alter ego

What

AlterEgo is a Ruby implementation of the State pattern as described by the Gang of Four. It differs from other Ruby state machine libraries in that it focuses on providing polymorphic behavior based on object state. In effect, it makes it easy to give an object different “personalities” depending on the state it is in.

Installing

sudo gem install alter-ego

Rationale

Sometimes you want a single object to behave differently depending on what state it’s in. For instance, a traffic light has a different color depending on whether it is informing people that they should stop, slow down, or proceed. Or a user account object might behave differently depending on whether the account is in the “confirmed” state. AlterEgo can help you set up these state-based object “personalities”.

AlterEgo will help you to define a set of valid states for your objects, and to define the allowable transitions between states. The real strength of AlterEgo, however, is in defining separate behaviors for each state that the object is in. Underneath the covers, AlterEgo will arrange for your object to delegate to a different state object depending on what state it is in. You can either explicitly define these state objects as classes, or use a concise DSL provided by AlterEgo.

Synopsis

  class TrafficLight
    include AlterEgo

    state :proceed, :default => true do
      handle :color do
	"green"
      end
      transition :to => :caution, :on => :cycle!
    end

    state :caution do
      handle :color do
	"yellow"
      end
      transition :to => :stop, :on => :cycle!
    end

    state :stop do
      handle :color do
	"red"
      end 
      transition :to => :proceed, :on => :cycle!
    end
  end

  light = TrafficLight.new
  light.color                     # => "green"
  light.cycle!
  light.color                     # => "yellow"
  light.cycle!
  light.color                     # => "red"
  light.cycle!
  light.color                     # => "green"

Features

• Implemented as a module which can be included in any Ruby class.
• Fully tested with literate RSpec
• Guard clauses may be defined for each transition.
• Enter/exit actions may be defined for each state.
• For more advanced scenarios, arbitrary “request filters” may be defined with full control over which requests are filtered.
• Uses dynamic module generation and delegation instead of method rewriting.
• Pervasive contract-checking catches mistakes in library usage early.
• Storing and reading current state is completely customizable, making it easier to add AlterEgo to legacy classes.

Documentation

See the project’s RDoc

Development

You can fetch the source from RubyForge using Git:

http://rubyforge.org/scm/?group_id=7389

git clone git://rubyforge.org/alter-ego.git

Acknowledgements

Thank you to MDlogix, where this code was initially developed, for kindly permitting me to release AlterEgo to the public.

License

This code is free to use under the terms of the MIT license.

Contact

Questions, comments, suggestions: Email Avdi Grimm

Avdi Grimm, 28th November 2008
Theme extended from Paul Battley