Class: StateMachine::Machine

Inherits:
Object
  • Object
show all
Includes:
Assertions, EvalHelpers, MatcherHelpers
Defined in:
lib/state_machine/machine.rb

Overview

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.

Class Attribute Summary collapse

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Methods included from MatcherHelpers

#all, #same

Methods included from EvalHelpers

#evaluate_method

Methods included from Assertions

#assert_exclusive_keys, #assert_valid_keys

Constructor Details

#initialize(owner_class, *args, &block) ⇒ Machine

Creates a new state machine for the given attribute



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# File 'lib/state_machine/machine.rb', line 539

def initialize(owner_class, *args, &block)
  options = args.last.is_a?(Hash) ? args.pop : {}
  assert_valid_keys(options, :attribute, :initial, :initialize, :action, :plural, :namespace, :integration, :messages, :use_transactions)
  
  # Find an integration that matches this machine's owner class
  if options.include?(:integration)
    @integration = StateMachine::Integrations.find_by_name(options[:integration]) if options[:integration]
  else
    @integration = StateMachine::Integrations.match(owner_class)
  end
  
  if @integration
    extend @integration
    options = (@integration.defaults || {}).merge(options)
  end
  
  # Add machine-wide defaults
  options = {:use_transactions => true, :initialize => true}.merge(options)
  
  # Set machine configuration
  @name = args.first || :state
  @attribute = options[:attribute] || @name
  @events = EventCollection.new(self)
  @states = StateCollection.new(self)
  @callbacks = {:before => [], :after => [], :failure => []}
  @namespace = options[:namespace]
  @messages = options[:messages] || {}
  @action = options[:action]
  @use_transactions = options[:use_transactions]
  @initialize_state = options[:initialize]
  self.owner_class = owner_class
  self.initial_state = options[:initial] unless sibling_machines.any?
  
  # Merge with sibling machine configurations
  add_sibling_machine_configs
  
  # Define class integration
  define_helpers
  define_scopes(options[:plural])
  after_initialize
  
  # Evaluate DSL
  instance_eval(&block) if block_given?
end

Class Attribute Details

.default_messagesObject

Returns the value of attribute default_messages



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# File 'lib/state_machine/machine.rb', line 488

def default_messages
  @default_messages
end

.ignore_method_conflictsObject

Returns the value of attribute ignore_method_conflicts



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# File 'lib/state_machine/machine.rb', line 497

def ignore_method_conflicts
  @ignore_method_conflicts
end

Instance Attribute Details

#actionObject (readonly)

The action to invoke when an object transitions



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# File 'lib/state_machine/machine.rb', line 528

def action
  @action
end

#callbacksObject (readonly)

The callbacks to invoke before/after a transition is performed

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



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# File 'lib/state_machine/machine.rb', line 525

def callbacks
  @callbacks
end

#eventsObject (readonly)

The events that trigger transitions. These are sorted, by default, in the order in which they were defined.



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# File 'lib/state_machine/machine.rb', line 509

def events
  @events
end

#nameObject (readonly)

The name of the machine, used for scoping methods generated for the machine as a whole (not states or events)



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# File 'lib/state_machine/machine.rb', line 505

def name
  @name
end

#namespaceObject (readonly)

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.



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# File 'lib/state_machine/machine.rb', line 533

def namespace
  @namespace
end

#owner_classObject

The class that the machine is defined in



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# File 'lib/state_machine/machine.rb', line 501

def owner_class
  @owner_class
end

#statesObject (readonly)

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.



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# File 'lib/state_machine/machine.rb', line 520

def states
  @states
end

#use_transactionsObject (readonly)

Whether the machine will use transactions when firing events



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# File 'lib/state_machine/machine.rb', line 536

def use_transactions
  @use_transactions
end

Class Method Details

.draw(class_names, options = {}) ⇒ Object

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

Raises:

  • (ArgumentError)


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# File 'lib/state_machine/machine.rb', line 464

def draw(class_names, options = {})
  raise ArgumentError, 'At least one class must be specified' unless class_names && class_names.split(',').any?
  
