source]

      # File lib/state_machine/machine.rb, line 1730
1730:     def after_failure(*args, &block)
1731:       options = (args.last.is_a?(Hash) ? args.pop : {})
1732:       options[:do] = args if args.any?
1733:       assert_valid_keys(options, :on, :do, :if, :unless)
1734:       
1735:       add_callback(:failure, options, &block)
1736:     end
after_transition(*args, &block)

Creates a callback that will be invoked after a transition is performed so long as the given requirements match the transition.

See before_transition for a description of the possible configurations for defining callbacks.

[Source]

      # File lib/state_machine/machine.rb, line 1635
1635:     def after_transition(*args, &block)
1636:       options = (args.last.is_a?(Hash) ? args.pop : {})
1637:       options[:do] = args if args.any?
1638:       add_callback(:after, options, &block)
1639:     end
around_transition(*args, &block)

Creates a callback that will be invoked around a transition so long as the given requirements match the transition.

The callback

Around callbacks wrap transitions, executing code both before and after. These callbacks are defined in the exact same manner as before / after callbacks with the exception that the transition must be yielded to in order to finish running it.

If defining around callbacks using blocks, you must yield within the transition by directly calling the block (since yielding is not allowed within blocks).

For example, 

  class Vehicle
    state_machine do
      around_transition do |block|
        Benchmark.measure { block.call }
      end

      around_transition do |vehicle, block|
        logger.info "vehicle was #{state}..."
        block.call
        logger.info "...and is now #{state}"
      end

      around_transition do |vehicle, transition, block|
        logger.info "before #{transition.event}: #{vehicle.state}"
        block.call
        logger.info "after #{transition.event}: #{vehicle.state}"
      end
    end
  end

Notice that referencing the block is similar to doing so within an actual method definition in that it is always the last argument.

On the other hand, if you‘re defining around callbacks using method references, you can yield like normal: 

  class Vehicle
    state_machine do
      around_transition :benchmark
      ...
    end

    def benchmark
      Benchmark.measure { yield }
    end
  end

See before_transition for a description of the possible configurations for defining callbacks.

[Source]

      # File lib/state_machine/machine.rb, line 1696
1696:     def around_transition(*args, &block)
1697:       options = (args.last.is_a?(Hash) ? args.pop : {})
1698:       options[:do] = args if args.any?
1699:       add_callback(:around, options, &block)
1700:     end
attribute(name = :state)

Gets the actual name of the attribute on the machine‘s owner class that stores data with the given name.

[Source]

     # File lib/state_machine/machine.rb, line 702
702:     def attribute(name = :state)
703:       name == :state ? @attribute : "#{self.name}_#{name}""#{self.name}_#{name}"
704:     end
before_transition(*args, &block)

Creates a callback that will be invoked before a transition is performed so long as the given requirements match the transition.

The callback

Callbacks must be defined as either an argument, in the :do option, or as a block. For example, 

  class Vehicle
    state_machine do
      before_transition :set_alarm
      before_transition :set_alarm, all => :parked
      before_transition all => :parked, :do => :set_alarm
      before_transition all => :parked do |vehicle, transition|
        vehicle.set_alarm
      end
      ...
    end
  end

Notice that the first three callbacks are the same in terms of how the methods to invoke are defined. However, using the :do can provide for a more fluid DSL.

In addition, multiple callbacks can be defined like so: 

  class Vehicle
    state_machine do
      before_transition :set_alarm, :lock_doors, all => :parked
      before_transition all => :parked, :do => [:set_alarm, :lock_doors]
      before_transition :set_alarm do |vehicle, transition|
        vehicle.lock_doors
      end
    end
  end

Notice that the different ways of configuring methods can be mixed.

State requirements

Callbacks can require that the machine be transitioning from and to specific states. These requirements use a Hash syntax to map beginning states to ending states. For example, 

  before_transition :parked => :idling, :idling => :first_gear, :do => :set_alarm

In this case, the set_alarm callback will only be called if the machine is transitioning from parked to idling or from idling to parked.

To help define state requirements, a set of helpers are available for slightly more complex matching:

  • all - Matches every state/event in the machine
  • all - [:parked, :idling, …] - Matches every state/event except those specified
  • any - An alias for all (matches every state/event in the machine)
  • same - Matches the same state being transitioned from

See StateMachine::MatcherHelpers for more information.

Examples: 

  before_transition :parked => [:idling, :first_gear], :do => ...     # Matches from parked to idling or first_gear
  before_transition all - [:parked, :idling] => :idling, :do => ...   # Matches from every state except parked and idling to idling
  before_transition all => :parked, :do => ...                        # Matches all states to parked
  before_transition any => same, :do => ...                           # Matches every loopback

Event requirements

In addition to state requirements, an event requirement can be defined so that the callback is only invoked on specific events using the on option. This can also use the same matcher helpers as the state requirements.

Examples: 

  before_transition :on => :ignite, :do => ...                        # Matches only on ignite
  before_transition :on => all - :ignite, :do => ...                  # Matches on every event except ignite
  before_transition :parked => :idling, :on => :ignite, :do => ...    # Matches from parked to idling on ignite

Verbose Requirements

Requirements can also be defined using verbose options rather than the implicit Hash syntax and helper methods described above.

Configuration options:

  • :from - One or more states being transitioned from. If none are specified, then all states will match.
  • :to - One or more states being transitioned to. If none are specified, then all states will match.
  • :on - One or more events that fired the transition. If none are specified, then all events will match.
  • :except_from - One or more states not being transitioned from
  • :except_to - One more states not being transitioned to
  • :except_on - One or more events that *did not* fire the transition

Examples: 

  before_transition :from => :ignite, :to => :idling, :on => :park, :do => ...
  before_transition :except_from => :ignite, :except_to => :idling, :except_on => :park, :do => ...

Conditions

In addition to the state/event requirements, a condition can also be defined to help determine whether the callback should be invoked.

Configuration options:

  • :if - A method, proc or string to call to determine if the callback should occur (e.g. :if => :allow_callbacks, or :if => lambda {|user| user.signup_step > 2}). The method, proc or string should return or evaluate to a true or false value.
  • :unless - A method, proc or string to call to determine if the callback should not occur (e.g. :unless => :skip_callbacks, or :unless => lambda {|user| user.signup_step <= 2}). The method, proc or string should return or evaluate to a true or false value.

Examples: 

  before_transition :parked => :idling, :if => :moving?, :do => ...
  before_transition :on => :ignite, :unless => :seatbelt_on?, :do => ...

Accessing the transition

In addition to passing the object being transitioned, the actual transition describing the context (e.g. event, from, to) can be accessed as well. This additional argument is only passed if the callback allows for it.

For example, 

  class Vehicle
    # Only specifies one parameter (the object being transitioned)
    before_transition all => :parked do |vehicle|
      vehicle.set_alarm
    end

    # Specifies 2 parameters (object being transitioned and actual transition)
    before_transition all => :parked do |vehicle, transition|
      vehicle.set_alarm(transition)
    end
  end

Note that the object in the callback will only be passed in as an argument if callbacks are configured to not be bound to the object involved. This is the default and may change on a per-integration basis.

