memento

Memento Pattern

Intent

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

Applicability

Use the Memento pattern when

• a snapshot of (some portion of) an object's state must be saved so that it can be restored to that state later, and
• a direct interface to obtaining the state would expose implementation details and break the object's encapsulation.

Structure

Participants

• Memento
  • stores internal state of the Originator object.
  • protects against access by objects other than the originator.
• 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.

Example

A NotebookMemento is a snapshot of a Notebook. Notebook can restore its state and provide undo operation according to the NotebookMemento.

Participants in this example:

• Notebook is the Originator.
• NotebookMemento is the Memento, and NotebookMementoInternal is the ConcreteMemento.
• App is the Caretaker.

Scala Tips

N/A

Reference

• Design Patterns: Elements of Reusable Object-Oriented Software