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.
class TrafficLight include AlterEgo state :proceed, :default => true do handle :color do
end transition :to => :caution, :on => :cycle! end state :caution do handle :color do
end transition :to => :stop, :on => :cycle! end state :stop do handle :color do
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"
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.
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.
The context is the class which should have state-based behavior. In the example above, the TrafficLight class is the context.
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.
A request refers to a message (method) sent to the context. In the example given above, the supported requests are color and cycle.
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”.
Gamma, Erich; Richard Helm, Ralph Johnson, John M. Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley, 395. ISBN 0201633612.
sudo gem install alter-ego
(The MIT License)
Copyright © 2008 Avdi Grimm
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