See StateMachine::Transition for more information about the attributes available on the transition.

Examples

Below is an example of a class with one state machine and various types of before transitions defined for it: 

  class Vehicle
    state_machine do
      # Before all transitions
      before_transition :update_dashboard

      # Before specific transition:
      before_transition [:first_gear, :idling] => :parked, :on => :park, :do => :take_off_seatbelt

      # With conditional callback:
      before_transition all => :parked, :do => :take_off_seatbelt, :if => :seatbelt_on?

      # Using helpers:
      before_transition all - :stalled => same, :on => any - :crash, :do => :update_dashboard
      ...
    end
  end

As can be seen, any number of transitions can be created using various combinations of configuration options.

[Source]

      # File lib/state_machine/machine.rb, line 1624
1624:     def before_transition(*args, &block)
1625:       options = (args.last.is_a?(Hash) ? args.pop : {})
1626:       options[:do] = args if args.any?
1627:       add_callback(:before, options, &block)
1628:     end
define_helper(scope, method, *args, &block)

Defines a new helper method in an instance or class scope with the given name. If the method is already defined in the scope, then this will not override it.

If passing in a block, there are two side effects to be aware of

  1. The method cannot be chained, meaning that the block cannot call super
  2. If the method is already defined in an ancestor, then it will not get overridden and a warning will be output.

Example: 

  # Instance helper
  machine.define_helper(:instance, :state_name) do |machine, object|
    machine.states.match(object).name
  end

  # Class helper
  machine.define_helper(:class, :state_machine_name) do |machine, klass|
    "State"
  end

You can also define helpers using string evaluation like so: 

  # Instance helper
  machine.define_helper :instance, <<-end_eval, __FILE__, __LINE__ + 1
    def state_name
      self.class.state_machine(:state).states.match(self).name
    end
  end_eval

  # Class helper
  machine.define_helper :class, <<-end_eval, __FILE__, __LINE__ + 1
    def state_machine_name
      "State"
    end
  end_eval

[Source]

     # File lib/state_machine/machine.rb, line 742
742:     def define_helper(scope, method, *args, &block)
743:       helper_module = @helper_modules.fetch(scope)
744:       
745:       if block_given?
746:         if !self.class.ignore_method_conflicts && conflicting_ancestor = owner_class_ancestor_has_method?(scope, method)
747:           ancestor_name = conflicting_ancestor.name && !conflicting_ancestor.name.empty? ? conflicting_ancestor.name : conflicting_ancestor.to_s
748:           warn "#{scope == :class ? 'Class' : 'Instance'} method \"#{method}\" is already defined in #{ancestor_name}, use generic helper instead or set StateMachine::Machine.ignore_method_conflicts = true."
749:         else
750:           name = self.name
751:           helper_module.class_eval do
752:             define_method(method) do |*args|
753:               block.call((scope == :instance ? self.class : self).state_machine(name), self, *args)
754:             end
755:           end
756:         end
757:       else
758:         helper_module.class_eval(method, *args)
759:       end
760:     end
draw(options = {})

Draws a directed graph of the machine for visualizing the various events, states, and their transitions.

This requires both the Ruby graphviz gem and the graphviz library be installed on the system.

Configuration options:

  • :name - The name of the file to write to (without the file extension). Default is "#{owner_class.name}_#{name}"
  • :path - The path to write the graph file to. Default is the current directory (".").
  • :format - The image format to generate the graph in. Default is "png’.
  • :font - The name of the font to draw state names in. Default is "Arial".
  • :orientation - The direction of the graph ("portrait" or "landscape"). Default is "portrait".
  • :output - Whether to generate the output of the graph

[Source]

      # File lib/state_machine/machine.rb, line 1867
1867:     def draw(options = {})
1868:       options = {
1869:         :name => "#{owner_class.name}_#{name}",
1870:         :path => '.',
1871:         :format => 'png',
1872:         :font => 'Arial',
1873:         :orientation => 'portrait'
1874:       }.merge(options)
1875:       assert_valid_keys(options, :name, :path, :format, :font, :orientation)
1876:       
1877:       begin
1878:         # Load the graphviz library
1879:         require 'rubygems'
1880:         gem 'ruby-graphviz', '>=0.9.0'
1881:         require 'graphviz'
1882:         
1883:         graph = GraphViz.new('G', :rankdir => options[:orientation] == 'landscape' ? 'LR' : 'TB')
1884:         
1885:         # Add nodes
1886:         states.by_priority.each do |state|
1887:           node = state.draw(graph)
1888:           node.fontname = options[:font]
1889:         end
1890:         
1891:         # Add edges
1892:         events.each do |event|
1893:           edges = event.draw(graph)
1894:           edges.each {|edge| edge.fontname = options[:font]}
1895:         end
1896:         
1897:         # Generate the graph
1898:         graphvizVersion = Constants::RGV_VERSION.split('.')
1899:         file = File.join(options[:path], "#{options[:name]}.#{options[:format]}")
1900:         
1901:         if graphvizVersion[0] == '0' && graphvizVersion[1] == '9' && graphvizVersion[2] == '0'
1902:           outputOptions = {:output => options[:format], :file => file}
1903:         else
1904:           outputOptions = {options[:format] => file}
1905:         end
1906:         
1907:         graph.output(outputOptions)
1908:         graph
1909:       rescue LoadError => ex
1910:         $stderr.puts "Cannot draw the machine (#{ex.message}). `gem install ruby-graphviz` >= v0.9.0 and try again."
1911:         false
1912:       end
1913:     end
dynamic_initial_state?()

Whether a dynamic initial state is being used in the machine

[Source]

     # File lib/state_machine/machine.rb, line 675
675:     def dynamic_initial_state?
676:       @initial_state.is_a?(Proc)
677:     end
errors_for(object)

Gets a description of the errors for the given object. This is used to provide more detailed information when an InvalidTransition exception is raised.

[Source]

      # File lib/state_machine/machine.rb, line 1820
1820:     def errors_for(object)
1821:       ''
1822:     end
event(*names, &block)

Defines one or more events for the machine and the transitions that can be performed when those events are run.

This method is also aliased as on for improved compatibility with using a domain-specific language.

Configuration options:

  • :human_name - The human-readable version of this event‘s name. By default, this is either defined by the integration or stringifies the name and converts underscores to spaces.

Instance methods

The following instance methods are generated when a new event is defined (the "park" event is used as an example):

  • park(…, run_action = true) - Fires the "park" event, transitioning from the current state to the next valid state. If the last argument is a boolean, it will control whether the machine‘s action gets run.
  • park!(…, run_action = true) - Fires the "park" event, transitioning from the current state to the next valid state. If the transition fails, then a StateMachine::InvalidTransition error will be raised. If the last argument is a boolean, it will control whether the machine‘s action gets run.
  • can_park?(requirements = {}) - Checks whether the "park" event can be fired given the current state of the object. This will not run validations or callbacks in ORM integrations. It will only determine if the state machine defines a valid transition for the event. To check whether an event can fire and passes validations, use event attributes (e.g. state_event) as described in the "Events" documentation of each ORM integration.
  • park_transition(requirements = {}) - Gets the next transition that would be performed if the "park" event were to be fired now on the object or nil if no transitions can be performed. Like can_park? this will also not run validations or callbacks. It will only determine if the state machine defines a valid transition for the event.

