Memento pattern

From Wikipedia, the free encyclopedia

The memento pattern is a software design pattern that provides the ability to restore an object to its previous state (undo via rollback).

The memento pattern is used by two objects: the originator and a caretaker. The originator is some object that has an internal state. The caretaker is going to do something to the originator, but wants to be able to undo the change. The caretaker first asks the originator for a memento object. Then it does whatever operation (or sequence of operations) it was going to do. To roll back to the state before the operations, it returns the memento object to the originator. The memento object itself is an opaque object (one which the caretaker can not, or should not, change). When using this pattern, care should be taken if the originator may change other objects or resources - the memento pattern operates on a single object.

Classic examples of the memento pattern include the seed of a pseudorandom number generator and the state in a finite state machine.

Contents

[edit] Examples

[edit] Java

The following Java program illustrates the "undo" usage of the Memento Pattern.

import java.util.*;  
 
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) );
    }
}

The output is:

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

[edit] Ruby

The following Ruby program illustrates the same pattern.

#!/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


[edit] See also

[edit] External links

v • d • e
Design patterns in the book Design Patterns
Creational
Structural
Behavioral