Memento Design Pattern

The Memento Design Pattern is a behavioral design pattern that allows an object's state to be captured and restored at a later time without exposing its internal structure. It involves three main components: the originator (object whose state needs to be saved), the memento (object that stores the state of the originator), and the caretaker (object that keeps track of and manages multiple mementos). This pattern helps in implementing undo mechanisms, restoring previous states, or providing a history of changes in an application.

Key points of the Memento Design Pattern:

1. Intent:

   • The main intent of the Memento pattern is to capture an object's internal state in a memento object so that the object can be restored to this state later.

2. Key Components:

   • Originator:
     • The object whose state needs to be saved. It creates and uses a Memento to capture its internal state.
   • Memento:
     • Represents the snapshot of the originator's state. It provides a way to retrieve the state of the originator and may also allow the originator to restore its state.
   • Caretaker:
     • Manages the mementos and is responsible for keeping track of the history of an object's state. It does not have direct access to the originator's state but can request mementos from the originator.

3. State Capture and Restoration:

   • The Memento pattern allows the originator to capture its state in a memento object and later restore its state using that memento.

4. Encapsulation:

   • The internal state of the originator is encapsulated within the memento object. This prevents external objects (such as the caretaker) from accessing the originator's state directly.

5. Multiple States:

   • The originator can create and store multiple mementos, allowing it to restore its state to different points in time.

6. Caretaker Responsibility:

   • The caretaker is responsible for keeping track of the mementos, managing the history of the originator's state, and deciding when to request the originator to restore its state.

7. Advantages:

   • Undo Mechanism: The Memento pattern is often used to implement undo functionality by keeping a history of an object's state changes.
   • Isolation: It isolates the internal state of the originator, preventing direct access from other objects.

8. Use Cases:

   • The Memento pattern is useful when you need to provide an undo mechanism or maintain a history of an object's state changes.
   • It is applicable when the internal state of an object is complex or has a significant impact on the object's behavior.

9. Example:

   • Consider a text editor where you want to implement an undo feature. The Memento pattern could be used to capture snapshots of the editor's state, allowing users to undo changes.