With a namespace of "car", the above names map to the following methods:

  • can_park_car?
  • park_car_transition
  • park_car
  • park_car!

The can_park? and park_transition helpers both take an optional set of requirements for determining what transitions are available for the current object. These requirements include:

  • :from - One or more states to transition from. If none are specified, then this will be the object‘s current state.
  • :to - One or more states to transition to. If none are specified, then this will match any to state.
  • :guard - Whether to guard transitions with the if/unless conditionals defined for each one. Default is true.

Defining transitions

event requires a block which allows you to define the possible transitions that can happen as a result of that event. For example, 

  event :park, :stop do
    transition :idling => :parked
  end

  event :first_gear do
    transition :parked => :first_gear, :if => :seatbelt_on?
    transition :parked => same # Allow to loopback if seatbelt is off
  end

See StateMachine::Event#transition for more information on the possible options that can be passed in.

Note that this block is executed within the context of the actual event object. As a result, you will not be able to reference any class methods on the model without referencing the class itself. For example, 

  class Vehicle
    def self.safe_states
      [:parked, :idling, :stalled]
    end

    state_machine do
      event :park do
        transition Vehicle.safe_states => :parked
      end
    end
  end

Overriding the event method

By default, this will define an instance method (with the same name as the event) that will fire the next possible transition for that. Although the before_transition, after_transition, and around_transition hooks allow you to define behavior that gets executed as a result of the event‘s transition, you can also override the event method in order to have a little more fine-grained control.

For example: 

  class Vehicle
    state_machine do
      event :park do
        ...
      end
    end

    def park(*)
      take_deep_breath  # Executes before the transition (and before_transition hooks) even if no transition is possible
      if result = super # Runs the transition and all before/after/around hooks
        applaud         # Executes after the transition (and after_transition hooks)
      end
      result
    end
  end

There are a few important things to note here. First, the method signature is defined with an unlimited argument list in order to allow callers to continue passing arguments that are expected by state_machine. For example, it will still allow calls to park with a single parameter for skipping the configured action.

Second, the overridden event method must call super in order to run the logic for running the next possible transition. In order to remain consistent with other events, the result of super is returned.

Third, any behavior defined in this method will not get executed if you‘re taking advantage of attribute-based event transitions. For example: 

  vehicle = Vehicle.new
  vehicle.state_event = 'park'
  vehicle.save

In this case, the park event will run the before/after/around transition hooks and transition the state, but the behavior defined in the overriden park method will not be executed.

Defining additional arguments

Additional arguments can be passed into events and accessed by transition hooks like so: 

  class Vehicle
    state_machine do
      after_transition :on => :park do |vehicle, transition|
        kind = *transition.args # :parallel
        ...
      end
      after_transition :on => :park, :do => :take_deep_breath

      event :park do
        ...
      end

      def take_deep_breath(transition)
        kind = *transition.args # :parallel
        ...
      end
    end
  end

  vehicle = Vehicle.new
  vehicle.park(:parallel)

Remember that if the last argument is a boolean, it will be used as the run_action parameter to the event action. Using the park action example from above, you can might call it like so: 

  vehicle.park                    # => Uses default args and runs machine action
  vehicle.park(:parallel)         # => Specifies the +kind+ argument and runs the machine action
  vehicle.park(:parallel, false)  # => Specifies the +kind+ argument and *skips* the machine action

If you decide to override the park event method and define additional arguments, you can do so as shown below: 

  class Vehicle
    state_machine do
      event :park do
        ...
      end
    end

    def park(kind = :parallel, *args)
      take_deep_breath if kind == :parallel
      super
    end
  end

Note that super is called instead of super(*args). This allow the entire arguments list to be accessed by transition callbacks through StateMachine::Transition#args.

Using matchers

The all / any matchers can be used to easily execute blocks for a group of events. Note, however, that you cannot use these matchers to set configurations for events. Blocks using these matchers can be defined at any point in the state machine and will always get applied to the proper events.

For example: 

  state_machine :initial => :parked do
    ...

    event all - [:crash] do
      transition :stalled => :parked
    end
  end

Example

  class Vehicle
    state_machine do
      # The park, stop, and halt events will all share the given transitions
      event :park, :stop, :halt do
        transition [:idling, :backing_up] => :parked
      end

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

      event :ignite do
        transition :parked => :idling
        transition :idling => same # Allow ignite while still idling
      end
    end
  end

[Source]

      # File lib/state_machine/machine.rb, line 1326
1326:     def event(*names, &block)
1327:       options = names.last.is_a?(Hash) ? names.pop : {}
1328:       assert_valid_keys(options, :human_name)
1329:       
1330:       # Store the context so that it can be used for / matched against any event
1331:       # that gets added
1332:       @events.context(names, &block) if block_given?
1333:       
1334:       if names.first.is_a?(Matcher)
1335:         # Add any events referenced in the matcher.  When matchers are used,
1336:         # events are not allowed to be configured.
1337:         raise ArgumentError, "Cannot configure events when using matchers (using #{options.inspect})" if options.any?
1338:         events = add_events(names.first.values)
1339:       else
1340:         events = add_events(names)
1341:         
1342:         # Update the configuration for the event(s)
1343:         events.each do |event|
1344:           event.human_name = options[:human_name] if options.include?(:human_name)
1345:           
1346:           # Add any states that may have been referenced within the event
1347:           add_states(event.known_states)
1348:         end
1349:       end
1350:       
1351:       events.length == 1 ? events.first : events
1352:     end
generate_message(name, values = [])

Generates the message to use when invalidating the given object after failing to transition on a specific event

[Source]

      # File lib/state_machine/machine.rb, line 1832
1832:     def generate_message(name, values = [])
1833:       (@messages[name] || self.class.default_messages[name]) % values.map {|value| value.last}
1834:     end
initial_state(object)

Gets the initial state of the machine for the given object. If a dynamic initial state was configured for this machine, then the object will be passed into the lambda block to help determine the actual state.

