Memento

Usage

• Also known as: Snapshot; Token
• Capture and restore an object's internal state
• Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later

Diagram

Participants

Memento

• Stores internal state of the Originator object.
• The memento may store as much or as little of the originator's internal state as necessary at its originator's discretion.
• Protect against access by objects of other than the originator.
• Mementos have effectively two interfaces:

• Caretaker sees a narrow interface to the Memento -- it can only pass the memento to the other objects.
• Originator, in contrast, sees a wide interface, one that lets it access all the data necessary to restore itself to its previous state.

• Ideally, only the originator that Produces the memento would be permitted to access the memento's internal state.

Originator

• Creates a memento containing a snapshot of its current internal state.
• Uses the memento to restore its internal state

Caretaker

• Is responsible for the memento's safekeeping
• Never operates on or examines the contents of a memento.