#Memento

#Overview

Capture and externalize an object's internal state so it can be restored later.

#When to use

• You need undo/redo.
• You want to keep state history without exposing internals.

#Java example

class EditorMemento {
    private final String content;
    EditorMemento(String content) { this.content = content; }
    String getContent() { return content; }
}

class Editor {
    private String content = "";
    void type(String text) { content += text; }
    EditorMemento save() { return new EditorMemento(content); }
    void restore(EditorMemento memento) { content = memento.getContent(); }
}

#TypeScript example

class EditorMemento {
  constructor(readonly content: string) {}
}

class Editor {
  private content = "";
  type(text: string): void { this.content += text; }
  save(): EditorMemento { return new EditorMemento(this.content); }
  restore(m: EditorMemento): void { this.content = m.content; }
}

#Pros and cons

Pros:

• Enables undo/redo without exposing internals.
• Clear separation of state snapshots.

Cons:

• Memory usage can grow quickly.
• Snapshot creation may be expensive.

#Common pitfalls

• Saving too frequently without limits.
• Storing large, deep object graphs.

#Related patterns

• Command
• State