Command en Ruby
Command es un patrón de diseño de comportamiento que convierte solicitudes u operaciones simples en objetos.
La conversión permite la ejecución diferida de comandos, el almacenamiento del historial de comandos, etc.
Complejidad:
Popularidad:
Ejemplos de uso: El patrón Command es muy común en el código Ruby. La mayoría de las veces se utiliza como alternativa a las retrollamadas (callbacks) para parametrizar elementos UI con acciones. También se utiliza para poner tareas en cola, realizar el seguimiento del historial de operaciones, etc.
Identificación: El patrón Command es reconocible por los métodos de comportamiento en un tipo de clase abstracta/interfaz (emisora) que invoca un método en una implementación de un tipo de clase abstracta/interfaz diferente (receptora) que la implementación del comando ha implementado durante su creación. Las clases de comando se limitan normalmente a acciones específicas.
Ejemplo conceptual
Este ejemplo ilustra la estructura del patrón de diseño Command. Se centra en responder las siguientes preguntas:
- ¿De qué clases se compone?
- ¿Qué papeles juegan esas clases?
- ¿De qué forma se relacionan los elementos del patrón?
main.rb: Ejemplo conceptual
# The Command interface declares a method for executing a command. class Command # @abstract def execute raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'" end end # Some commands can implement simple operations on their own. class SimpleCommand < Command # @param [String] payload def initialize(payload) @payload = payload end def execute puts "SimpleCommand: See, I can do simple things like printing (#{@payload})" end end # However, some commands can delegate more complex operations to other objects, # called "receivers". class ComplexCommand < Command # Complex commands can accept one or several receiver objects along with any # context data via the constructor. def initialize(receiver, a, b) @receiver = receiver @a = a @b = b end # Commands can delegate to any methods of a receiver. def execute print 'ComplexCommand: Complex stuff should be done by a receiver object' @receiver.do_something(@a) @receiver.do_something_else(@b) end end # The Receiver classes contain some important business logic. They know how to # perform all kinds of operations, associated with carrying out a request. In # fact, any class may serve as a Receiver. class Receiver # @param [String] a def do_something(a) print "\nReceiver: Working on (#{a}.)" end # @param [String] b def do_something_else(b) print "\nReceiver: Also working on (#{b}.)" end end # The Invoker is associated with one or several commands. It sends a request to # the command. class Invoker # Initialize commands. # @param [Command] command def on_start=(command) @on_start = command end # @param [Command] command def on_finish=(command) @on_finish = command end # The Invoker does not depend on concrete command or receiver classes. The # Invoker passes a request to a receiver indirectly, by executing a command. def do_something_important puts 'Invoker: Does anybody want something done before I begin?' @on_start.execute if @on_start.is_a? Command puts 'Invoker: ...doing something really important...' puts 'Invoker: Does anybody want something done after I finish?' @on_finish.execute if @on_finish.is_a? Command end end # The client code can parameterize an invoker with any commands. invoker = Invoker.new invoker.on_start = SimpleCommand.new('Say Hi!') receiver = Receiver.new invoker.on_finish = ComplexCommand.new(receiver, 'Send email', 'Save report') invoker.do_something_important output.txt: Resultado de la ejecución
Invoker: Does anybody want something done before I begin? SimpleCommand: See, I can do simple things like printing (Say Hi!) Invoker: ...doing something really important... Invoker: Does anybody want something done after I finish? ComplexCommand: Complex stuff should be done by a receiver object Receiver: Working on (Send email.) Receiver: Also working on (Save report.) 
