johnnystefan/pattern-memento

Skill: Implement Memento Pattern

Purpose

Captures and externalizes an object's internal state without violating encapsulation, so the object can be restored to this state later.

Procedural Instructions

1. Define Originator: The class whose state needs saving. It creates mementos and uses them to restore itself.
2. Define Memento: A value object that stores the Originator's state. It should be immutable.
3. Define Caretaker: Responsible for keeping track of the mementos (history) but never modifies them.
4. Encapsulation Strategy: Use nested classes (if supported) or restricted interfaces to keep the state private from the Caretaker.

TypeScript Example

class Memento {
  // Immutable
  constructor(private readonly state: string) {}
  getState() {
    return this.state;
  }
}

class Originator {
  private state: string = '';
  save(): Memento {
    return new Memento(this.state);
  }
  restore(m: Memento) {
    this.state = m.getState();
  }
}

class Caretaker {
  private history: Memento[] = [];
  // Manages the history array
}

Payoff

• Safety: Allows producing snapshots of an object's state without violating its encapsulation.
• Clean Originator: Offloads history management to the Caretaker.