Examples

With a static initial state: 

  class Vehicle
    state_machine :initial => :parked do
      ...
    end
  end

  vehicle = Vehicle.new
  Vehicle.state_machine.initial_state(vehicle)  # => #<StateMachine::State name=:parked value="parked" initial=true>

With a dynamic initial state: 

  class Vehicle
    attr_accessor :force_idle

    state_machine :initial => lambda {|vehicle| vehicle.force_idle ? :idling : :parked} do
      ...
    end
  end

  vehicle = Vehicle.new

  vehicle.force_idle = true
  Vehicle.state_machine.initial_state(vehicle)  # => #<StateMachine::State name=:idling value="idling" initial=false>

  vehicle.force_idle = false
  Vehicle.state_machine.initial_state(vehicle)  # => #<StateMachine::State name=:parked value="parked" initial=false>

[Source]

     # File lib/state_machine/machine.rb, line 670
670:     def initial_state(object)
671:       states.fetch(dynamic_initial_state? ? evaluate_method(object, @initial_state) : @initial_state) if instance_variable_defined?('@initial_state')
672:     end
initial_state=(new_initial_state)

Sets the initial state of the machine. This can be either the static name of a state or a lambda block which determines the initial state at creation time.

[Source]

     # File lib/state_machine/machine.rb, line 628
628:     def initial_state=(new_initial_state)
629:       @initial_state = new_initial_state
630:       add_states([@initial_state]) unless dynamic_initial_state?
631:       
632:       # Update all states to reflect the new initial state
633:       states.each {|state| state.initial = (state.name == @initial_state)}
634:     end
initialize_state(object, options = {})

Initializes the state on the given object. Initial values are only set if the machine‘s attribute hasn‘t been previously initialized.

Configuration options:

  • :force - Whether to initialize the state regardless of its current value
  • :to - A hash to set the initial value in instead of writing directly to the object

[Source]

     # File lib/state_machine/machine.rb, line 687
687:     def initialize_state(object, options = {})
688:       state = initial_state(object)
689:       if state && (options[:force] || initialize_state?(object))
690:         value = state.value
691:         
692:         if hash = options[:to]
693:           hash[attribute.to_s] = value
694:         else
695:           write(object, :state, value)
696:         end
697:       end
698:     end
invalidate(object, attribute, message, values = [])

Marks the given object as invalid with the given message.

By default, this is a no-op.

[Source]

      # File lib/state_machine/machine.rb, line 1814
1814:     def invalidate(object, attribute, message, values = [])
1815:     end
on(*names, &block)

Alias for event

other_states(*names, &block)

Alias for state

owner_class=(klass)

Sets the class which is the owner of this state machine. Any methods generated by states, events, or other parts of the machine will be defined on the given owner class.

[Source]

     # File lib/state_machine/machine.rb, line 600
600:     def owner_class=(klass)
601:       @owner_class = klass
602:       
603:       # Create modules for extending the class with state/event-specific methods
604:       @helper_modules = helper_modules = {:instance => HelperModule.new(self, :instance), :class => HelperModule.new(self, :class)}
605:       owner_class.class_eval do
606:         extend helper_modules[:class]
607:         include helper_modules[:instance]
608:       end
609:       
610:       # Add class-/instance-level methods to the owner class for state initialization
611:       unless owner_class < StateMachine::InstanceMethods
612:         owner_class.class_eval do
613:           extend StateMachine::ClassMethods
614:           include StateMachine::InstanceMethods
615:         end
616:         
617:         define_state_initializer if @initialize_state
618:       end
619:       
620:       # Record this machine as matched to the name in the current owner class.
621:       # This will override any machines mapped to the same name in any superclasses.
622:       owner_class.state_machines[name] = self
623:     end
paths_for(object, requirements = {})

Generates a list of the possible transition sequences that can be run on the given object. These paths can reveal all of the possible states and events that can be encountered in the object‘s state machine based on the object‘s current state.

Configuration options:

  • from - The initial state to start all paths from. By default, this is the object‘s current state.
  • to - The target state to end all paths on. By default, paths will end when they loop back to the first transition on the path.
  • deep - Whether to allow the target state to be crossed more than once in a path. By default, paths will immediately stop when the target state (if specified) is reached. If this is enabled, then paths can continue even after reaching the target state; they will stop when reaching the target state a second time.

Note that the object is never modified when the list of paths is generated.

Examples

  class Vehicle
    state_machine :initial => :parked do
      event :ignite do
        transition :parked => :idling
      end

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

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

  vehicle = Vehicle.new   # => #<Vehicle:0xb7c27024 @state="parked">
  vehicle.state           # => "parked"

  vehicle.state_paths
  # => [
  #     [#<StateMachine::Transition attribute=:state event=:ignite from="parked" from_name=:parked to="idling" to_name=:idling>,
  #      #<StateMachine::Transition attribute=:state event=:shift_up from="idling" from_name=:idling to="first_gear" to_name=:first_gear>,
  #      #<StateMachine::Transition attribute=:state event=:shift_up from="first_gear" from_name=:first_gear to="second_gear" to_name=:second_gear>,
  #      #<StateMachine::Transition attribute=:state event=:shift_down from="second_gear" from_name=:second_gear to="first_gear" to_name=:first_gear>,
  #      #<StateMachine::Transition attribute=:state event=:shift_down from="first_gear" from_name=:first_gear to="idling" to_name=:idling>],
  #
  #     [#<StateMachine::Transition attribute=:state event=:ignite from="parked" from_name=:parked to="idling" to_name=:idling>,
  #      #<StateMachine::Transition attribute=:state event=:shift_up from="idling" from_name=:idling to="first_gear" to_name=:first_gear>,
  #      #<StateMachine::Transition attribute=:state event=:shift_down from="first_gear" from_name=:first_gear to="idling" to_name=:idling>]
  #    ]

  vehicle.state_paths(:from => :parked, :to => :second_gear)
  # => [
  #     [#<StateMachine::Transition attribute=:state event=:ignite from="parked" from_name=:parked to="idling" to_name=:idling>,
  #      #<StateMachine::Transition attribute=:state event=:shift_up from="idling" from_name=:idling to="first_gear" to_name=:first_gear>,
  #      #<StateMachine::Transition attribute=:state event=:shift_up from="first_gear" from_name=:first_gear to="second_gear" to_name=:second_gear>]
  #    ]

In addition to getting the possible paths that can be accessed, you can also get summary information about the states / events that can be accessed at some point along one of the paths. For example: 

  # Get the list of states that can be accessed from the current state
  vehicle.state_paths.to_states # => [:idling, :first_gear, :second_gear]

  # Get the list of events that can be accessed from the current state
  vehicle.state_paths.events    # => [:ignite, :shift_up, :shift_down]

[Source]

      # File lib/state_machine/machine.rb, line 1807
1807:     def paths_for(object, requirements = {})
1808:       PathCollection.new(object, self, requirements)
1809:     end
read(object, attribute, ivar = false)

Gets the current value stored in the given object‘s attribute.

For example, 

  class Vehicle
    state_machine :initial => :parked do
      ...
    end
  end

  vehicle = Vehicle.new                           # => #<Vehicle:0xb7d94ab0 @state="parked">
  Vehicle.state_machine.read(vehicle, :state)     # => "parked" # Equivalent to vehicle.state
  Vehicle.state_machine.read(vehicle, :event)     # => nil      # Equivalent to vehicle.state_event

[Source]

      # File lib/state_machine/machine.rb, line 1075
1075:     def read(object, attribute, ivar = false)
1076:       attribute = self.attribute(attribute)
1077:       ivar ? object.instance_variable_get("@#{attribute}") : object.send(attribute)
1078:     end
reset(object)

Resets any errors previously added when invalidating the given object.

