The memento pattern is a software design pattern that provides the ability to restore an object to its previous state (undo via rollback).
Contents
The memento pattern is implemented with three objects: the originator, a caretaker and a memento. 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.
Java example
The following Java program illustrates the "undo" usage of the memento pattern.
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: State3This example uses a String as the state, which is an immutable object in Java. In real-life scenarios the state will almost always be an object, in which case a copy of the state must be done.
It must be said that the implementation shown has a drawback: it declares an internal class. It would be better if this memento strategy could apply to more than one originator.
There are mainly three other ways to achieve Memento:
- Serialization.
- A class declared in the same package.
- The object can also be accessed via a proxy, which can achieve any save/restore operation on the object.
C# Example
The memento pattern allows one to capture the internal state of an object without violating encapsulation such that later one can undo/revert the changes if required. Here one can see that the memento object is actually used to revert the changes made in the object.