transitions

State machine extracted from ActiveModel

troessner
533
91
Ruby

Overview

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Synopsis

transitions is a ruby state machine implementation.

Installation

Rails

This goes into your Gemfile:

gem "transitions", :require => ["transitions", "active_model/transitions"]

… and this into your ORM model:

include ActiveModel::Transitions

Standalone

gem install transitions

… and this into your class:

include Transitions

Using transitions

class Product
  include ActiveModel::Transitions

  state_machine do
    state :available # first one is initial state
    state :out_of_stock, :exit => :exit_out_of_stock
    state :discontinued, :enter => lambda { |product| product.cancel_orders }

    event :discontinued do
      transitions :to => :discontinued, :from => [:available, :out_of_stock], :on_transition => :do_discontinue
    end
    event :out_of_stock, :success => :reorder do
      transitions :to => :out_of_stock, :from => [:available, :discontinued]
    end
    event :available do
      transitions :to => :available, :from => [:out_of_stock], :guard => lambda { |product| product.in_stock > 0 }
    end
  end
end

In this example we assume that you are in a rails project using Bundler, which
would automatically require transitions. If this is not the case then you
have to add

require 'transitions'

wherever you load your dependencies in your application.

Known limitations:

  • You can only use one state machine per model. While in theory you can
    define two or more, this won’t work as you would expect. Not supporting
    this was intentional, if you’re interested in the rational look up version
    0.1.0 in the CHANGELOG.

  • Use symbols, not strings for declaring the state machine. Using strings is
    not supported as is using whitespace in names (because transitions
    possibly generates methods out of this).

Features

Getting and setting the current state

Use the (surprise ahead) current_state method - in case you didn’t set a
state explicitly you’ll get back the state that you defined as initial state.

>> Product.new.current_state
=> :available

You can also set a new state explicitly via update_current_state(new_state, persist = true / false) but you should never do this unless you really know
what you’re doing and why - rather use events / state transitions (see below).

Predicate methods are also available using the name of the state.

>> Product.new.available?
=> true

Events

When you declare an event, say discontinue, three methods are declared for
you: discontinue, discontinue! and can_discontinue?. The first two
events will modify the state attribute on successful transition, but only
the bang(!)-version will call save!. The can_discontinue? method will not
modify state but instead returns a boolean letting you know if a given
transition is possible.

In addition, a can_transition? method is added to the object that expects one or more event names as arguments. This semi-verbose method name is used to avoid collisions with [https://github.com/ryanb/cancan](the authorization gem CanCan).

>> Product.new.can_transition? :out_of_stock
=> true

If you need to get all available transitions for the current state you can simply call:

>> Product.new.available_transitions
=> [:discontinued, :out_of_stock]

Callback overview

transitions offers you the possibility to define a couple of callbacks during the different stages / places of transitioning from one state to another. So let’s say you have an event discontinue which transitions the current state from in_stock to sold_out. The callback sequence would look like this:

  | discontinue event |
        |
        |
        |
  | current_state `in_stock` | ----> executes `exit` callback
        |
        |
        |
  | current_state `in_stock` | ----> executes `on_transition` callback if and only the `guard` check was successfull. If not successfull, the chain aborts here and the `event_failed` callback is executed
        |
        |
        |
  | current_state `in_stock` | ----> executes `enter` callback for new state `sold_out`
        |
        |
        |
  | current_state `in_stock` | ----> executes `event_fired` callback
        |
        |
        |
  | current_state `in_stock` | ----> move state from `in_stock` to `sold_out`
        |
        |
        |
  | current_state `sold_out` | ----> executes `success` callback of the `discontinue` event

This all looks very complicated (I know), but don’t worry, in 99% of all cases you don’t have to care about the details and the usage itself is straightforward as you can see in the examples below where each callback is explained a little more throrough.

Callback # 1: State callbacks enter and exit

If you want to trigger a method call when the object enters or exits a state regardless
of the transition that made that happen, use enter and exit.

exit will be called before the transition out of the state is executed. If you want the method
to only be called if the transition is successful, then use another approach.

enter will be called after the transition has been made but before the object is persisted. If you want
the method to only be called after a successful transition to a new state including persistence,
use the success argument to an event instead.

An example:

class Motor < ActiveRecord::Base
  include ActiveModel::Transitions

  state_machine do
    state :off, enter: :turn_power_off
    state :on,  exit: :prepare_shutdown
  end
end

Callback # 2: Transition callback on_transition

Each event definition takes an optional on_transition argument, which allows
you to execute code on transition. This callback is executed after the exit callback of the former state (if it has been defined) but before the enter callback of the new state and only if the guard check succeeds. There is no check if the callback itself succeeds (meaning that transitions does not evaluate its return value somewhere). However, you can easily add some properly abstracted error handling yourself by raising an exception in this callback and then handling this exception in the (also defined by you) event_failed callback (see below and / or the wonderful ascii diagram above).

You can pass in a Symbol, a String, a Proc or an Array containing method names
as Symbol or String like this:

event :discontinue do
  transitions :to => :discontinued, :from => [:available, :out_of_stock], :on_transition => [:do_discontinue, :notify_clerk]
end

Any arguments passed to the event method will be passed on to the on_transition callback.