By default, this is a no-op.

[Source]

      # File lib/state_machine/machine.rb, line 1827
1827:     def reset(object)
1828:     end
state(*names, &block)

Customizes the definition of one or more states in the machine.

Configuration options:

  • :value - The actual value to store when an object transitions to the state. Default is the name (stringified).
  • :cache - If a dynamic value (via a lambda block) is being used, then setting this to true will cache the evaluated result
  • :if - Determines whether an object‘s value matches the state (e.g. :value => lambda {Time.now}, :if => lambda {|state| !state.nil?}). By default, the configured value is matched.
  • :human_name - The human-readable version of this state‘s name. By default, this is either defined by the integration or stringifies the name and converts underscores to spaces.

Customizing the stored value

Whenever a state is automatically discovered in the state machine, its default value is assumed to be the stringified version of the name. For example, 

  class Vehicle
    state_machine :initial => :parked do
      event :ignite do
        transition :parked => :idling
      end
    end
  end

In the above state machine, there are two states automatically discovered: :parked and :idling. These states, by default, will store their stringified equivalents when an object moves into that state (e.g. "parked" / "idling").

For legacy systems or when tying state machines into existing frameworks, it‘s oftentimes necessary to need to store a different value for a state than the default. In order to continue taking advantage of an expressive state machine and helper methods, every defined state can be re-configured with a custom stored value. For example, 

  class Vehicle
    state_machine :initial => :parked do
      event :ignite do
        transition :parked => :idling
      end

      state :idling, :value => 'IDLING'
      state :parked, :value => 'PARKED
    end
  end

This is also useful if being used in association with a database and, instead of storing the state name in a column, you want to store the state‘s foreign key: 

  class VehicleState < ActiveRecord::Base
  end

  class Vehicle < ActiveRecord::Base
    state_machine :attribute => :state_id, :initial => :parked do
      event :ignite do
        transition :parked => :idling
      end

      states.each do |state|
        self.state(state.name, :value => lambda { VehicleState.find_by_name(state.name.to_s).id }, :cache => true)
      end
    end
  end

In the above example, each known state is configured to store it‘s associated database id in the state_id attribute. Also, notice that a lambda block is used to define the state‘s value. This is required in situations (like testing) where the model is loaded without any existing data (i.e. no VehicleState records available).

One caveat to the above example is to keep performance in mind. To avoid constant db hits for looking up the VehicleState ids, the value is cached by specifying the :cache option. Alternatively, a custom caching strategy can be used like so: 

  class VehicleState < ActiveRecord::Base
    cattr_accessor :cache_store
    self.cache_store = ActiveSupport::Cache::MemoryStore.new

    def self.find_by_name(name)
      cache_store.fetch(name) { find(:first, :conditions => {:name => name}) }
    end
  end

Dynamic values

In addition to customizing states with other value types, lambda blocks can also be specified to allow for a state‘s value to be determined dynamically at runtime. For example, 

  class Vehicle
    state_machine :purchased_at, :initial => :available do
      event :purchase do
        transition all => :purchased
      end

      event :restock do
        transition all => :available
      end

      state :available, :value => nil
      state :purchased, :if => lambda {|value| !value.nil?}, :value => lambda {Time.now}
    end
  end

In the above definition, the :purchased state is customized with both a dynamic value and a value matcher.

When an object transitions to the purchased state, the value‘s lambda block will be called. This will get the current time and store it in the object‘s purchased_at attribute.

Note that the custom matcher is very important here. Since there‘s no way for the state machine to figure out an object‘s state when it‘s set to a runtime value, it must be explicitly defined. If the :if option were not configured for the state, then an ArgumentError exception would be raised at runtime, indicating that the state machine could not figure out what the current state of the object was.

Behaviors

Behaviors define a series of methods to mixin with objects when the current state matches the given one(s). This allows instance methods to behave a specific way depending on what the value of the object‘s state is.

For example, 

  class Vehicle
    attr_accessor :driver
    attr_accessor :passenger

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

      state :parked do
        def speed
          0
        end

        def rotate_driver
          driver = self.driver
          self.driver = passenger
          self.passenger = driver
          true
        end
      end

      state :idling, :first_gear do
        def speed
          20
        end

        def rotate_driver
          self.state = 'parked'
          rotate_driver
        end
      end

      other_states :backing_up
    end
  end

In the above example, there are two dynamic behaviors defined for the class:

  • speed
  • rotate_driver

Each of these behaviors are instance methods on the Vehicle class. However, which method actually gets invoked is based on the current state of the object. Using the above class as the example: 

  vehicle = Vehicle.new
  vehicle.driver = 'John'
  vehicle.passenger = 'Jane'

  # Behaviors in the "parked" state
  vehicle.state             # => "parked"
  vehicle.speed             # => 0
  vehicle.rotate_driver     # => true
  vehicle.driver            # => "Jane"
  vehicle.passenger         # => "John"

  vehicle.ignite            # => true

  # Behaviors in the "idling" state
  vehicle.state             # => "idling"
  vehicle.speed             # => 20
  vehicle.rotate_driver     # => true
  vehicle.driver            # => "John"
  vehicle.passenger         # => "Jane"

As can be seen, both the speed and rotate_driver instance method implementations changed how they behave based on what the current state of the vehicle was.

Invalid behaviors

If a specific behavior has not been defined for a state, then a NoMethodError exception will be raised, indicating that that method would not normally exist for an object with that state.

Using the example from before: 

  vehicle = Vehicle.new
  vehicle.state = 'backing_up'
  vehicle.speed               # => NoMethodError: undefined method 'speed' for #<Vehicle:0xb7d296ac> in state "backing_up"

Using matchers

The all / any matchers can be used to easily define behaviors for a group of states. Note, however, that you cannot use these matchers to set configurations for states. Behaviors using these matchers can be defined at any point in the state machine and will always get applied to the proper states.

For example: 

  state_machine :initial => :parked do
    ...

    state all - [:parked, :idling, :stalled] do
      validates_presence_of :speed

      def speed
        gear * 10
      end
    end
  end

State-aware class methods

In addition to defining scopes for instance methods that are state-aware, the same can be done for certain types of class methods.

Some libraries have support for class-level methods that only run certain behaviors based on a conditions hash passed in. For example: 

  class Vehicle < ActiveRecord::Base
    state_machine do
      ...
      state :first_gear, :second_gear, :third_gear do
        validates_presence_of   :speed
        validates_inclusion_of  :speed, :in => 0..25, :if => :in_school_zone?
      end
    end
  end

In the above ActiveRecord model, two validations have been defined which will only run when the Vehicle object is in one of the three states: first_gear, second_gear, or +third_gear. Notice, also, that if/unless conditions can continue to be used.

This functionality is not library-specific and can work for any class-level method that is defined like so: 

  def validates_presence_of(attribute, options = {})
    ...
  end

The minimum requirement is that the last argument in the method be an options hash which contains at least :if condition support.

