memento

Memento

Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

Structure

The formal structure of the Memento pattern can be explained with the class diagram below:

classDiagram

class Originator {
    state
    SetMemento(Memento m)
    CreateMemento()
}

class Memento {
    state
    GetState()
    SetState()
}

class CareTaker

Originator --> Memento : manipulates-a
CareTaker --> Memento : holds-a-reference-of

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It contains a Originator class, which is responsible for creating and updating its own state based on the Memento. The CareTaker usually manipulates a Originator, eventually requesting it to create a snapshot of its state. This snapshot can be kept in memory, for instance, for a certain period of time, and then handed back to the Originator to restore its state, using originator.SetMemento.

The sequence diagram below shows how aCareTaker interacts with anOriginator to create aMemento:

sequenceDiagram

activate aCareTaker
aCareTaker ->> anOriginator: CreateMemento()
activate anOriginator
anOriginator -->> aMemento: new Memento()
activate aMemento
deactivate aMemento
anOriginator ->> aMemento: SetState()
activate aMemento
deactivate aMemento
deactivate anOriginator
deactivate aCareTaker

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And after some time, the careTaker could request the originator to set its state to a previously generated memento:

sequenceDiagram

activate aCareTaker
aCareTaker ->> anOriginator: SetMemento(aMemento)
anOriginator ->> aMemento: GetState()
activate aMemento
deactivate aMemento

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Working example

The working example for this pattern is a simple Tic Tac Toe game. The game uses the Memento Pattern to provide the "undo" functionality for users, allowing to take back a the last move. Check out TicTacToe for implementation details.