Memento-1© Gunnar Gotshalks

Memento Pattern – Behavioural

• Intent» Without invalidating encapsulation, capture and

externalize an objectʼs internal state so that theobject can be restored to this state later

• Also known as» Token

Memento-2© Gunnar Gotshalks

Motivation

• Sometimes it is necessary to record the internal stateof an object» Implementing checkpoints» Undo mechanisms

• Save the state to be able to restore it later

• Do not want to comprise an objectʼs reliability andextensibility by exposing the internal state

Memento-3© Gunnar Gotshalks

Example Graphic

• Consider a graphical editor that supports connectivitybetween objects» As the grey rectangle is moved the line stretches

and moves its end points» A ConstraintSolver object is used to encapsulate

this functionality» Supporting an undo function for this is not easy

Memento-4© Gunnar Gotshalks

Example Graphic – 2

• Just recording the original position of the grey box isinsufficient» UNDO: moves the box to original position and re-

computing the connections may not work> There usually is often slack in the computation> Not all connections may be as in the original

Memento-5© Gunnar Gotshalks

Example Graphic – 3

• Memento pattern solves the problem» Memento stores a snapshot of the internal state of

another object – the originator» The undo mechanism requests a memento from

the originator when it needs to checkpoint the state» The original initializes the memento with its current

state> Only the originator can store and retrieve

information from the memento

• In the example, the ConstraintSolver acts as anoriginator

Memento-6© Gunnar Gotshalks

Abstract Architecture

Memento-7© Gunnar Gotshalks

Scenario

1 Request memento originator.createMemento2 Create and set memento state3 Set state from memento originator.setFrom(memento)4 get_state

Scenario: Create & use memento

Memento-8© Gunnar Gotshalks

Participants

• Memento (SolverState)» Stores internal state of the Originator object

> Enough state for the purpose

» Has two interfaces> Caretaker sees narrow interface

– Can only pass memento to other objects> Originator sees a wide interface

– To be able to store & access all necessaryinformation

Memento-9© Gunnar Gotshalks

Participants – 2

• Originator (ConstraintSolver)» Creates a memento containing a snapshot of its

current internal state» Uses a memento to restore its internal state

• Caretaker (undo mechanism)» Is responsible for the mementoʼs safekeeping» Never operates on or examines the contents of a

memento

Memento-10© Gunnar Gotshalks

Applicability

• Use when» A snapshot of some portion of an objectʼs state

must be saved so that it can be restored to thatstate later

» A direct interface to obtaining the state wouldexpose implementation details and break theobjectʼs encapsulation

AND

Memento-11© Gunnar Gotshalks

Consequences

• Preserving encapsulation boundaries» Avoids exposing information that only an

originator should manage but must be storedelsewhere

• Simplifies Originator» In other encapsulating-preserving designs

Originator state is only in the Originator putting astorage and retrieval burden on the Originator

Memento-12© Gunnar Gotshalks

Consequences – 2

• Mementos might be expensive» There may be considerable overhead if the state is

large

» Clients create and return mementos frequently

• Defining narrow and wide interfaces» May be difficult in some languages to restrict

internal state to only Originator access

OR

Memento-13© Gunnar Gotshalks

Consequences – 3

• Hidden costs in Caretaker» Is responsible for saving mementos» Does not know how large mementos may be» Otherwise lightweight Caretakers may incur large

storage costs

Memento-14© Gunnar Gotshalks

Related Patterns

• Command can use mementos to maintain state forundoable operations

• Mementos can be used in iterators» Memento-based iteration doesnʼt require breaking

the collectionʼs encapsulation> All information is interpreted only by the

collection