Multiglom Logistics provides a tool for order management to its clients. Originally planned as a workaround the software has become quite popular due to its simplicity and focus. However, clients are missing an undo/redo for the text input component.
It was decided to add this functionality with the upcoming release.
The purpose of the new flexible text processor is to provide the usual undo and redo functionality known from common text processors.
Main Features
- There shall be an undo-button for every text input action of a user.
- A corresponding redo-button shall allow the user to compensate the last undo-action.
- A sequence of redo-clicks shall compensate the previous sequence of undo-clicks (if any).
Table 1. Quality Goals
| No. | Quality | Motivation |
|---|---|---|
| 1 | Usability | The feature shall work as in any other common application. |
In this scenario we want to apply the Command Pattern to encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations (GoF).
We have identified "typing text" (append/delete letters) as actions (commands) that shall support undo.
Each command has an execute() and a corresponding executeUndo() operation to compensate the action.
Open CommandTest.java to start playing with this pattern. By setting the log-level for this pattern to DEBUG in logback.xml you can watch the pattern working step by step.
See also Command Processor.
- The Command pattern looks simple in the first place and gets tricky when it comes to the details. A command encapsulates what to do in what context. This means: commands include or assume state.
- Commands usually defer execution. The latter is an interesting aspect when you use the Command pattern in more complex scenarios. A good example is the user context: A properly authenticated and authorized user performs an action. Now you want to encapsulate that action in a command to be stored in a queue or database to eventually execute it later leveraging a scheduler. How do you restore the user context at the later execution time of that command?
- Technically, Lambda expressions (closures) are a kind of Command. Programming languages have found different ways to handle the context problem of the deferred execution when accessing variables (see also Java Lambda Expressions).
- (GoF) Gamma, E., Helm, R., Johnson, R., Vlissides, J.: Design Patterns – Elements of Reusable Object-Oriented Software. Addison-Wesley (1995).
- (Martin) Martin, R.C.: Agile Software Development: Principles, Patterns and Practices. Addison-Wesley (2002)