Memento Design Pattern in Scala

1. Definition

The Memento Design Pattern provides a way to capture an object's internal state in a manner that it can be restored to that state at a later point. It essentially allows for "undo" capabilities without violating the object's encapsulation.

2. Problem Statement

In many applications, especially those involving user interactions like editors or graphic tools, it's essential to offer an "undo" functionality. Implementing this in a way that doesn't expose the internal details or states of the objects can be challenging.

3. Solution

The Memento pattern involves three key roles:

1. Originator: the object whose state needs to be saved and restored.

2. Memento: a class that represents a snapshot of the Originator's state.

3. Caretaker: responsible for keeping track of the mementos but does not modify them.

4. Real-World Use Cases

1. Undo and redo functionality in text editors.

2. Game save and load mechanism in video games.

3. Snapshot functionality in virtualization tools.

5. Implementation Steps

1. Define the Memento class that will hold the state of the Originator.

2. Modify the Originator to produce Memento objects and also set its state from a Memento.

3. Implement the Caretaker that holds and manages Mementos.

6. Implementation in Scala Programming

// Step 1: Define the Memento class
case class Memento(state: String)
// Step 2: Define the Originator
class Originator(var state: String) {
  // Creates memento
  def createMemento: Memento = Memento(state)
  // Restores state from memento
  def restore(memento: Memento): Unit = {
    state = memento.state
  }
}
// Step 3: Implement the Caretaker
class Caretaker {
  private var mementos: List[Memento] = List()
  def addMemento(m: Memento): Unit = mementos ::= m
  def getMemento(index: Int): Memento = mementos(index)
}
// Client and Tester
object MementoClient extends App {
  val originator = new Originator("Initial State")
  val caretaker = new Caretaker()
  caretaker.addMemento(originator.createMemento)
  originator.state = "State 2"
  caretaker.addMemento(originator.createMemento)
  originator.state = "State 3"
  println(s"Current State: ${originator.state}")
  originator.restore(caretaker.getMemento(1))
  println(s"Restored to: ${originator.state}")
}

Output:

Current State: State 3
Restored to: State 2

Explanation:

1. The Memento class is a case class, capturing the state as a simple String.

2. The Originator can produce Mementos with its current state and can also set its state from a given Memento.

3. The Caretaker is responsible for managing the list of mementos but does not modify their content.

4. In the client code, we simulate changing the state of the originator and using the caretaker to save and restore states.

7. When to use?

The Memento pattern is useful when:

1. You need to implement a snapshot of an object's state so that it can be restored to that state later.

2. Direct access to the object's internal state would violate its encapsulation, but you still need to save it externally.

3. A significant part of the object's state can be made independent, making storing it less costly.