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machine.rb
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machine.rb
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require 'state_machine/extensions'
require 'state_machine/assertions'
require 'state_machine/integrations'
require 'state_machine/helper_module'
require 'state_machine/state'
require 'state_machine/event'
require 'state_machine/callback'
require 'state_machine/node_collection'
require 'state_machine/state_collection'
require 'state_machine/event_collection'
require 'state_machine/path_collection'
require 'state_machine/matcher_helpers'
module StateMachine
# 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 (<tt>:ignite</tt> 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 <tt>before :create</tt> 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 <tt>:pending</tt> 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 <tt>:pending</tt> 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 Machine
include Assertions
include EvalHelpers
include MatcherHelpers
class << self
# 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).
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
# 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:
# * <tt>:file</tt> - A comma-delimited string of files to load that
# contain the state machine definitions to draw
# * <tt>:path</tt> - The path to write the graph file to
# * <tt>:format</tt> - The image format to generate the graph in
# * <tt>:font</tt> - The name of the font to draw state names in
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
end
# Default messages to use for validation errors in ORM integrations
class << self; attr_accessor :default_messages; end
@default_messages = {
:invalid => 'is invalid',
:invalid_event => 'cannot transition when %s',
:invalid_transition => 'cannot transition via "%s"'
}
# Whether to ignore any conflicts that are detected for helper methods that
# get generated for a machine's owner class. Default is false.
class << self; attr_accessor :ignore_method_conflicts; end
@ignore_method_conflicts = false
# The class that the machine is defined in
attr_accessor :owner_class
# The name of the machine, used for scoping methods generated for the
# machine as a whole (not states or events)
attr_reader :name
# The events that trigger transitions. These are sorted, by default, in
# the order in which they were defined.
attr_reader :events
# 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.
attr_reader :states
# The callbacks to invoke before/after a transition is performed
#
# Maps :before => callbacks and :after => callbacks
attr_reader :callbacks
# The action to invoke when an object transitions
attr_reader :action
# 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.
attr_reader :namespace
# Whether the machine will use transactions when firing events
attr_reader :use_transactions
# Creates a new state machine for the given attribute
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?
Rails.logger.error("initializing the state machine")
Rails.logger.error("init use_transactions=#{@use_transactions}")
# 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
# 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.
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
# Sets the class which is the owner of this state machine. Any methods
# generated by states, events, or other parts of the machine will be defined
# on the given owner class.
def owner_class=(klass)
@owner_class = klass
# Create modules for extending the class with state/event-specific methods
@helper_modules = helper_modules = {:instance => HelperModule.new(self, :instance), :class => HelperModule.new(self, :class)}
owner_class.class_eval do
extend helper_modules[:class]
include helper_modules[:instance]
end
# Add class-/instance-level methods to the owner class for state initialization
unless owner_class < StateMachine::InstanceMethods
owner_class.class_eval do
extend StateMachine::ClassMethods
include StateMachine::InstanceMethods
end
define_state_initializer if @initialize_state
end
# Record this machine as matched to the name in the current owner class.
# This will override any machines mapped to the same name in any superclasses.
owner_class.state_machines[name] = self
end
# 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.
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
# 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>
def initial_state(object)
states.fetch(dynamic_initial_state? ? evaluate_method(object, @initial_state) : @initial_state) if instance_variable_defined?('@initial_state')
end
# Whether a dynamic initial state is being used in the machine
def dynamic_initial_state?
@initial_state.is_a?(Proc)
end
# Initializes the state on the given object. Initial values are only set if
# the machine's attribute hasn't been previously initialized.
#
# Configuration options:
# * <tt>:force</tt> - Whether to initialize the state regardless of its
# current value
# * <tt>:to</tt> - A hash to set the initial value in instead of writing
# directly to the object
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
# Gets the actual name of the attribute on the machine's owner class that
# stores data with the given name.
def attribute(name = :state)
name == :state ? @attribute : :"#{self.name}_#{name}"
end
# 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
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
# Customizes the definition of one or more states in the machine.
#
# Configuration options:
# * <tt>:value</tt> - The actual value to store when an object transitions
# to the state. Default is the name (stringified).
# * <tt>:cache</tt> - If a dynamic value (via a lambda block) is being used,
# then setting this to true will cache the evaluated result
# * <tt>:if</tt> - 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.
# * <tt>:human_name</tt> - 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 <tt>:cache</tt> 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 <tt>:purchased</tt> 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 <tt>:if</tt> 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