We've talked about an undo support implementation in the previous section on the Command Pattern, and found it was not easy to implement the mechanism purely based on reversing all the operations. However, if we take snapshots of objects as their history, we may manage to avoid accumulating mistakes and make the system more stable. But then we have a problem: we need to store the states of objects while the states are encapsulated with objects themselves.

Memento Pattern helps in this situation. While a memento carries the state of an object at a certain time point, it also controls the process of setting the state back to an object. This makes the internal state implementation hidden from the undo mechanism in the following example:

We have the instances of the memento controlling the state restoration in the preceding structure. It can also be controlled by the caretaker, namely the undo mechanism, for simple state restoring cases.