memento pattern

The memento pattern is a software design pattern.

Name: memento pattern
Need: ability to restore an object to its previous state (undo by rollback)
Solution: 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 rollback 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 or should not change).
Consequences: 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. See also: Facade pattern, Facade memento strategy, Design patterns

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