How do you implement an "undo" feature or create a "save point" in a game? While our previous issue on the Command Pattern (issue 33) showed how to undo simple actions, what happens when an operation is huge and complex, like a "find and replace all"? Reversing that step-by-step would be a nightmare. The Memento Pattern offers a simpler, more powerful approach: it takes a "snapshot" of an object's state before the big change happens.
🔍 The Discovery
Name of the Technology: The Memento Pattern
Original Creator/Institution: A classic software design pattern, famously documented in the 1994 book "Design Patterns: Elements of Reusable Object-Oriented Software" by the "Gang of Four."
Year of Origin: circa 1994
License: A fundamental, public domain software design concept.
Think of it like giving a friend a locked box to hold for you.
The Object (Originator): This is the thing you want to save, like your document or your game character.
The Snapshot (Memento): The object creates a "snapshot" of its current state and puts it in a locked box.
The Holder (Caretaker): This is your history list or save file. It takes the locked box to hold onto. The holder can't see what's inside the box; it just keeps it safe.
Later, if you want to undo the change, you just ask the holder for the box back. You then give the box to the original object, which knows how to unlock it and restore itself to the exact state it was in when you took the snapshot. This allows you to perform huge, complex operations with a simple, reliable "rollback" option.
🛠️ Ready for Today: Why This Isn't Just Theory
The Memento pattern is the standard, clean way to implement state-saving and undo functionality, especially for complex objects. It works hand-in-hand with the Command pattern to create robust, transactional systems.
Status: The concept is in the public domain.
Implementations: This is a behavioral design pattern, not a library. It's a recipe for structuring code that is used in countless applications where state restoration is needed.
Undo/Redo Stacks: This is the classic use case. Before a major operation (like "replace all"), a Memento of the document's state is created and pushed onto an "undo" stack. To undo, the Memento is popped off the stack and used to restore the document.
Version History in Dev Platforms: Modern AI and coding platforms like Lovable or Replit often feature a "history" or "rollback" button. Every time you make a significant change, the system saves a Memento of your project's state. This allows you to rewind your entire codebase to any previous point in time, not just undo the last action.
Game Save Systems: When you save your game, the game's core objects (player stats, inventory, world state) create Mementos that are then serialized and written to a file. Loading the game involves restoring those objects from their Mementos.
💡 Creative Applications (Ideas To Get You Thinking)
The ability to snapshot and restore an object's state cleanly opens up many possibilities for creating more forgiving and powerful user experiences.
Idea 1 (A "Configuration Wizard" with Rollback): Imagine a multi-step wizard for setting up a complex piece of software. After each step, the configuration object creates a Memento. If the user makes a mistake on step 5, they can simply click a "Back" button, which restores the Memento from step 4, effectively undoing the last set of changes without having to cancel the entire process.
Idea 2 (A "Try-On" Feature for E-Commerce): An e-commerce site for customizable products (like a car or a computer) could use the Memento pattern. The user's current configuration is the Originator. Each time they add an expensive option, a Memento is saved. This would allow the site to offer a "Compare" or "Revert to Base Model" button that could instantly restore a previous configuration for easy comparison.
Idea 3 (A "Replayable" Bug Reporting Tool): A software testing tool could record a user's actions. Instead of just recording clicks, it could also save a Memento of the application's state at key moments. When a bug occurs, the developer gets not just a list of actions, but a series of "save points" they can restore to debug the application's state at the exact moment the bug happened.
🐰 The Rabbit Hole
Dive Deeper: The "Refactoring.Guru" website has a fantastic, clear explanation of the Memento pattern. It uses the example of a text editor and its history to perfectly illustrate the roles of the Originator, Memento, and Caretaker, complete with UML diagrams and code examples.
Join The Search
Our mission is to unearth the world's most powerful, overlooked ideas. If you know of a technology that is trapped in a niche, overshadowed by hype, or simply deserves a bigger spotlight, please submit it for a future issue here.
Till next time,