MicroMachine

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.

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.

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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