MicroMachine
Minimal Finite State Machine.
Description
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.
Usage
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 #=> :ignoredThe 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 #=> :ignoredIf 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 raisedIt can also have callbacks when entering some state:
machine.on(:confirmed) do
puts "Confirmed"
endOr callbacks on any transition:
machine.on(:any) do
puts "Transitioned..."
endNote that :any is a special key. Using it as a state when declaring
transitions will give you unexpected results.
You can also pass any data as the second argument for trigger and
trigger! which will be passed to every callback as the second
argument too:
machine.on(:any) do |_status, payload|
puts payload.inspect
end
machine.trigger(:cancel, from: :user)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.
Adding MicroMachine to your models
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
endThis 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
endNow, on any transition the confirmation_state attribute in the model
will be updated.
Installation
$ sudo gem install micromachine
License
Copyright (c) 2009 Michel Martens
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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