Memento (patró de disseny)

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

Actionscript 3

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

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

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

• Facade

Enllaços externs

• Descripció per Matthew Heaney (anglès)
• Diagrama de classes del Memento en UML Arxivat 2014-07-24 a Wayback Machine. amb exemples de codi en C# i .NET. (anglès)