  # Load any files
  if files = options.delete(:file)
    files.split(',').each {|file| require file}
  end
  
  class_names.split(',').each do |class_name|
    # Navigate through the namespace structure to get to the class
    klass = Object
    class_name.split('::').each do |name|
      klass = klass.const_defined?(name) ? klass.const_get(name) : klass.const_missing(name)
    end
    
    # Draw each of the class's state machines
    klass.state_machines.each_value do |machine|
      machine.draw(options)
    end
  end
end

.find_or_create(owner_class, *args, &block) ⇒ Object

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).



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# File 'lib/state_machine/machine.rb', line 431

def find_or_create(owner_class, *args, &block)
  options = args.last.is_a?(Hash) ? args.pop : {}
  name = args.first || :state
  
  # Find an existing machine
  if owner_class.respond_to?(:state_machines) && machine = owner_class.state_machines[name]
    # Only create a new copy if changes are being made to the machine in
    # a subclass
    if machine.owner_class != owner_class && (options.any? || block_given?)
      machine = machine.clone
      machine.initial_state = options[:initial] if options.include?(:initial)
      machine.owner_class = owner_class
    end
    
    # Evaluate DSL
    machine.instance_eval(&block) if block_given?
  else
    # No existing machine: create a new one
    machine = new(owner_class, name, options, &block)
  end
  
  machine
end

Instance Method Details

#action_hook?(self_only = false) ⇒ Boolean

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



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# File 'lib/state_machine/machine.rb', line 1917

def action_hook?(self_only = false)
  @action_hook_defined || !self_only && owner_class.state_machines.any? {|name, machine| machine.action == action && machine != self && machine.action_hook?(true)}
end

#after_failure(*args, &block) ⇒ Object

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


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# File 'lib/state_machine/machine.rb', line 1730

def after_failure(*args, &block)
  options = (args.last.is_a?(Hash) ? args.pop : {})
  options[:do] = args if args.any?
  assert_valid_keys(options, :on, :do, :if, :unless)
  
  add_callback(:failure, options, &block)
end

#after_transition(*args, &block) ⇒ Object

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.



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# File 'lib/state_machine/machine.rb', line 1635

def after_transition(*args, &block)
  options = (args.last.is_a?(Hash) ? args.pop : {})
  options[:do] = args if args.any?
  add_callback(:after, options, &block)
end

#around_transition(*args, &block) ⇒ Object

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.



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# File 'lib/state_machine/machine.rb', line 1696

def around_transition(*args, &block)
  options = (args.last.is_a?(Hash) ? args.pop : {})
  options[:do] = args if args.any?
  add_callback(:around, options, &block)
end

#attribute(name = :state) ⇒ Object

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



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# File 'lib/state_machine/machine.rb', line 702

def attribute(name = :state)
  name == :state ? @attribute : :#{self.name}_#{name}"
end

#before_transition(*args, &block) ⇒ Object

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.



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# File 'lib/state_machine/machine.rb', line 1624

def before_transition(*args, &block)
  options = (args.last.is_a?(Hash) ? args.pop : {})
  options[:do] = args if args.any?
  add_callback(:before, options, &block)
end

#define_helper(scope, method, *args, &block) ⇒ Object

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


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# File 'lib/state_machine/machine.rb', line 742

def define_helper(scope, method, *args, &block)
  helper_module = @helper_modules.fetch(scope)
  
  if block_given?
    if !self.class.ignore_method_conflicts && conflicting_ancestor = owner_class_ancestor_has_method?(scope, method)
      ancestor_name = conflicting_ancestor.name && !conflicting_ancestor.name.empty? ? conflicting_ancestor.name : conflicting_ancestor.to_s
      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."
    else
      name = self.name
      helper_module.class_eval do
        define_method(method) do |*args|
          block.call((scope == :instance ? self.class : self).state_machine(name), self, *args)
        end
      end
    end
  else
    helper_module.class_eval(method, *args)
  end
end