Callback #3 : Event callback success

In case you need to trigger a method call after a successful transition you
can use success. This will be called after the save! is complete (if you
use the state_name! method) and should be used for any methods that require
that the object be persisted.

event :discontinue, :success => :notify_admin do
  transitions :to => :discontinued, :from => [:available, :out_of_stock]
end

In addition to just specify the method name on the record as a symbol you can
pass a lambda to perfom some more complex success callbacks:

event :discontinue, :success => lambda { |order| AdminNotifier.notify_about_discontinued_order(order) } do
  transitions :to => :discontinued, :from => [:available, :out_of_stock]
end

If you need it, you can even call multiple methods or lambdas just passing an
array:

event :discontinue, :success => [:notify_admin, lambda { |order| AdminNotifier.notify_about_discontinued_order(order) }] do
  transitions :to => :discontinued, :from => [:available, :out_of_stock]
end

Callback caveats

Since callbacks will not be called by you but by transitions the scope is different when they are called and you’ll run into problems if you use classes / modules in those callbacks that have the same names like transitions ones, e.g. “Event”:

def event_fired(current_state, new_state, event)
  Event.create!
end

This will crash because transitions uses an Event class as well, and, since the scope has changed when transitions calls this method, transitions will use it’s own Event class here, not yours.
In this case you can try to prefix your models with the “::” operator and see if that solves your problems. See https://github.com/troessner/transitions/issues/123 for details.

Automatic scope generation

transitions will automatically generate scopes for you if you are using
ActiveRecord and tell it to do so via the auto_scopes option:

Given a model like this:

class Order < ActiveRecord::Base
  include ActiveModel::Transitions
  state_machine :auto_scopes => true do
    state :pick_line_items
    state :picking_line_items
    event :move_cart do
      transitions to: :pick_line_items, from: :picking_line_items
    end
  end
end

you can use this feature a la:

>> Order.pick_line_items
=> []
>> Order.create!
=> #<Order id: 3, state: "pick_line_items", description: nil, created_at: "2011-08-23 15:48:46", updated_at: "2011-08-23 15:48:46">
>> Order.pick_line_items
=> [#<Order id: 3, state: "pick_line_items", description: nil, created_at: "2011-08-23 15:48:46", updated_at: "2011-08-23 15:48:46">]

Using guard

Each event definition takes an optional guard argument, which acts as a
predicate for the transition.

You can pass in Symbols, Strings, or Procs like this:

event :discontinue do
  transitions :to => :discontinued, :from => [:available, :out_of_stock], :guard => :can_discontinue
end

or

event :discontinue do
  transitions :to => :discontinued, :from => [:available, :out_of_stock], :guard => [:can_discontinue, :super_sure?]
end

Any arguments passed to the event method will be passed on to the guard
predicate.

Note that guards will not raise on failure on their own. This means that if you want to
treat the failure of a guard exceptional you’ll need to raise an exception yourself explicitly
in that guard (see here for the
corresponding discussion).

Timestamps

If you’d like to note the time of a state change, Transitions comes with
timestamps free! To activate them, simply pass the timestamp option to the
event definition with a value of either true or the name of the timestamp
column. NOTE - This should be either true, a String, a Symbol, or an Array of
these

# This will look for an attribute called exploded_at or exploded_on (in that order)
# If present, it will be updated
event :explode, :timestamp => true do
  transitions :from => :complete, :to => :exploded
end

# This will look for an attribute named repaired_on to update upon save
event :rebuild, :timestamp => :repaired_on do
  transitions :from => :exploded, :to => :rebuilt
end

Using event_fired and event_failed

In case you define event_fired and / or event_failed, transitions will
use those callbacks correspondingly.

You can use those callbacks like this:

def event_fired(current_state, new_state, event)
  MyLogger.info "Event fired #{event.inspect}"
end

def event_failed(event)
  MyLogger.warn "Event failed #{event.inspect}"
end

Listing all the available states and events

You can easily get a listing of all available states:

Order.available_states # Uses the <tt>default</tt> state machine
# => [:pick_line_items, :picking_line_items]

Same goes for the available events:

Order.available_events
# => [:move_cart]

Explicitly setting the initial state with the initial option

state_machine :initial => :closed do
  state :open
  state :closed
end

The explicitly specified state must be one of the states listed in the state definition below, otherwise transitions will raise a rather unhelpful exception like “NoMethodError: undefined method `call_action’ for nil:NilClass” (there’s a ticket to fix this already: https://github.com/troessner/transitions/issues/112)

Configuring a different column name with ActiveRecord

To use a different column than state to track it’s value simply do this:

class Product < ActiveRecord::Base
  include Transitions

  state_machine :attribute_name => :different_column do

    ...

  end
end

Ruby Compatibility

Supported versions:

  • 2.1
  • 2.2

Supported implementations:

  • MRI
  • Rubinius
  • Jruby

Supported Rails versions:

  • 3
  • 4

Known bugs / limitations

  • Right now it seems like transitions does not play well with mongoid. A
    possible fix had to be rolled back due to other side effects:
    https://github.com/troessner/transitions/issues/76. Since I know virtually
    zero about mongoid, a pull request would be highly appreciated.
  • Multiple state machines are not and will not be supported. For the rationale
    behind this see the Changelog.

Associated projects

  • GraphvizTransitions - Adds support for generating graphs based on states, events and transitions

Documentation, Guides & Examples

Copyright

Copyright © 2010 Jakub Kuźma, Timo Rößner. See LICENSE for details.