Memento

Memento

The Memento pattern provides the abilitiy to restore an object to its previous state later.

Problem

• The internal state of an object should be saved externally so that the object can be restored to this state later.
• The object's encapsulation must not be violated.

Solution

Make an object (originator) itself responsible for

• saving its internal state to a memento object and
• restoring to a previous state from a memento object.

Only the originator that created a memento is allowed to access it.

A client (caretaker) can request a memento from the originator to save the internal state. It can also pass a memento back to the originator to restore to a previous state.

Common Structure

• Memento
  • is a value object that stores internal state of the Originator object.
  • It is common practice to make Memento immutable and pass it data only once, via constructor.
• Originator
  • creates a memento containing a snapshot of its current internal state.
  • uses the memento to restore its internal state
• Caretaker
  • keeps a history of originator's state by storing a stack of mementos
  • never operates on or examines the contents of a memento
  • When originator has to go back in history, the caretaker passes the last memento to the originator's restoration method.

Collaboration

• Mementos are passive. Only the originator that created a memento will assign or retrieve its state.

Benefits

• Does not violate the originator's encapsulation.
• Keeping the saved state external from the originator helps to maintain cohesion.
• Provides easy-to-implement recovery capability.

Drawbacks

• Saving and restoring state can be time consuming.
• It may require lots of memory if clients create mementors too often.
• Clients should track the originator's lifecycle in order to destroy obsolete mementos.

Example

Memento

[Serializable]
class Memento
{
	public string State { get; }

	public Memento(String state)
	{
		this.State = state;
	}
}

Originator

class Originator
{
	private String _state;

	public void SetState(String state)
	{
		this._state = state;
		Console.WriteLine($"State is set to {state}");
	}

	public Memento Save()
	{
		Console.WriteLine($"Saving state to Memento");
		return new Memento(this._state);
	}

	public void Restore(Memento m)
	{
		this._state = m.State;
		Console.WriteLine($"State is restored from Memento: {_state}");
	}
}

CareTaker

IList<Memento> mementoes = new List<Memento>();

Originator originator = new Originator();
originator.SetState("State 1");
originator.SetState("State 2");
mementoes.Add(originator.Save()); // checkpoint 1

originator.SetState("State 3");
mementoes.Add(originator.Save()); // checkpoint 2

originator.SetState("State 4");
originator.Restore(mementoes[0]); // restore to checkpoint 1

Output

State is set to State 1
State is set to State 2
Saving state to Memento
State is set to State 3
Saving state to Memento
State is set to State 4
State is restored from Memento: State 2

Relations with Other Patterns

• Command can use Mementos to maintain state for undoable operations.

• Iterator - Mementos can be used for iteration as described earlier.