The memento pattern is a software design pattern in the field of object-oriented programming that allows reverting the state of an object. Uses of this design pattern include undo, version control, and serialization.
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 easily bring back the prior state. 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 immutable. When using this pattern, care should be taken if the originator may change other objects or resourcesâÂÂthe memento pattern operates on a single object.
One classic example of this pattern is the pseudorandom number generator (PRGN). In this case, each consumer of the PRNG serves as a caretaker who can initialize the PRNG (the originator) with a particular seed (the memento) to produce an identical sequence of pseudorandom numbers.
In the above UML class diagram, the <code>Caretaker</code> class refers to the <code>Originator</code> class for saving (<code>createMemento()</code>) and restoring (<code>restore(memento)</code>) originator's internal state. <br> The <code>Originator</code> class implements <br> (1) <code>createMemento()</code> by creating and returning a <code>Memento</code> object that stores originator's current internal state and <br> (2) <code>restore(memento)</code> by restoring state from the passed in <code>Memento</code> object. <br>
The UML sequence diagram shows the run-time interactions: <br> (1) Saving originator's internal state: The <code>Caretaker</code> object calls <code>createMemento()</code> on the <code>Originator</code> object, which creates a <code>Memento</code> object, saves its current internal state (<code>setState()</code>), and returns the <code>Memento</code> to the <code>Caretaker</code>. <br> (2) Restoring originator's internal state: The <code>Caretaker</code> calls <code>restore(memento)</code> on the <code>Originator</code> object and specifies the <code>Memento</code> object that stores the state that should be restored. The <code>Originator</code> gets the state (<code>getState()</code>) from the <code>Memento</code> to set its own state.
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: State3
This example uses a String as the state, which is an immutable object in Java. In real-life scenarios the state will almost always be a mutable object, in which case a copy of the state must be made.
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:
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.