Class: StateMachines::Machine

Inherits:
Object
  • Object
show all
Extended by:
ClassMethods
Includes:
EvalHelpers, ActionHooks, AsyncExtensions, Callbacks, Configuration, EventMethods, HelperGenerators, Integration, Parsing, Rendering, Scoping, StateMethods, Utilities, Validation, MatcherHelpers
Defined in:
lib/state_machines/machine.rb,
lib/state_machines/machine/parsing.rb,
lib/state_machines/machine/scoping.rb,
lib/state_machines/machine/callbacks.rb,
lib/state_machines/machine/rendering.rb,
lib/state_machines/machine/utilities.rb,
lib/state_machines/machine/validation.rb,
lib/state_machines/machine/integration.rb,
lib/state_machines/machine/action_hooks.rb,
lib/state_machines/machine/class_methods.rb,
lib/state_machines/machine/configuration.rb,
lib/state_machines/machine/event_methods.rb,
lib/state_machines/machine/state_methods.rb,
lib/state_machines/machine/async_extensions.rb,
lib/state_machines/machine/helper_generators.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 StateMachines::Machine#before_transition and StateMachines::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!       # => StateMachines::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 StateMachines::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:

StateMachines::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:

StateMachines::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 StateMachines::Integrations namespace.

Defined Under Namespace

Modules: ActionHooks, AsyncExtensions, Callbacks, ClassMethods, Configuration, EventMethods, HelperGenerators, Integration, Parsing, Rendering, Scoping, StateMethods, Utilities, Validation

Constant Summary

Constants included from Validation

Validation::DANGEROUS_PATTERNS

Instance Attribute Summary collapse

Attributes included from ClassMethods

#ignore_method_conflicts

Instance Method Summary collapse

Methods included from ClassMethods

default_messages, default_messages=, find_or_create, replace_messages

Methods included from AsyncExtensions

#async_mode_enabled?, #configure_async_mode!, #read_safely, #run_callbacks_safely, #write_safely

Methods included from Callbacks

#after_failure, #after_transition, #around_transition, #before_transition

Methods included from EventMethods

#event, #paths_for, #transition

Methods included from StateMethods

#dynamic_initial_state?, #initial_state, #initialize_state, #read, #state, #write

Methods included from Configuration

#attribute, #initial_state=, #initialize, #initialize_copy, #owner_class=

Methods included from MatcherHelpers

#all, #same

Methods included from EvalHelpers

#evaluate_method, #evaluate_method_with_event_args

Instance Attribute Details

#action ⇒ Object (readonly)

The action to invoke when an object transitions



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# File 'lib/state_machines/machine.rb', line 469

def action
  @action
end

#callbacks ⇒ Object (readonly)

The callbacks to invoke before/after a transition is performed

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



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# File 'lib/state_machines/machine.rb', line 466

def callbacks
  @callbacks
end

#events ⇒ Object (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_machines/machine.rb', line 450

def events
  @events
end

#name ⇒ Object (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_machines/machine.rb', line 446

def name
  @name
end

#namespace ⇒ Object (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_machines/machine.rb', line 474

def namespace
  @namespace
end

#owner_class ⇒ Object (readonly)

The class that the machine is defined in



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# File 'lib/state_machines/machine.rb', line 442

def owner_class
  @owner_class
end

#states ⇒ Object (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_machines/machine.rb', line 461

def states
  @states
end

#use_transactions ⇒ Object (readonly)

Whether the machine will use transactions when firing events



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# File 'lib/state_machines/machine.rb', line 477

def use_transactions
  @use_transactions
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.

Returns:

  • (Boolean) 


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# File 'lib/state_machines/machine.rb', line 589

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

#define_helper(scope, method, &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_machines/machine.rb', line 515

def define_helper(scope, method, *, **, &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 StateMachines::Machine.ignore_method_conflicts = true."
    else
      name = self.name
      helper_module.class_eval do
        define_method(method) do |*args, **kwargs|
          block.call((scope == :instance ? self.class : self).state_machine(name), self, *args, **kwargs)
        end
      end
    end
  else
    helper_module.class_eval(method, __FILE__, __LINE__)
  end
end

#draw ⇒ Object 



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# File 'lib/state_machines/machine.rb', line 583

def draw(**)
  renderer.draw_machine(self, **)
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_machines/machine.rb', line 543

def errors_for(_object)
  ''
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_machines/machine.rb', line 554

def generate_message(name, values = [])
  message = @messages[name] || self.class.default_messages[name]

  # Check whether there are actually any values to interpolate to avoid
  # any warnings
  if message.scan(/%./).any? { |match| match != '%%' }
    message % values.map(&:last)
  else
    message
  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_machines/machine.rb', line 538

def invalidate(_object, _attribute, _message, _values = []); end

#renderer ⇒ Object 



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# File 'lib/state_machines/machine.rb', line 579

def renderer
  self.class.renderer
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_machines/machine.rb', line 550

def reset(_object); 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_machines/machine.rb', line 571

def within_transaction(object, &)
  if use_transactions
    transaction(object, &)
  else
    yield
  end
end