memento

Memento Design Pattern

Overview

Throughout the lifecycle of an application, an objects state may change. You might want to store a copy of the current state in case of later retrieval. E.g., when writing a document, you may want to auto save the current state every 10 minutes. Or you have a game, and you want to save the current position of your player in the level, with its score and current inventory.

You can use the Memento pattern for saving a copy of state and for later retrieval if necessary.

The Memento pattern, like the Command pattern, is also commonly used for implementing UNDO/REDO functionality within your application.

The difference between the Command and the Memento patterns for UNDO/REDO, is that in the Command pattern, you re-execute commands in the same order that changed attributes of a state, and with the Memento, you completely replace the state by retrieving from a cache/store.

Terminology

• Originator: The originator is an object with an internal state that changes on occasion.
• Caretaker: (Guardian) A Class that asks the Originator to create or restore Mementos. The Caretaker than saves them into a cache or store of mementos.
• Memento: A copy of the internal state of the Originator that can later be restored back into the Originator to replace its current state.

Memento UML Diagram

Memento Use Case

There is a game, and the character is progressing through the levels. It has acquired several new items in its inventory, the score is very good, and you want to save your progress and continue later.

You then decide you made a mistake and need to go back to a previous save because you took a wrong turn.

Memento Example UML Diagram

Summary

• You don't need to create a new Memento each time an Originators state changes. You can do it only when considered necessary. E.g., an occasional backup to a file.

• Mementos can be stored in memory or saved/cached externally. The Caretaker will abstract the complications of storing and retrieving Mementos from the Originator.

• Consider the Command pattern for fine-grained changes to an objects state to manage UNDO/REDO between memento saves. Or even save command history into a Memento that can be later replayed.

• In my examples, the whole state is recorded and changed with the Memento. You can use the Memento to record and change partial states instead if required.

• When copying state, be aware of shallow/deep copying. In complicated projects, your restore functionality will probably contain a combination of both the Command and Memento patterns.