[Source]

      # File lib/state_machine/machine.rb, line 1029
1029:     def state(*names, &block)
1030:       options = names.last.is_a?(Hash) ? names.pop : {}
1031:       assert_valid_keys(options, :value, :cache, :if, :human_name)
1032:       
1033:       # Store the context so that it can be used for / matched against any state
1034:       # that gets added
1035:       @states.context(names, &block) if block_given?
1036:       
1037:       if names.first.is_a?(Matcher)
1038:         # Add any states referenced in the matcher.  When matchers are used,
1039:         # states are not allowed to be configured.
1040:         raise ArgumentError, "Cannot configure states when using matchers (using #{options.inspect})" if options.any?
1041:         states = add_states(names.first.values)
1042:       else
1043:         states = add_states(names)
1044:         
1045:         # Update the configuration for the state(s)
1046:         states.each do |state|
1047:           if options.include?(:value)
1048:             state.value = options[:value]
1049:             self.states.update(state)
1050:           end
1051:           
1052:           state.human_name = options[:human_name] if options.include?(:human_name)
1053:           state.cache = options[:cache] if options.include?(:cache)
1054:           state.matcher = options[:if] if options.include?(:if)
1055:         end
1056:       end
1057:       
1058:       states.length == 1 ? states.first : states
1059:     end
transition(options)

Creates a new transition that determines what to change the current state to when an event fires.

Defining transitions

The options for a new transition uses the Hash syntax to map beginning states to ending states. For example, 

  transition :parked => :idling, :idling => :first_gear, :on => :ignite

In this case, when the ignite event is fired, this transition will cause the state to be idling if it‘s current state is parked or first_gear if it‘s current state is idling.

To help define these implicit transitions, a set of helpers are available for slightly more complex matching:

  • all - Matches every state in the machine
  • all - [:parked, :idling, …] - Matches every state except those specified
  • any - An alias for all (matches every state in the machine)
  • same - Matches the same state being transitioned from

See StateMachine::MatcherHelpers for more information.

Examples: 

  transition all => nil, :on => :ignite                               # Transitions to nil regardless of the current state
  transition all => :idling, :on => :ignite                           # Transitions to :idling regardless of the current state
  transition all - [:idling, :first_gear] => :idling, :on => :ignite  # Transitions every state but :idling and :first_gear to :idling
  transition nil => :idling, :on => :ignite                           # Transitions to :idling from the nil state
  transition :parked => :idling, :on => :ignite                       # Transitions to :idling if :parked
  transition [:parked, :stalled] => :idling, :on => :ignite           # Transitions to :idling if :parked or :stalled

  transition :parked => same, :on => :park                            # Loops :parked back to :parked
  transition [:parked, :stalled] => same, :on => [:park, :stall]      # Loops either :parked or :stalled back to the same state on the park and stall events
  transition all - :parked => same, :on => :noop                      # Loops every state but :parked back to the same state

  # Transitions to :idling if :parked, :first_gear if :idling, or :second_gear if :first_gear
  transition :parked => :idling, :idling => :first_gear, :first_gear => :second_gear, :on => :shift_up

Verbose transitions

Transitions can also be defined use an explicit set of configuration options:

  • :from - A state or array of states that can be transitioned from. If not specified, then the transition can occur for any state.
  • :to - The state that‘s being transitioned to. If not specified, then the transition will simply loop back (i.e. the state will not change).
  • :except_from - A state or array of states that cannot be transitioned from.

These options must be used when defining transitions within the context of a state.

Examples: 

  transition :to => nil, :on => :park
  transition :to => :idling, :on => :ignite
  transition :except_from => [:idling, :first_gear], :to => :idling, :on => :ignite
  transition :from => nil, :to => :idling, :on => :ignite
  transition :from => [:parked, :stalled], :to => :idling, :on => :ignite

Conditions

In addition to the state requirements for each transition, a condition can also be defined to help determine whether that transition is available. These options will work on both the normal and verbose syntax.

Configuration options:

  • :if - A method, proc or string to call to determine if the transition should occur (e.g. :if => :moving?, or :if => lambda {|vehicle| vehicle.speed > 60}). The condition should return or evaluate to true or false.
  • :unless - A method, proc or string to call to determine if the transition should not occur (e.g. :unless => :stopped?, or :unless => lambda {|vehicle| vehicle.speed <= 60}). The condition should return or evaluate to true or false.

Examples: 

  transition :parked => :idling, :on => :ignite, :if => :moving?
  transition :parked => :idling, :on => :ignite, :unless => :stopped?
  transition :idling => :first_gear, :first_gear => :second_gear, :on => :shift_up, :if => :seatbelt_on?

  transition :from => :parked, :to => :idling, :on => ignite, :if => :moving?
  transition :from => :parked, :to => :idling, :on => ignite, :unless => :stopped?

Order of operations

Transitions are evaluated in the order in which they‘re defined. As a result, if more than one transition applies to a given object, then the first transition that matches will be performed.

[Source]

      # File lib/state_machine/machine.rb, line 1444
1444:     def transition(options)
1445:       raise ArgumentError, 'Must specify :on event' unless options[:on]
1446:       
1447:       branches = []
1448:       event(*Array(options.delete(:on))) { branches << transition(options) }
1449:       
1450:       branches.length == 1 ? branches.first : branches
1451:     end
within_transaction(object) {|| ...}

Runs a transaction, rolling back any changes if the yielded block fails.

This is only applicable to integrations that involve databases. By default, this will not run any transactions since the changes aren‘t taking place within the context of a database.

[Source]

      # File lib/state_machine/machine.rb, line 1841
1841:     def within_transaction(object)
1842:       if use_transactions
1843:         transaction(object) { yield }
1844:       else
1845:         yield
1846:       end
1847:     end
write(object, attribute, value, ivar = false)

Sets a new value in the given object‘s attribute.

For example, 

  class Vehicle
    state_machine :initial => :parked do
      ...
    end
  end

  vehicle = Vehicle.new                                   # => #<Vehicle:0xb7d94ab0 @state="parked">
  Vehicle.state_machine.write(vehicle, :state, 'idling')  # => Equivalent to vehicle.state = 'idling'
  Vehicle.state_machine.write(vehicle, :event, 'park')    # => Equivalent to vehicle.state_event = 'park'
  vehicle.state                                           # => "idling"
  vehicle.event                                           # => "park"

[Source]

      # File lib/state_machine/machine.rb, line 1095
1095:     def write(object, attribute, value, ivar = false)
1096:       attribute = self.attribute(attribute)
1097:       ivar ? object.instance_variable_set("@#{attribute}", value) : object.send("#{attribute}=", value)
1098:     end

Protected Instance methods

action_hook()

The method to hook into for triggering transitions when invoked. By default, this is the action configured for the machine.

Since the default hook technique relies on module inheritance, the action must be defined in an ancestor of the owner classs in order for it to be the action hook.

