A state machine is an interesting design pattern.

The state_machine gem is a great library. It provides a nice structure to declare states and transitions. It also provides nice callback hooks that can be utilized to run specific actions before or after a transition happens.

It has hooks into ActiveRecord and saves the model when the transition from one state to another is successful. The gem also works just fine with plain old ruby objects as well. Here is a simple example of a state machine.

class Ticket < ActiveRecord::Base
state_machine :state, :initial => :stopped do
state :stopped
state :started

event :start do
transition :stopped => :started
end

event :stop do
transition :started => :stopped
end
end
end


To interact with the state machine, you can do the following:

ticket = Ticket.create(subject: 'New issue with post', content: 'Some text')
ticket.start #=> true
ticket.state #=> 'started'
ticket.start #=> false
ticket.state #=> 'started'
ticket.stop  #=> true
ticket.state #=> 'stopped'


Earlier in Dumb Data Objects I talked very briefly about how to integrate a state machine into a Dumb Data Object. What this gem brings to the table is another way back into “Callback Hell”, and that is something everyone should avoid.

The transition callbacks are there to be used sparingly. If the need arises to apply a callback to the state machine, think about the implications this can have on the object through out its lifetime. Will this callback make interaction easier or will it complicate the model and make it difficult for someone else to pick up?

One callback that is typically rife with code smell is the after_transition callback. This callback will trigger an action after a transition from one state to another has taken place. Why not just execute that method after the event was triggered? It’s very easy to do.

State machines are supposed to be simple. If transitions become complex, then the state machine flow becomes disruptive and difficult to ascertain. The ultimate goal is to move crazy logic out of the model and push that off into service objects and command objects like the following:

class StartTicket
def initialize(ticket)
@ticket = ticket
end

def execute
if @ticket.start
notify = NotifyTicketSubscribers.new(@ticket)
notify.send_ticket_started_email
true
else
false
end
end
end


If I were to mix in the Signals Gem and implement a command pattern, it would look like the following:

class StartTicket
include Signals::Publisher

def initialize(ticket)
@ticket = ticket
end

def execute
if @ticket.start
else
end
end
end


Testing StartTicket becomes really easy at this point.

# Use 'rspec', >= '2.14.0.rc1' in order to
# utilize test spies
describe StartTicket do
describe '#execute' do
it 'should start the ticket' do
# Mock
ticket = double('Ticket', start: true)
command = StartTicket.new(ticket)

# Excercise
command.execute

# Verify
command.should(
with(:start_ticket_successful, ticket)
).once
end
end
end


The after_transition callbacks will be pushed off onto the listeners and before_transition calls will be done before the event is ever triggered in the StartTicket#execute method.

class TicketListener
include Signals::Subscriber

listen_for :start_ticket_successful => :send_ticket_started_email
listen_for :stop_ticket_successful => :send_ticket_stopped_email

def send_ticket_started_email(ticket)
# Send out emails
end

def send_ticket_stopped_email(ticket)
# Send out emails
end
end


Piecing it all together, results in this simple use case.

ticket  = Ticket.find(1)
command = StartTicket.new(ticket)
command.subscribe(TicketListener.new)
command.execute


Yes there is a lot of boiler-plate code, but trust me when I say the benefits greatly out weighs the drawbacks.

For the love that is programming and refactoring, stay away from callbacks!