Memento pattern

• Memento Design Pattern

Intent

    Without violating encapsulation, capture and externalize an object’s internal state so that the object can be returned to this state later.
    A magic cookie that encapsulates a “check point” capability.
    Promote undo or rollback to full object status.


Rules of thumb


    Command and Memento act as magic tokens to be passed around and invoked at a later time. In Command, the token represents a request; in Memento, it represents the internal state of an object at a particular time. Polymorphism is important to Command, but not to Memento because its interface is so narrow that a memento can only be passed as a value.
    Command can use Memento to maintain the state required for an undo operation.
    Memento is often used in conjunction with Iterator. An Iterator can use a Memento to capture the state of an iteration. The Iterator stores the Memento internally.
http://sourcemaking.com/design_patterns/memento

• Memento pattern

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 implemented with 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 cannot, 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 (it will always produce the same sequence thereafter when initialized with the seed state) and the state in a finite state machine.

http://en.wikipedia.org/wiki/Memento_pattern