[Source]

      # File lib/state_machine/machine.rb, line 2084
2084:       def action_hook
2085:         action && owner_class_ancestor_has_method?(:instance, action) ? action : nil
2086:       end
add_callback(type, options, &block)

Adds a new transition callback of the given type.

[Source]

      # File lib/state_machine/machine.rb, line 2211
2211:       def add_callback(type, options, &block)
2212:         callbacks[type == :around ? :before : type] << callback = Callback.new(type, options, &block)
2213:         add_states(callback.known_states)
2214:         callback
2215:       end
add_events(new_events)

Tracks the given set of events in the list of all known events for this machine

[Source]

      # File lib/state_machine/machine.rb, line 2240
2240:       def add_events(new_events)
2241:         new_events.map do |new_event|
2242:           # Check for other states that use a different class type for their name.
2243:           # This typically prevents string / symbol misuse.
2244:           if conflict = events.detect {|event| event.name.class != new_event.class}
2245:             raise ArgumentError, "#{new_event.inspect} event defined as #{new_event.class}, #{conflict.name.inspect} defined as #{conflict.name.class}; all events must be consistent"
2246:           end
2247:           
2248:           unless event = events[new_event]
2249:             events << event = Event.new(self, new_event)
2250:           end
2251:           
2252:           event
2253:         end
2254:       end
add_sibling_machine_configs()

Updates this machine based on the configuration of other machines in the owner class that share the same target attribute.

[Source]

      # File lib/state_machine/machine.rb, line 2203
2203:       def add_sibling_machine_configs
2204:         # Add existing states
2205:         sibling_machines.each do |machine|
2206:           machine.states.each {|state| states << state unless states[state.name]}
2207:         end
2208:       end
add_states(new_states)

Tracks the given set of states in the list of all known states for this machine

[Source]

      # File lib/state_machine/machine.rb, line 2219
2219:       def add_states(new_states)
2220:         new_states.map do |new_state|
2221:           # Check for other states that use a different class type for their name.
2222:           # This typically prevents string / symbol misuse.
2223:           if new_state && conflict = states.detect {|state| state.name && state.name.class != new_state.class}
2224:             raise ArgumentError, "#{new_state.inspect} state defined as #{new_state.class}, #{conflict.name.inspect} defined as #{conflict.name.class}; all states must be consistent"
2225:           end
2226:           
2227:           unless state = states[new_state]
2228:             states << state = State.new(self, new_state)
2229:             
2230:             # Copy states over to sibling machines
2231:             sibling_machines.each {|machine| machine.states << state}
2232:           end
2233:           
2234:           state
2235:         end
2236:       end
after_initialize()

Runs additional initialization hooks. By default, this is a no-op.

[Source]

      # File lib/state_machine/machine.rb, line 1923
1923:       def after_initialize
1924:       end
create_with_scope(name)

Creates a scope for finding objects with a particular value or values for the attribute.

By default, this is a no-op.

[Source]

      # File lib/state_machine/machine.rb, line 2186
2186:       def create_with_scope(name)
2187:       end
create_without_scope(name)

Creates a scope for finding objects without a particular value or values for the attribute.

By default, this is a no-op.

[Source]

      # File lib/state_machine/machine.rb, line 2193
2193:       def create_without_scope(name)
2194:       end
define_action_helpers()

Adds helper methods for automatically firing events when an action is invoked

[Source]

      # File lib/state_machine/machine.rb, line 2052
2052:       def define_action_helpers
2053:         if action_hook
2054:           @action_hook_defined = true
2055:           define_action_hook
2056:         end
2057:       end
define_action_helpers?()

Determines whether action helpers should be defined for this machine. This is only true if there is an action configured and no other machines have process this same configuration already.

[Source]

      # File lib/state_machine/machine.rb, line 2046
2046:       def define_action_helpers?
2047:         action && !owner_class.state_machines.any? {|name, machine| machine.action == action && machine != self}
2048:       end
define_action_hook()

Hooks directly into actions by defining the same method in an included module. As a result, when the action gets invoked, any state events defined for the object will get run. Method visibility is preserved.

[Source]

      # File lib/state_machine/machine.rb, line 2062
2062:       def define_action_hook
2063:         action_hook = self.action_hook
2064:         action = self.action
2065:         private_action_hook = owner_class.private_method_defined?(action_hook)
2066:         
2067:         # Only define helper if it hasn't 
2068:         define_helper :instance, "def \#{action_hook}(*)\nself.class.state_machines.transitions(self, \#{action.inspect}).perform { super }\nend\n\nprivate \#{action_hook.inspect} if \#{private_action_hook}\n", __FILE__, __LINE__ + 1
2069:       end
define_event_helpers()

Adds helper methods for getting information about this state machine‘s events

[Source]

      # File lib/state_machine/machine.rb, line 1993
1993:       def define_event_helpers
1994:         # Gets the events that are allowed to fire on the current object
1995:         define_helper(:instance, attribute(:events)) do |machine, object, *args|
1996:           machine.events.valid_for(object, *args).map {|event| event.name}
1997:         end
1998:         
1999:         # Gets the next possible transitions that can be run on the current
2000:         # object
2001:         define_helper(:instance, attribute(:transitions)) do |machine, object, *args|
2002:           machine.events.transitions_for(object, *args)
2003:         end
2004:         
2005:         # Fire an arbitrary event for this machine
2006:         define_helper(:instance, "fire_#{attribute(:event)}") do |machine, object, event, *args|
2007:           machine.events.fetch(event).fire(object, *args)
2008:         end
2009:         
2010:         # Add helpers for tracking the event / transition to invoke when the
2011:         # action is called
2012:         if action
2013:           event_attribute = attribute(:event)
2014:           define_helper(:instance, event_attribute) do |machine, object|
2015:             # Interpret non-blank events as present
2016:             event = machine.read(object, :event, true)
2017:             event && !(event.respond_to?(:empty?) && event.empty?) ? event.to_sym : nil
2018:           end
2019:           
2020:           # A roundabout way of writing the attribute is used here so that
2021:           # integrations can hook into this modification
2022:           define_helper(:instance, "#{event_attribute}=") do |machine, object, value|
2023:             machine.write(object, :event, value, true)
2024:           end
2025:           
2026:           event_transition_attribute = attribute(:event_transition)
2027:           define_helper :instance, "protected; attr_accessor \#{event_transition_attribute.inspect}\n", __FILE__, __LINE__ + 1
2028:         end
2029:       end
define_helpers()

Adds helper methods for interacting with the state machine, including for states, events, and transitions

[Source]

      # File lib/state_machine/machine.rb, line 1950
1950:       def define_helpers
1951:         define_state_accessor
1952:         define_state_predicate
1953:         define_event_helpers
1954:         define_path_helpers
1955:         define_action_helpers if define_action_helpers?
1956:         define_name_helpers
1957:       end
define_name_helpers()

Adds helper methods for accessing naming information about states and events on the owner class

[Source]

