Programming - Memento Behavioral Design Pattern

Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

The classes and/or objects participating in this pattern are:

Memento
- stores internal state of the Originator object. The memento may store as much or as little of the originator's internal state as necessary at its originator's discretion.
- protect against access by objects of other than the originator. Mementos have effectively two interfaces. Caretaker sees a narrow interface to the Memento -- it can only pass the memento to the other objects. Originator, in contrast, sees a wide interface, one that lets it access all the data necessary to restore itself to its previous state. Ideally, only the originator that produces the memento would be permitted to access the memento's internal state.
Originator
and
- creates a memento containing a snapshot of its current internal state.
- uses the memento to restore its internal state
Caretaker
- is responsible for the memento's safekeeping
- never operates on or examines the contents of a memento.

// Memento pattern -- Structural example

                using
              
              System;

               namespace

DoFactory.GangOfFour.Memento.Structural
{

// MainApp test application

class MainApp
{
static void Main()
{
Originator o = new Originator();
o.State = "On";

// Store internal state
Caretaker c = new Caretaker();
c.Memento = o.CreateMemento();

// Continue changing originator
o.State = "Off";

// Restore saved state
o.SetMemento(c.Memento);

// Wait for user
Console.Read();
}
}

// "Originator"

class Originator
{
private string state;

// Property
public string State
{
get{ return state; }
set
{
state = value;
Console.WriteLine("State = " + state);
}
}

public Memento CreateMemento()
{
return (new Memento(state));
}

public void SetMemento(Memento memento)
{
Console.WriteLine("Restoring state:");
State = memento.State;
}
}

// "Memento"

class Memento
{
private string state;

// Constructor
public Memento(string state)
{
this.state = state;
}

// Property
public string State
{
get{ return state; }
}
}

// "Caretaker"

class Caretaker
{
private Memento memento;

// Property
public Memento Memento
{
set{ memento = value; }
get{ return memento; }
}
}
}

and

Output expected:

              State = On

               State = Off

               Restoring state:

               State = On

Other pages of interest:

[+]	Programming - Java - Conditional Operators

[+]	Object Oriented Programming