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Memento

Memento en Ruby

Memento es un patrón de diseño de comportamiento que permite tomar instantáneas del estado de un objeto y restaurarlo en el futuro.

El patrón Memento no compromete la estructura interna del objeto con el que trabaja, ni la información que se encuentra dentro de las instantáneas.

Uso del patrón en Ruby

Complejidad:

Popularidad:

Ejemplos de uso: El principio del patrón Memento puede cumplirse utilizando la serialización, que es bastante habitual en Ruby. Aunque no es la única forma ni la más efectiva de realizar instantáneas del estado de un objeto, permite almacenar copias de seguridad del estado mientras protege de otros objetos la estructura del originador.

Ejemplo conceptual

Este ejemplo ilustra la estructura del patrón de diseño Memento. 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 Originator holds some important state that may change over time. It also
# defines a method for saving the state inside a memento and another method for
# restoring the state from it.
class Originator
  # For the sake of simplicity, the originator's state is stored inside a single
  # variable.
  attr_accessor :state
  private :state

  # @param [String] state
  def initialize(state)
    @state = state
    puts "Originator: My initial state is: #{@state}"
  end

  # The Originator's business logic may affect its internal state. Therefore,
  # the client should backup the state before launching methods of the business
  # logic via the save() method.
  def do_something
    puts 'Originator: I\'m doing something important.'
    @state = generate_random_string(30)
    puts "Originator: and my state has changed to: #{@state}"
  end

  private def generate_random_string(length = 10)
    ascii_letters = [*'a'..'z', *'A'..'Z']
    (0...length).map { ascii_letters.sample }.join
  end

  # Saves the current state inside a memento.
  def save
    ConcreteMemento.new(@state)
  end

  # Restores the Originator's state from a memento object.
  def restore(memento)
    @state = memento.state
    puts "Originator: My state has changed to: #{@state}"
  end
end

# The Memento interface provides a way to retrieve the memento's metadata, such
# as creation date or name. However, it doesn't expose the Originator's state.
class Memento
  # @abstract
  #
  # @return [String]
  def name
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end

  # @abstract
  #
  # @return [String]
  def date
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end
end

class ConcreteMemento < Memento
  # @param [String] state
  def initialize(state)
    @state = state
    @date = Time.now.strftime('%F %T')
  end

  # The Originator uses this method when restoring its state.
  attr_reader :state

  # The rest of the methods are used by the Caretaker to display metadata.
  def name
    "#{@date} / (#{@state[0, 9]}...)"
  end

  # @return [String]
  attr_reader :date
end

# The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
# doesn't have access to the originator's state, stored inside the memento. It
# works with all mementos via the base Memento interface.
class Caretaker
  # @param [Originator] originator
  def initialize(originator)
    @mementos = []
    @originator = originator
  end

  def backup
    puts "\nCaretaker: Saving Originator's state..."
    @mementos << @originator.save
  end

  def undo
    return if @mementos.empty?

    memento = @mementos.pop
    puts "Caretaker: Restoring state to: #{memento.name}"

    begin
      @originator.restore(memento)
    rescue StandardError
      undo
    end
  end

  def show_history
    puts 'Caretaker: Here\'s the list of mementos:'

    @mementos.each { |memento| puts memento.name }
  end
end

originator = Originator.new('Super-duper-super-puper-super.')
caretaker = Caretaker.new(originator)

caretaker.backup
originator.do_something

caretaker.backup
originator.do_something

caretaker.backup
originator.do_something

puts "\n"
caretaker.show_history

puts "\nClient: Now, let's rollback!\n"
caretaker.undo

puts "\nClient: Once more!\n"
caretaker.undo

output.txt: Resultado de la ejecución

Originator: My initial state is: Super-duper-super-puper-super.

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: BFDECxFORrlPIMDCDfQuRwHcuvpbTv

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: xjyrOYzoBYIPYHhGKlvbQrsRvKSRzY

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: xHfPozLWyhsamFVUUPfhIaGBhaBBvK

Caretaker: Here's the list of mementos:
2019-03-06 23:04:13 / (Super-dup...)
2019-03-06 23:04:13 / (BFDECxFOR...)
2019-03-06 23:04:13 / (xjyrOYzoB...)

Client: Now, let's rollback!
Caretaker: Restoring state to: 2019-03-06 23:04:13 / (xjyrOYzoB...)
Originator: My state has changed to: xjyrOYzoBYIPYHhGKlvbQrsRvKSRzY

Client: Once more!
Caretaker: Restoring state to: 2019-03-06 23:04:13 / (BFDECxFOR...)
Originator: My state has changed to: BFDECxFORrlPIMDCDfQuRwHcuvpbTv

Memento en otros lenguajes

Patrones de diseño: Memento en Java Patrones de diseño: Memento en C# Patrones de diseño: Memento en C++ Patrones de diseño: Memento en PHP Patrones de diseño: Memento en Python Patrones de diseño: Memento en Swift Patrones de diseño: Memento en TypeScript