      # File lib/state_machine/machine.rb, line 2119
2119:       def define_name_helpers
2120:         # Gets the humanized version of a state
2121:         define_helper(:class, "human_#{attribute(:name)}") do |machine, klass, state|
2122:           machine.states.fetch(state).human_name(klass)
2123:         end
2124:         
2125:         # Gets the humanized version of an event
2126:         define_helper(:class, "human_#{attribute(:event_name)}") do |machine, klass, event|
2127:           machine.events.fetch(event).human_name(klass)
2128:         end
2129:         
2130:         # Gets the state name for the current value
2131:         define_helper(:instance, attribute(:name)) do |machine, object|
2132:           machine.states.match!(object).name
2133:         end
2134:         
2135:         # Gets the human state name for the current value
2136:         define_helper(:instance, "human_#{attribute(:name)}") do |machine, object|
2137:           machine.states.match!(object).human_name(object.class)
2138:         end
2139:       end
define_path_helpers()

Adds helper methods for getting information about this state machine‘s available transition paths

[Source]

      # File lib/state_machine/machine.rb, line 2036
2036:       def define_path_helpers
2037:         # Gets the paths of transitions available to the current object
2038:         define_helper(:instance, attribute(:paths)) do |machine, object, *args|
2039:           machine.paths_for(object, *args)
2040:         end
2041:       end
define_scopes(custom_plural = nil)

Defines the with/without scope helpers for this attribute. Both the singular and plural versions of the attribute are defined for each scope helper. A custom plural can be specified if it cannot be automatically determined by either calling pluralize on the attribute name or adding an "s" to the end of the name.

[Source]

      # File lib/state_machine/machine.rb, line 2146
2146:       def define_scopes(custom_plural = nil)
2147:         plural = custom_plural || pluralize(name)
2148:         
2149:         [name, plural].uniq.each do |name|
2150:           [:with, :without].each do |kind|
2151:             method = "#{kind}_#{name}"
2152:             
2153:             if scope = send("create_#{kind}_scope", method)
2154:               # Converts state names to their corresponding values so that they
2155:               # can be looked up properly
2156:               define_helper(:class, method) do |machine, klass, *states|
2157:                 run_scope(scope, machine, klass, states)
2158:               end
2159:             end
2160:           end
2161:         end
2162:       end
define_state_accessor()

Adds reader/writer methods for accessing the state attribute

[Source]

      # File lib/state_machine/machine.rb, line 1972
1972:       def define_state_accessor
1973:         attribute = self.attribute
1974:         
1975:         @helper_modules[:instance].class_eval { attr_reader attribute } unless owner_class_ancestor_has_method?(:instance, attribute)
1976:         @helper_modules[:instance].class_eval { attr_writer attribute } unless owner_class_ancestor_has_method?(:instance, "#{attribute}=")
1977:       end
define_state_initializer()

Defines the initial values for state machine attributes. Static values are set prior to the original initialize method and dynamic values are set after the initialize method in case it is dependent on it.

[Source]

      # File lib/state_machine/machine.rb, line 1962
1962:       def define_state_initializer
1963:         define_helper :instance, "def initialize(*)\nself.class.state_machines.initialize_states(self) { super }\nend\n", __FILE__, __LINE__ + 1
1964:       end
define_state_predicate()

Adds predicate method to the owner class for determining the name of the current state

[Source]

      # File lib/state_machine/machine.rb, line 1981
1981:       def define_state_predicate
1982:         call_super = !!owner_class_ancestor_has_method?(:instance, "#{name}?")
1983:         define_helper :instance, "def \#{name}?(*args)\nargs.empty? && (\#{call_super} || defined?(super)) ? super : self.class.state_machine(\#{name.inspect}).states.matches?(self, *args)\nend\n", __FILE__, __LINE__ + 1
1984:       end
initialize_state?(object)

Determines if the machine‘s attribute needs to be initialized. This will only be true if the machine‘s attribute is blank.

[Source]

      # File lib/state_machine/machine.rb, line 1943
1943:       def initialize_state?(object)
1944:         value = read(object, :state)
1945:         (value.nil? || value.respond_to?(:empty?) && value.empty?) && !states[value, :value]
1946:       end
owner_class_ancestor_has_method?(scope, method)

Determines whether there‘s already a helper method defined within the given scope. This is true only if one of the owner‘s ancestors defines the method and is further along in the ancestor chain than this machine‘s helper module.

[Source]

      # File lib/state_machine/machine.rb, line 2092
2092:       def owner_class_ancestor_has_method?(scope, method)
2093:         superclasses = owner_class.ancestors[1..-1].select {|ancestor| ancestor.is_a?(Class)}
2094:         
2095:         if scope == :class
2096:           # Use singleton classes
2097:           current = (class << owner_class; self; end)
2098:           superclass = superclasses.first
2099:         else
2100:           current = owner_class
2101:           superclass = owner_class.superclass
2102:         end
2103:         
2104:         # Generate the list of modules that *only* occur in the owner class, but
2105:         # were included *prior* to the helper modules, in addition to the
2106:         # superclasses
2107:         ancestors = current.ancestors - superclass.ancestors + superclasses
2108:         ancestors = ancestors[ancestors.index(@helper_modules[scope])..-1].reverse
2109:         
2110:         # Search for for the first ancestor that defined this method
2111:         ancestors.detect do |ancestor|
2112:           ancestor = (class << ancestor; self; end) if scope == :class && ancestor.is_a?(Class)
2113:           ancestor.method_defined?(method) || ancestor.private_method_defined?(method)
2114:         end
2115:       end
pluralize(word)

Pluralizes the given word using pluralize (if available) or simply adding an "s" to the end of the word

[Source]

      # File lib/state_machine/machine.rb, line 2173
2173:       def pluralize(word)
2174:         word = word.to_s
2175:         if word.respond_to?(:pluralize)
2176:           word.pluralize
2177:         else
2178:           "#{name}s"
2179:         end
2180:       end
run_scope(scope, machine, klass, states)

Generates the results for the given scope based on one or more states to filter by

[Source]

      # File lib/state_machine/machine.rb, line 2166
2166:       def run_scope(scope, machine, klass, states)
2167:         values = states.flatten.map {|state| machine.states.fetch(state).value}
2168:         scope.call(klass, values)
2169:       end
sibling_machines()

Looks up other machines that have been defined in the owner class and are targeting the same attribute as this machine. When accessing sibling machines, they will be automatically copied for the current class if they haven‘t been already. This ensures that any configuration changes made to the sibling machines only affect this class and not any base class that may have originally defined the machine.

[Source]

      # File lib/state_machine/machine.rb, line 1932
1932:       def sibling_machines
1933:         owner_class.state_machines.inject([]) do |machines, (name, machine)|
1934:           if machine.attribute == attribute && machine != self
1935:             machines << (owner_class.state_machine(name) {})
1936:           end
1937:           machines
1938:         end
1939:       end
transaction(object) {|| ...}

Always yields

[Source]

      # File lib/state_machine/machine.rb, line 2197
2197:       def transaction(object)
2198:         yield
2199:       end

[Validate]