#draw(options = {}) ⇒ Object

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 “#StateMachine::Machine.owner_classowner_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



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# File 'lib/state_machine/machine.rb', line 1867

def draw(options = {})
  options = {
    :name => "#{owner_class.name}_#{name}",
    :path => '.',
    :format => 'png',
    :font => 'Arial',
    :orientation => 'portrait'
  }.merge(options)
  assert_valid_keys(options, :name, :path, :format, :font, :orientation)
  
  begin
    # Load the graphviz library
    require 'rubygems'
    gem 'ruby-graphviz', '>=0.9.0'
    require 'graphviz'
    
    graph = GraphViz.new('G', :rankdir => options[:orientation] == 'landscape' ? 'LR' : 'TB')
    
    # Add nodes
    states.by_priority.each do |state|
      node = state.draw(graph)
      node.fontname = options[:font]
    end
    
    # Add edges
    events.each do |event|
      edges = event.draw(graph)
      edges.each {|edge| edge.fontname = options[:font]}
    end
    
    # Generate the graph
    graphvizVersion = Constants::RGV_VERSION.split('.')
    file = File.join(options[:path], "#{options[:name]}.#{options[:format]}")
    
    if graphvizVersion[0] == '0' && graphvizVersion[1] == '9' && graphvizVersion[2] == '0'
      outputOptions = {:output => options[:format], :file => file}
    else
      outputOptions = {options[:format] => file}
    end
    
    graph.output(outputOptions)
    graph
  rescue LoadError => ex
    $stderr.puts "Cannot draw the machine (#{ex.message}). `gem install ruby-graphviz` >= v0.9.0 and try again."
    false
  end
end

#dynamic_initial_state?Boolean

Whether a dynamic initial state is being used in the machine



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# File 'lib/state_machine/machine.rb', line 675

def dynamic_initial_state?
  @initial_state.is_a?(Proc)
end

#errors_for(object) ⇒ 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.



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# File 'lib/state_machine/machine.rb', line 1820

def errors_for(object)
  ''
end

#event(*names, &block) ⇒ Object Also known as: on

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


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# File 'lib/state_machine/machine.rb', line 1326

def event(*names, &block)
  options = names.last.is_a?(Hash) ? names.pop : {}
  assert_valid_keys(options, :human_name)
  
  # Store the context so that it can be used for / matched against any event
  # that gets added
  @events.context(names, &block) if block_given?
  
  if names.first.is_a?(Matcher)
    # Add any events referenced in the matcher.  When matchers are used,
    # events are not allowed to be configured.
    raise ArgumentError, "Cannot configure events when using matchers (using #{options.inspect})" if options.any?
    events = add_events(names.first.values)
  else
    events = add_events(names)
    
    # Update the configuration for the event(s)
    events.each do |event|
      event.human_name = options[:human_name] if options.include?(:human_name)
      
      # Add any states that may have been referenced within the event
      add_states(event.known_states)
    end
  end
  
  events.length == 1 ? events.first : events
end

#generate_message(name, values = []) ⇒ Object

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



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# File 'lib/state_machine/machine.rb', line 1832

def generate_message(name, values = [])
  (@messages[name] || self.class.default_messages[name]) % values.map {|value| value.last}
end

#initial_state(object) ⇒ 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>


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# File 'lib/state_machine/machine.rb', line 670

def initial_state(object)
  states.fetch(dynamic_initial_state? ? evaluate_method(object, @initial_state) : @initial_state) if instance_variable_defined?('@initial_state')
end

#initial_state=(new_initial_state) ⇒ Object

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.



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# File 'lib/state_machine/machine.rb', line 628

def initial_state=(new_initial_state)
  @initial_state = new_initial_state
  add_states([@initial_state]) unless dynamic_initial_state?
  
