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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.