AlterEgo

DESCRIPTION:

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.

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.

DETAILS:

AlterEgo differs from other Finite State Machine implementations in Ruby in that where other libraries focus on describing a set of valid state transitions, AlterEgo focuses on varying behavior based on state. In other words, it provides state-based polymorphism.

AlterEgo draws heavily on the State Pattern as published in the book Design Patterns. A summary of the pattern can be found on Wikipedia. Because AlterEgo uses the terminology set forth in the State Pattern, it is useful to have some familiarity with the pattern in order to understand the library.

In the State Pattern, states of an object are represented as discrete objects. At any given time an object with state-based behavior has-a state object. The object with state-based behavior delegates certain method calls to its current state. In this way, the implementation of those methods can vary with the state of the object. Or in certain states some methods may not be supported at all.

The AlterEgo library provides both an object model for manually setting up explicit state classes, and a concise DSL built on top of that model which simplifies building classes with state-based behavior.

This file only scratches the surface of AlterEgo functionality. For complete tutorial documentation, see the file spec/alter_ego_spec.rb. It contains the annotated specification, written in the style of a step-by-step tutorial.

Terminology:

Context

The context is the class which should have state-based behavior. In the example above, the TrafficLight class is the context.

State

Each state the context might exist in is represented by a class, In the example given above, the available states are caution, stop, and proceed.

Request

A request refers to a message (method) sent to the context. In the example given above, the supported requests are color and cycle.

Handler

A handler is a method on a state which implements a request for that state. For instance, in the example above, when the TrafficLight is in the caution state, the handler for the request color returns “yellow”.

Footnotes:

  1. Gamma, Erich; Richard Helm, Ralph Johnson, John M. Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley, 395. ISBN 0201633612.

REQUIREMENTS:

  • ActiveSupport

INSTALL:

  • sudo gem install alter-ego

LICENSE:

(The MIT License)

Copyright © 2008 Avdi Grimm

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.