Minimal Finite State Machine.
There are many finite state machine implementations for Ruby, and they all provide a nice DSL for declaring events, exceptions, callbacks, and all kinds of niceties in general.
But if all you want is a finite state machine, look no further: this has less than 50 lines of code and provides everything a finite state machine must have, and nothing more.
require 'micromachine'
machine = MicroMachine.new(:new) # Initial state.
# Define the possible transitions for each event.
machine.when(:confirm, :new => :confirmed)
machine.when(:ignore, :new => :ignored)
machine.when(:reset, :confirmed => :new, :ignored => :new)
machine.trigger(:confirm) #=> true
machine.state #=> :confirmed
machine.trigger(:ignore) #=> false
machine.state #=> :confirmed
machine.trigger(:reset) #=> true
machine.state #=> :new
machine.trigger(:ignore) #=> true
machine.state #=> :ignored
The when
helper is syntactic sugar for assigning to the
transitions_for
hash. This code is equivalent:
machine.transitions_for[:confirm] = { :new => :confirmed }
machine.transitions_for[:ignore] = { :new => :ignored }
machine.transitions_for[:reset] = { :confirmed => :new, :ignored => :new }
You can also ask if an event will trigger a change in state. Following the example above:
machine.state #=> :ignored
machine.trigger?(:ignore) #=> false
machine.trigger?(:reset) #=> true
# And the state is preserved, because you were only asking.
machine.state #=> :ignored
If you want to force an Exception when trying to trigger a event from a
non compatible state use the trigger!
method:
machine.trigger?(:ignore) #=> false
machine.trigger!(:ignore) #=> MicroMachine::InvalidState raised
It can also have callbacks when entering some state:
machine.on(:confirmed) do
puts "Confirmed"
end
Or callbacks on any transition:
machine.on(:any) do
puts "Transitioned..."
end
Note that :any
is a special key. Using it as a state when declaring
transitions will give you unexpected results.
Finally, you can list possible events or states:
# All possible events
machine.events #=> [:confirm, :ignore, :reset]
# All events triggerable from the current state
machine.triggerable_events #=> [:confirm, :ignore]
# All possible states
machine.states #=> [:new, :confirmed, :ignored]
Check the examples directory for more information.
The most popular pattern among Ruby libraries that tackle this problem is to extend the model and transform it into a finite state machine. Instead of working as a mixin, MicroMachine's implementation is by composition: you instantiate a finite state machine (or many!) inside your model and you are in charge of querying and persisting the state. Here's an example of how to use it with an ActiveRecord model:
class Event < ActiveRecord::Base
before_save :persist_confirmation
def confirm!
confirmation.trigger(:confirm)
end
def cancel!
confirmation.trigger(:cancel)
end
def reset!
confirmation.trigger(:reset)
end
def confirmation
@confirmation ||= begin
fsm = MicroMachine.new(confirmation_state || "pending")
fsm.when(:confirm, "pending" => "confirmed")
fsm.when(:cancel, "confirmed" => "cancelled")
fsm.when(:reset, "confirmed" => "pending", "cancelled" => "pending")
fsm
end
end
private
def persist_confirmation
self.confirmation_state = confirmation.state
end
end
This example asumes you have a :confirmation_state
attribute in your
model. This may look like a very verbose implementation, but you gain a
lot in flexibility.
An alternative approach, using callbacks:
class Event < ActiveRecord::Base
def confirm!
confirmation.trigger(:confirm)
end
def cancel!
confirmation.trigger(:cancel)
end
def reset!
confirmation.trigger(:reset)
end
def confirmation
@confirmation ||= begin
fsm = MicroMachine.new(confirmation_state || "pending")
fsm.when(:confirm, "pending" => "confirmed")
fsm.when(:cancel, "confirmed" => "cancelled")
fsm.when(:reset, "confirmed" => "pending", "cancelled" => "pending")
fsm.on(:any) { self.confirmation_state = confirmation.state }
fsm
end
end
end
Now, on any transition the confirmation_state
attribute in the model
will be updated.
$ sudo gem install micromachine
Copyright (c) 2009 Michel Martens
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