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Minimal Finite State Machine.

There are many finite state machine implementations for Ruby, and theyall 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: thishas less than 50 lines of code and provides everything a finite statemachine 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)#=> truemachine.state#=> :confirmedmachine.trigger(:ignore)#=> falsemachine.state#=> :confirmedmachine.trigger(:reset)#=> truemachine.state#=> :newmachine.trigger(:ignore)#=> truemachine.state#=> :ignored

Thewhen helper is syntactic sugar for assigning to thetransitions_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. Followingthe example above:

machine.state#=> :ignoredmachine.trigger?(:ignore)#=> falsemachine.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 anon compatible state use thetrigger! method:

machine.trigger?(:ignore)#=> falsemachine.trigger!(:ignore)#=> MicroMachine::InvalidState raised

It can also have callbacks when entering some state:

machine.on(:confirmed)doputs"Confirmed"end

Or callbacks on any transition:

machine.on(:any)doputs"Transitioned..."end

Note that:any is a special key. Using it as a state when declaringtransitions will give you unexpected results.

You can also pass any data as the second argument fortrigger andtrigger! which will be passed to every callback as the secondargument too:

machine.on(:any)do |_status,payload|putspayload.inspectendmachine.trigger(:cancel,from::user)

Finally, you can list possible events or states:

# All possible eventsmachine.events#=> [:confirm, :ignore, :reset]# All events triggerable from the current statemachine.triggerable_events#=> [:confirm, :ignore]# All possible statesmachine.states#=> [:new, :confirmed, :ignored]

Check the examples directory for more information.

The most popular pattern among Ruby libraries that tackle this problemis to extend the model and transform it into a finite state machine.Instead of working as a mixin, MicroMachine's implementation is bycomposition: you instantiate a finite state machine (or many!) insideyour 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:

classEvent <ActiveRecord::Basebefore_save:persist_confirmationdefconfirm!confirmation.trigger(:confirm)enddefcancel!confirmation.trigger(:cancel)enddefreset!confirmation.trigger(:reset)enddefconfirmation@confirmation ||=beginfsm=MicroMachine.new(confirmation_state ||"pending")fsm.when(:confirm,"pending"=>"confirmed")fsm.when(:cancel,"confirmed"=>"cancelled")fsm.when(:reset,"confirmed"=>"pending","cancelled"=>"pending")fsmendendprivatedefpersist_confirmationself.confirmation_state=confirmation.stateendend

This example asumes you have a:confirmation_state attribute in yourmodel. This may look like a very verbose implementation, but you gain alot in flexibility.

An alternative approach, using callbacks:

classEvent <ActiveRecord::Basedefconfirm!confirmation.trigger(:confirm)enddefcancel!confirmation.trigger(:cancel)enddefreset!confirmation.trigger(:reset)enddefconfirmation@confirmation ||=beginfsm=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}fsmendendend

Now, on any transition theconfirmation_state attribute in the modelwill be updated.

$ sudo gem install micromachine

Copyright (c) 2009 Michel Martens

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