Memento (patró de disseny)

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El Memento o patró de record és un patró de disseny de programari que ofereix la possibilitat de restaurar un objecte al seu estat anterior (desfer a través d'un rollback).

El patró Memento es compon de dos objectes: l' iniciador i un cuidador. L'iniciador és un objecte que té un estat intern. El cuidador li vol fer una cosa a l'iniciador, però vol ser capaç de desfer el canvi. El cuidador primer li demana a l'iniciador un objecte de record (memento). A continuació, es fa l'operació (o la seqüència d'operacions), que es volien fer. Per desfer aquestes operacions, ens retorna l'objecte de memento. El memento en si és un objecte opac (un que el cuidador no pot o no hauria de canviar). A l'usar aquest patró, s'ha de tenir cura si l'iniciador pot canviar altres objectes o recursos - el patró memento opera només amb un objecte.


Codis d'exemple[modifica | modifica el codi]

Actionscript 3[modifica | modifica el codi]

El següent programa en Actionscript 3 implementa el patró Memento.

package 
{
	import flash.display.Sprite;
 
	/**
	* Memento
	*/
	public class As3Memento extends Sprite
	{
		function As3Memento()
		{
			var ct:CareTaker = new CareTaker();
			var originator:Originator = new Originator();
 
				originator.A = 'letter A';
				originator.B = 'letter B';
				originator.C = 'letter C';
 
				ct.addMementoFrom(originator);
 
				originator.A = 'anything...';
				originator.B = 'blah blah...';
				originator.C = 'etc.';
				ct.addMementoFrom(originator);
 
				originator.restoreFromMemento(ct.getMemento(0));
 
				trace(originator.A, originator.B, originator.C);
 
				originator.restoreFromMemento(ct.getMemento(1));
 
				trace(originator.A, originator.B, originator.C);
 
		}
	}
 
}
 
interface IMemento
{
	function getMemento():Object;
	function restoreFromMemento(obj:Object):void;
}
 
class Originator implements IMemento
{
	public var A:String;
	public var B:String;
	public var C:String;
 
	function Originator()
	{ }
 
	/* INTERFACE IMemento */
 
	public function getMemento():Object
	{
		return { _A:A, _B:B, _C:C };
	}
 
	public function restoreFromMemento(obj:Object):void
	{
		A = obj._A;
		B = obj._B;
		C = obj._C;
	}
 
}
 
class CareTaker 
{
	private var mementos:Array;
	function CareTaker()
	{
		mementos = new Array();
	}
 
	public function addMementoFrom( object:IMemento ):void
	{
		mementos.push( object.getMemento() );
	}
 
	public function getMemento(index:uint):Object
	{
		return mementos[index];
	}
}

Java[modifica | modifica el codi]

El següent programa de Java ensenya com fa el "desfés" usant el patró "Memento".

class Originator {
 
    private String state;
    /* lots of memory consumptive private data that is not necessary to define the
     * state and should thus not be saved. Hence the small memento object. */
 
    public void set(String state) {
        System.out.println("Originator: Setting state to " + state);
        this.state = state;
    }
 
    public Object saveToMemento() {
        System.out.println("Originator: Saving to Memento.");
        return new Memento(state);
    }
 
    public void restoreFromMemento(Object m) {
        if (m instanceof Memento) {
            Memento memento = (Memento) m;
            state = memento.getSavedState();
            System.out.println("Originator: State after restoring from Memento: " + state);
        }
    }
 
    private static class Memento {
 
        private String state;
 
        public Memento(String stateToSave) {
            state = stateToSave;
        }
 
        public String getSavedState() {
            return state;
        }
 
    }
 
}
 
class Caretaker {
 
    private List<Object> savedStates = new ArrayList<Object>();
 
    public void addMemento(Object m) {
        savedStates.add(m);
    }
 
    public Object getMemento(int index) {
        return savedStates.get(index);
    }
 
}
 
class MementoExample {
 
    public static void main(String[] args) {
 
        Caretaker caretaker = new Caretaker();
 
        Originator originator = new Originator();
        originator.set("State1");
        originator.set("State2");
        caretaker.addMemento(originator.saveToMemento());
        originator.set("State3");
        caretaker.addMemento(originator.saveToMemento());
        originator.set("State4");
 
        originator.restoreFromMemento(caretaker.getMemento(1));
 
    }
 
}

La sortida és:

Originator: Setting state to State1
Originator: Setting state to State2
Originator: Saving to Memento.
Originator: Setting state to State3
Originator: Saving to Memento.
Originator: Setting state to State4
Originator: State after restoring from Memento: State3

Ruby[modifica | modifica el codi]

El següent codi en Ruby implementa el mateix patró.

#!/usr/bin/env ruby -KU
 
require 'rubygems'
require 'spec'
 
class Originator
  class Memento
    def initialize(state)
      # dup required so that munging of the Originator's original state doesn't mess up
      # this Memento for first or subsequent restore
      @state = state.dup
    end
 
    def state
      # dup required so that munging of the Originator's restored state doesn't mess up
      # this Memento for a second restore
      @state.dup
    end
  end
 
  attr_accessor :state
 
  # pretend there's lots of additional memory-heavy data, which can be reconstructed
  # from state
  def save_to_memento
    Memento.new(@state)
  end
 
  def restore_from_memento(m)
    @state = m.state
  end
 
end
 
class Caretaker < Array; end
 
describe Originator do
  before(:all) do
    @caretaker = Caretaker.new
    @originator = Originator.new
 
    @originator.state = "State1"
  end
 
  it "should have original state" do 
    @originator.state.should == 'State1'
  end
 
  it "should update state" do
    @originator.state = "State2"
    @originator.state.should == 'State2'
  end
 
  it "should save memento" do
    @caretaker << @originator.save_to_memento
    @caretaker.size.should == 1
  end
 
  it "should update state after save to memento" do
    @originator.state = "State3"
    @originator.state.should == 'State3'
  end
 
  it "should save to memento again" do
    @caretaker << @originator.save_to_memento
    @caretaker.size.should == 2
  end
 
  it "should update state after save to memento again" do
    @originator.state = "State4";
    @originator.state.should == 'State4'
  end
 
  it "should restore to original save point" do
    @originator.restore_from_memento @caretaker[0]
    @originator.state.should == 'State2'
  end
 
  it "should restore to second save point" do
    @originator.restore_from_memento @caretaker[1]
    @originator.state.should == 'State3'
  end
 
  it "should restore after pathological munging of restored state" do
    @originator.state[-1] = '5'
    @originator.state.should == 'State5'
    @originator.restore_from_memento @caretaker[1]
    @originator.state.should == 'State3'
  end
 
  it "should restore after pathological munging of original state" do
    @originator.state = "State6"
    @originator.state.should == 'State6'
    @caretaker << @originator.save_to_memento
    @originator.state[-1] = '7'
    @originator.state.should == 'State7'
    @originator.restore_from_memento @caretaker[2]
    @originator.state.should == 'State6'
  end
end

Vegeu també[modifica | modifica el codi]

Enllaços externs[modifica | modifica el codi]