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              #=> :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.

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
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.

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.