  # Update all states to reflect the new initial state
  states.each {|state| state.initial = (state.name == @initial_state)}
end

#initialize_copy(orig) ⇒ Object

Creates a copy of this machine in addition to copies of each associated event/states/callback, so that the modifications to those collections do not affect the original machine.



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# File 'lib/state_machine/machine.rb', line 587

def initialize_copy(orig) #:nodoc:
  super
  
  @events = @events.dup
  @events.machine = self
  @states = @states.dup
  @states.machine = self
  @callbacks = {:before => @callbacks[:before].dup, :after => @callbacks[:after].dup, :failure => @callbacks[:failure].dup}
end

#initialize_state(object, options = {}) ⇒ Object

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



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# File 'lib/state_machine/machine.rb', line 687

def initialize_state(object, options = {})
  state = initial_state(object)
  if state && (options[:force] || initialize_state?(object))
    value = state.value
    
    if hash = options[:to]
      hash[attribute.to_s] = value
    else
      write(object, :state, value)
    end
  end
end

#invalidate(object, attribute, message, values = []) ⇒ Object

Marks the given object as invalid with the given message.

By default, this is a no-op.



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# File 'lib/state_machine/machine.rb', line 1814

def invalidate(object, attribute, message, values = [])
end

#paths_for(object, requirements = {}) ⇒ Object

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]


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# File 'lib/state_machine/machine.rb', line 1807

def paths_for(object, requirements = {})
  PathCollection.new(object, self, requirements)
end

#read(object, attribute, ivar = false) ⇒ Object

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


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# File 'lib/state_machine/machine.rb', line 1075

def read(object, attribute, ivar = false)
  attribute = self.attribute(attribute)
  ivar ? object.instance_variable_get("@#{attribute}") : object.send(attribute)
end

#reset(object) ⇒ Object

Resets any errors previously added when invalidating the given object.

By default, this is a no-op.



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# File 'lib/state_machine/machine.rb', line 1827

def reset(object)
end

#state(*names, &block) ⇒ Object Also known as: other_states

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.



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# File 'lib/state_machine/machine.rb', line 1029

def state(*names, &block)
  options = names.last.is_a?(Hash) ? names.pop : {}
  assert_valid_keys(options, :value, :cache, :if, :human_name)
  
  # Store the context so that it can be used for / matched against any state
  # that gets added
  @states.context(names, &block) if block_given?
  
  if names.first.is_a?(Matcher)
    # Add any states referenced in the matcher.  When matchers are used,
    # states are not allowed to be configured.
    raise ArgumentError, "Cannot configure states when using matchers (using #{options.inspect})" if options.any?
    states = add_states(names.first.values)
  else
    states = add_states(names)
    
    # Update the configuration for the state(s)
    states.each do |state|
      if options.include?(:value)
        state.value = options[:value]
        self.states.update(state)
      end
      
      state.human_name = options[:human_name] if options.include?(:human_name)
      state.cache = options[:cache] if options.include?(:cache)
      state.matcher = options[:if] if options.include?(:if)
    end
  end
  
  states.length == 1 ? states.first : states
end

#transition(options) ⇒ Object

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.

Raises:

  • (ArgumentError)


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# File 'lib/state_machine/machine.rb', line 1444

def transition(options)
  raise ArgumentError, 'Must specify :on event' unless options[:on]
  
  branches = []
  event(*Array(options.delete(:on))) { branches << transition(options) }
  
  branches.length == 1 ? branches.first : branches
end

#within_transaction(object) ⇒ 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.



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# File 'lib/state_machine/machine.rb', line 1841

def within_transaction(object)
  if use_transactions
    transaction(object) { yield }
  else
    yield
  end
end

#write(object, attribute, value, ivar = false) ⇒ Object

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"


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# File 'lib/state_machine/machine.rb', line 1095

def write(object, attribute, value, ivar = false)
  attribute = self.attribute(attribute)
  ivar ? object.instance_variable_set("@#{attribute}", value) : object.send("#{attribute}=", value)
end