Building an Undo Engine Using Design Patterns

Creating a robust structural modeling tool for civil engineering involves handling numerous complex actions, particularly when it comes to tracking changes. One common dilemma developers encounter is how to manage an undo engine effectively. In this post, we will explore this challenge and provide a comprehensive solution using design patterns, particularly focusing on the Command Pattern.

The Problem with Traditional Undo Mechanisms

When managing objects in a structural model, such as nodes and line elements, a typical approach might be to capture the entire state of the model after each modification. This method, while functional, can be inefficient and cumbersome because:

• Memory Intensive: Deep copies of the entire model can consume a lot of memory, especially if changes are frequent.
• Complex Management: Keeping a large number of copies may complicate the undo/redo process, causing delays and inefficiencies in application performance.

Considering a New Approach

Instead of saving deep-copies of your model after each change, you can implement an action-based undo engine. The idea is to maintain a list of actions (commands) performed on your model, along with their corresponding reverse actions. This approach enables you to efficiently undo or redo actions without duplicating the entire state.

Introducing the Command Pattern

Utilizing the Command Pattern is a widely-accepted strategy for implementing an undo engine. The core principle of this design pattern is as follows:

• Each user action that requires undo functionality is encapsulated in its own command instance.
• Every command contains all the necessary data to execute and to reverse the action.

Components of the Command Pattern

• Command Interface: An interface that defines the methods for executing and undoing actions.
• Concrete Command Classes: Separate classes for each command that implement the command interface. Every command will have:
  • An execute method to perform the action.
  • An undo method to reverse the action.
• Invoker: This is responsible for keeping track of the commands executed and their history. When undoing or redoing actions, the invoker will call the respective methods on the command objects.
• Receiver: The actual object that contains the data and logic for performing actions. The receiver will modify its state based on the commands it receives from the invoker.

Implementing Complex Commands

For complex commands—like inserting new node objects and creating references—you can ensure each command encapsulates all necessary information to both execute and undo the modifications effectively.

Example Steps to Create a Complex Command

1. Define the Command: Create a new class that implements the command interface and contains the specific data required for the operation (e.g., details about the node being added).
2. Implement Execute Method: This method should add the new node to your model and set up any necessary references.
3. Implement Undo Method: This method should remove the added node and clean up any references.

Handling References

To manage references associated with nodes:

• When adding a node, the command should:
  • Store a reference to both the new node and any line elements that reference it.
• The undo method for this command must ensure both the node and references are appropriately reverted.

Benefits of This Approach

• Efficiency: Only the details of the actions are saved, making memory use more efficient.
• Simplicity: You gain a clearer structure in managing undoable actions, enhancing code maintainability.
• Flexibility: Adding new commands to manage additional features can be done easily by creating new command classes.

Final Thoughts

Implementing an undo engine using the Command Pattern not only simplifies the design but also enhances the performance and maintainability of your modeling tool. By tracking actions and their reversals instead of the entire model state, you’ll have a more responsive and memory-efficient application. So, the next time you approach designing an undo function, consider leveraging the Command Pattern for a more streamlined solution.